Nuʻupia Ponds’ Water Circulation Characteristics: Exploring Water Exchange and Residence Time for Marine Ecosystem Management

Abstract

Nuʻupia Ponds, a traditional Hawaiian fishpond system, are located at Marine Corps Base Hawaiʻi (MCBH) and part of the Nuʻupia Ponds Wildlife Management Area, a wetland refuge for native, endangered, and protected birds and Hawaiian green sea turtles, as well as many native fish species. Currently, there is uncertainty regarding the ecological status and condition of the fishponds following prior modification of wetland habitats in and around the ponds. This study examines circulation dynamics and characterizes water exchange, pond volume, and residence time across the full tidal spectrum at the Nuʻupia fishponds. Our results indicate a general west to east gradient in current flow; with higher flushing rates and lower residence times of fishponds in the western ponds of the Nuʻupia system compared to the eastern ponds. We further found low flushing rates at several sites causing limited water exchange with Kāneʻohe Bay, as well as within the Nuʻupia Pond system. Sufficient water circulation plays a fundamental role in maintaining a healthy balance of fishpond flora and fauna and preserving ecosystem health. The results from this study provide a baseline for current physical water circulation dynamics and implications for ecosystem health, as well as informing science-based conservation and management strategies moving forward.

1. Introduction

The dynamics of physical processes within coastal ecosystems influence temporal and spatial scales of chemical and biological interactions and together substantially impact the health of coastal ecosystems [1]. In shallow, semi-enclosed systems such as traditional Hawaiian fishponds, water exchange and the coupled transport of dissolved and particulate materials play a fundamental role in shaping water quality, nutrient availability, pollution levels, and a healthy balance of fishpond flora and fauna [1,2,3]. Hence, understanding the temporal and spatial scales of transport and residence time is crucial for assessing how physical circulation patterns impact coastal ecosystems and provides practical information for resource management [4].

This intricate interplay presents a well-understood concept in traditional Hawaiian aquaculture: walled fishponds, known as Loko iʻa kuapa, were commonly built, across the islands centuries ago, within natural coastal embayments, where freshwater meets the ocean. This strategic placement leveraged the nutrient-rich freshwater influx from streams or submarine groundwater discharge—enhancing primary production within confined brackish environments [5,6]. In the traditional Hawaiian fishpond system, photosynthetic microorganisms form the base of the food web, creating ideal conditions for herbivorous fish species and crustaceans to thrive, rendering efficient generation of protein for human consumption [5]. Kuapā (fishpond wall) with mākāhā (size-slotted sluice gates) were strategically placed and designed to harness the natural flow of ocean tides while simultaneously preventing the escape of fish and creating a low wave energy environment within the fishponds. Mākāhā were used to harvest fish as well as regulate exchange with freshwater and ocean water, retaining a minimum water volume inside the fishponds at all times [5,6,7]. Water volume flux (m³ s⁻¹), which refers to the volume of water passing through each sluice gate per unit time, can flow into or out of the fishpond depending on sluice gate location, tidal phase, and other environmental conditions such as winds and precipitation [4]. Loko iʻa stewards utilized their understanding of juvenile fish migration to enhanced fish stocks capturing desired species behind sluice gates until they matured, while also thwarting the entrance of larger predators [5]. Loko iʻa reflected the deep-rooted wisdom and environmental stewardship of the Hawaiian culture, alleviated pressure on wild fish populations, provided juvenile fish habitat, and played an important role in nutrient cycling and sediment capture, while fostering a harmonious balance between marine and freshwater ecosystems. Native Hawaiian communities depended on fishponds as a source of protein: it is estimated that during the peak of fishpond activity, loko iʻa produced approximately 2 million pounds of fish annually [5,8].

Prior to western contact, at least thirty loko iʻa were located along Kāneʻohe Bay providing a rich fish and seaweed harvest from the ponds to the inhabitants of the Koʻolaupoko district, one of the largest populations in the Hawaiian Islands [9,10]. Nuʻupia Ponds was among them and historically supported its people, providing them with sustenance, resources, and an essential connection to their land. Archeological coring and testing research revealed the probable alignments of the original fishpond walls and estimated the age of the constructed fishpond at around A.D. 865-1230 [9,11]. Over time, a combination of land use changes (e.g., a shift from a subsistence to a plantation economy and disuse), physical changes (e.g., sedimentation and storm damage), and biological invasions of invasive species (e.g., mangroves) have led to dramatic alterations and the decline of most loko iʻa across the state of Hawaiʻi [12,13].

Today, Nuʻupia Ponds, located on the Marine Corps Base Hawaiʻi (MCBH), comprises a series of interconnected shallow water lagoons that serve as an essential habitat for a wide array of biodiversity, making it a vital breeding ground for bird species and native aquatic flora [9]. The ponds are home to a variety of migratory and endemic bird species, and provide crucial nesting and feeding grounds for waterfowl and shorebirds. Rare and endangered bird species, such as the Hawaiian stilt (aeʻo) and the Hawaiian coot (ʻalae keʻokeʻo), find sanctuary in this protected area, adding to its ecological significance [9,11]. Nuʻupia Ponds is not managed with the traditional goal of cultivating fish for food security at this time. Instead, Marine Corps Base Hawaiʻi’s management plans prioritize the enhancement of biodiversity and the promotion of optimal ecosystem health as their primary goals. Past MCBH management plans have included the eradication of invasive vegetation such as pickleweed (Batis maritima) and mangroves (Rhizophora mangle) from the Nuʻupia Ponds Wildlife Management Area, restoration of the mudflat habitat used by Hawaiian Stilt for feeding and nesting, and clearing of sediments, coral materials, and vegetation blocking culverts facilitating water exchange among ponds to improve water circulation [11]. Currently, there is uncertainty regarding the ecological status and condition of the fishponds following prior modification of wetland habitats in and around the ponds. In this study, we examine circulation dynamics at Nuʻupia fishponds using current meters, pressure sensors, and a bathymetric survey, and characterize water exchange, pond volume, and residence time across the full tidal spectrum at eight interconnected fishponds. Residence time and exchange patterns in shallow and coastal settings hold substantial practical significance, offering coastal managers important information on how physical processes might affect ecological and biogeochemical drivers within these environments. As such, sufficient water circulation in each of the ponds is central in order to maintain water quality, ensure oxygen saturation for fish, and preserve biodiversity [4,12]. The results from this study provide a baseline for current physical water circulation dynamics and allow MCBH to evaluate the effects of future modifications and activities on the ponds as well as guide science-based conservation and management strategies moving forward. To our current understanding, this study is the first that provides a holistic picture of the physical processes within Nuʻupia Ponds and its implications for ecosystem conservation and management.

2. Methods

2.1. Nuʻupia Ponds Study Site

Nuʻupia Ponds is located within the Kāneʻohe Bay on the Marine Corps Base Hawaiʻi (MCBH) on the windward side of the island of Oʻahu in Hawaiʻi (Figure 1A). The pond system is part of the Nuʻupia Ponds Wildlife Management Area (Nuʻupia Ponds WMA), a wetland refuge for native, endangered, and protected birds and Hawaiian green sea turtles, as well as many native fish species. Nuʻupia Ponds WMA straddles the Mōkapu Peninsula from east to west along the southern boundary of the base and encompasses approximately 483 acres. Nuʻupia Ponds is also one of the few remnants of a complex of ancient Hawaiian fishponds that formerly existed along the shorelines of Kāneʻohe Bay [5,9,10]. The eight historic Hawaiian walled fishponds (Loko iʻa kuapā) have a combined surface water area of 237 acres and associated wetlands areas of 377 acres that consist largely of mudflats, Batis maritima L. meadows, and monospecific stands of L. mangle [14]. The Mōkapu Peninsula is part of two traditional Hawaiian land divisions called ahupuaʻa: Kāneʻohe and Heʻeia ahupuaʻa. From ridge to reef, these ahupuʻa encompassing Nuʻupia Ponds provided its historical residents with everything needed to live in a self-sufficient way [9,10]. The ponds have been modified over the years, but some of the original pond walls and overall structure remain largely intact [11]. Nuʻupia Ponds consists of eight individual ponds: Nuʻupia ʻEkahi, Nuʻupia ʻElua, Nuʻupia ʻEkolu, Nuʻupia Ehā, Halekou, Heleloa, Paʻakai, and Kaluapuhi (Figure 1B). The pond system is interconnected through a set of fourteen exchange points (exchange point here describes a combination of concrete culverts and gaps that can facilitate water exchange between the eight individual ponds) that facilitate water exchange to varying degrees (Figure 1B;

Table 1. Summary of fishpond system exchange points, including a description, latitude and longitude, compass heading, total width (m), and instruments deployed at site.

Exchange Point	Ponds Connected	Latitude	Longitude	Heading	Total Width (m)	Instrument	Description
Site 1	Channel and Halekou	21.43745	−157.753614	90°/270°	39	CM, PS	3 Gaps
Site 2	Kāneʻohe Bay and Channel/Heleloa	21.43527	−157.756953	121°/318°	11	CM, PS	Bridge
Site 3	Kāneʻohe Bay and Nuʻupia ʻEkahi	21.43404	−157.757554	125°/305°	3.45	CM, PS	3 Culverts
Site 4	Kāneʻohe Bay and Nuʻupia ʻEkahi	21.43086	−157.760024	82°/na	2.2	CM, PS	2 Culverts
Site 5	NA	21.42960	−157.75913	na/na	na	PS	1 Culvert
Site 6	Nuʻupia ʻEkahi and Nuʻupia ʻElua	21.42948	−157.755293	na/na	1.4	PS	2 Culverts
Site 7	Nuʻupia ʻElua and Nuʻupia ʻEkolu	21.43208	−157.752659	80°/260°	68	CM, PS	3 Gaps
Site 8	Nuʻupia ʻEkahi and Nuʻupia ʻElua	21.43214	−157.755238	80°/285°	0.75	CM, PS	1 Culvert
Site 9	Nuʻupia ʻEkahi and Halekou	21.43298	−157.755018	na/na	0.85	PS	1 Culvert
Site 10	Halekou and Nuʻupia ʻElua	21.43284	−157.754489	150°/345°	10	CM, PS	1 Gap
Site 11	Halekou and Nuʻupia ʻEkolu	21.43486	−157.751989	130°/303°	12	CM, PS	1 Gap
Site 12	Nuʻupia ʻEkolu and Nuʻupia ʻEhā	21.43123	−157.743009	91°/na	1.2	CM, PS	2 Culverts
Site 13	Nuʻupia ʻEhā and Kaluapuhi	21.43116	−157.742126	82°/230°	0.8	CM, PS	2 Culverts
Site 14	Kaluapuhi and Paʻakai	21.43275	−157.739312	na/na	3	PS	1 Gap

).

Nuʻupia Ponds is connected to Kāneʻohe Bay in the west through a channel flowing under the John A. Burns Freeway Bridge at Sites 1 and 2 (Figure 2A). The channel feeds directly into Heleloa and Halekou Ponds. Culverts at Site 3 and Site 4 facilitate additional water exchange between Kāneʻohe Bay and Nuʻupia ʻEkahi Pond (Figure 2A, Table 1). Historically, Paʻakai Pond was connected to Kailua Bay in the east. However, today, the former channel is filled with sand and marshland and the connection is dry. Even at extreme high tides occurring 1–2 times a year (also called “King Tides”), the eastside ocean connection is not re-established (Figure 2B).

2.2. Water Volume Flux and Volume Change Calculations

To assess current direction (°), velocity (m s⁻¹), and water level (m) into and out of Nuʻupia Ponds, TCM-4 Shallow Water (SW) Tilt Current Meters (Lowell Instruments LLC, East Falmouth, MA, USA) and HOBO^® pressure sensors measuring water level (Onset, Bourne, MA, USA) were deployed in ten out of fourteen exchange points (Figure 1B; Table 1). Measuring current direction and velocities presented a challenge at this study site due to its shallow depth, as many commonly used current meters do not work in such shallow water environments. As the TCM-4 Shallow Water (SW) Tilt Current Meters require a minimum water depth of 30 cm, Sites 5, 6, 9, and 14 were solely equipped with pressure sensors due to shallow water depth (Table 1). Current meters and pressure sensors were mounted to a round concrete plate 16 inch in diameter and about 10 kg in weight and positioned at the center of each culvert channel or gap (Figure A1). For Sites 5, 6, 9, and 14, pressure sensors were zip-tied to bricks and placed at the center of the exchange point. All measurements were recorded at recommended meter configurations with a burst interval of 1 min, a burst rate of 8 Hz, and a burst duration of 20 s, rendering 160 samples per minute [15]. Thus, each minute velocity measurement was an average of 160 samples taken over a 20 s period. Current meters and pressure sensors were deployed for a duration of 15 days with the rationale of recording measurements throughout the entire tidal range and capturing one complete neap tide and one complete spring tide (See Figure A1 in Appendix A for images of the deployment set-up). The deployment period in July 2022 coincided with extreme high tides (also called “King Tides”).

Water volume flux and water velocity measurements (m s⁻¹) obtained from the TCM-4 Shallow Water (SW) Tilt Current Meters were utilized to create rating curves illustrating water volume flux and water level for each exchange point. To accommodate the bidirectional water flow in the channel caused by tidal forcing, water volume flux was calculated for a complete tidal cycle at the subsequent tidal stages: spring influx; spring outflux; neap influx; neap outflux. The cycle exhibiting the highest tidal amplitude was chosen to represent the spring tide, whereas the cycle with the lowest tidal amplitude was selected to represent the neap tide. The data were divided into influx and outflux segments based on flow direction as the switch in flow direction did not always occur at the pressure maximum/minimum. Rating curves were then generated using the following equation for spring influx, spring outflux, neap influx, and neap outflux:

φ = wdv

(1)

Water volume flux (φ) is a function of the respective channel width (w), the water level (d), and water velocity (m s⁻¹) changing over time (v) [4,16,17]. Adapting methodologies utilized at Heʻeia fishpond [4], rating curves were fitted using a polyfit function with a best-fit line and 95% confidence intervals in Matlab (with the exception of Site 2, for which the polyfit function did not provide a good fit) (The MathWorks Inc., Natick, MA, USA). The mean and maximum and relative water volume flux percentage for each exchange point were calculated for four tidal cycles (

Table 2. Water volume flux (WVF) dynamics at Nuʻupia Ponds: mean and peak water volume flux, average flow velocity, tidal cycle length, hourly rate of water volume flux, volume exchanged per tidal cycle, and percentage of relative water volume flux.

	Mean WVF (m3 s−1)	Peak WVF (m3 s−1)	Average vx (m s−1)	Tidal Cycle Length (h)	WVF Rate (m3 h−1)	Volume Exchanged per Tidal Cycle (m3)	Relative WVF (%)
Spring Influx
Site 1	6.97	10.74	0.33	6.17	25,082	154,753	30.1
Site 2	4.01	6.43	0.30	7.62	14,437	110,011	21.4
Site 3	1.02	1.44	0.68	6.5	3667	23,835	4.6
Site 4	0.39	0.56	0.35	5.01	1411	7070	1.4
Site 5	0.00	0.00	0.00	0	0	0	0
Site 6	na	na	na	na	na	na	<1
Site 7	3.69	7.10	0.13	7.1	13,273	94,237	18.3
Site 8	0.03	0.04	0.06	8.73	91	798	0.2
Site 9	na	na	na	na	na	na	<1
Site 10	2.39	3.49	0.34	6.9	8599	59,333	11.5
Site 11	2.18	3.06	0.41	7.68	7863	60,392	11.7
Site 12	0.09	0.18	0.22	9.16	337	3083	0.6
Site 13	0.02	0.03	0.08	10.89	65	710	0.1
Site 14	na	na	na	na	na	na	<1
Spring Outflux
Site 1	−2.87	−5.07	0.18	16.68	−10,333	−172,362	25.7
Site 2	−4.18	−6.07	0.54	16.72	−15,033	−251,350	37.5
Site 3	−0.26	−0.50	0.16	17.96	−942	−16,920	2.5
Site 4	no outflux	no outflux	no outflux	no outflux	no outflux	no outflux	0
Site 5	0.00	0.00	0.00	0	0	0	0
Site 6	na	na	na	na	na	na	<1
Site 7	−0.81	−4.77	0.03	17.55	−2921	−51,271	7.6
Site 8	no outflux	no outflux	no outflux	no outflux	no outflux	no outflux	0
Site 9	na	na	na	na	na	na	<1
Site 10	−1.51	−2.49	0.23	16.96	−5454	−92,494	13.8
Site 11	−1.45	−2.07	0.30	16.22	−5233	−84,884	12.7
Site 12	no outflux	no outflux	no outflux	no outflux	no outflux	no outflux	0
Site 13	−0.03	−0.04	0.18	10.13	−114	−1160	0.2
Site 14	na	na	na	na	na	na	<1
Neap Influx
Site 1	3.10	5.55	0.17	8.28	11,156	92,369	27.7
Site 2	2.28	4.01	0.20	8.71	8195	71,382	21.4
Site 3	0.56	0.91	0.43	6.88	2030	13,967	4.2
Site 4	0.01	0.01	0.01	12.53	19	244	0.1
Site 5	na	na	na	na	na	na	0
Site 6	na	na	na	na	na	na	<1
Site 7	2.28	4.20	0.10	8.12	8215	66,705	20.0
Site 8	0.05	0.11	0.12	12.4	165	2049	0.6
Site 9	na	na	na	na	na	na	<1
Site 10	1.77	2.55	0.26	8.03	6363	51,098	15.3
Site 11	0.93	1.61	0.20	10.38	3342	34,692	10.4
Site 12	0.03	0.06	0.09	7.48	118	886	0.3
Site 13	no influx	no influx	no influx	no influx	no influx	no influx	0
Site 14	na	na	na	na	na	na	<1
Neap Outflux
Site 1	−2.94	−1.60	0.19	7.5	−10,570	−79,275	27.2
Site 2	−2.76	−4.16	0.31	9.48	−9924	−94,077	32.2
Site 3	−0.11	−0.18	0.08	13.55	−388	−5254	1.8
Site 4	no outflux	no outflux	no outflux	no outflux	no outflux	no outflux	0
Site 5	na	na	na	na	na	na	0
Site 6	na	na	na	na	na	na	<1
Site 7	−0.83	−3.28	0.04	9.83	−2989	−29,386	10.1
Site 8	−0.01	−0.03	0.03	11.93	−44	−524	0.2
Site 9	na	na	na	na	na	na	<1
Site 10	−1.29	−2.12	0.20	8.96	−4637	−41,549	14.2
Site 11	−1.18	−1.68	0.26	9.5	−4230	−40,188	13.8
Site 12	no outflux	no outflux	no outflux	no outflux	no outflux	no outflux	0.0
Site 13	−0.03	−0.04	0.21	13.08	−125	−1629	0.6
Site 14	na	na	na	na	na	na	<1

). To account for varying tidal cycle length resulting from mixed semidiurnal tides in Kāneʻohe Bay, flow rates for each exchange point were normalized by calculating the hourly water volume flux rate. For exchange points with more than one culvert or opening, the area of all culverts/gaps combined (Table 1) was summed up in order to estimate total water volume flux for each respective site (Table 2). The following assumptions were made for this calculation: (1) flow was equally distributed at exchange points with multiple openings; (2) the openings of all exchange points remained in the same condition throughout the measurement periods. Regular visual observations during the measurement period confirmed that none of the exchange points became blocked by debris or otherwise.

Current meters deployed at Site 8 and Site 11 were flooded (damaged current meters showed significant bite marks, making it likely that they were bitten open by a marine organism (see Figure A2 in Appendix A for images)) during the first deployment in July and were redeployed in September 2022. Due to the similarity of the tidal pattern, we were able to adjust Site 11 at the spring influx and outflux back to the original deployment period in July. This could be accomplished by comparing pressure sensor measurements from the original and the redeployment period. Due to a near identical shape of the tidal curve, the rate of increase/decrease in water level and the associated water velocity measurements were hind-casted back to the original deployment time. For that, we made the assumption that current meter measurements of water velocity were in a comparable relationship to water level in both deployment periods given the similarity of the tidal curve. At Site 8, and during neap tide for both sites, the tidal curve was too different to confidently adjust times back to the original deployment period. For that reason, these sites could not be directly compared in the time lag and tidal onset order comparison (

Table 3. Time lag dynamics in Nuʻupia Ponds. Table shows the time of onset of influx and outflux for neap and spring tide, tidal duration, order of influx/outflux onset, and time lags since time 0:00 at Site 2.

Spring Influx	Time of Influx Start	Time of Influx End	Flood Duration Based on Influx/Outflux	Tidal Onset Order (Influx/Outflux)	Influx/Outflux Time Lag (Hours:Minutes)
Site 1	13 July 2022 12:30	13 July 2022 19:02	6.53	3	1:12
Site 2	13 July 2022 11:18	13 July 2022 18:55	7.62	1	0:00
Site 3	13 July 2022 12:13	13 July 2022 18:43	6.5	2	0:55
Site 4	13 July 2022 13:20	13 July 2022 18:22	5.01	5	2:02
Site 5	na	na	na	na	na
Site 6	na	na	na	na	na
Site 7	13 July 2022 12:39	13 July 2022 19:45	7.1	4	1:21
Site 8	6 September 2022 8:31	6 September 2022 17:14	8.73	na	na
Site 9	na	na	na	na	na
Site 10	15 July 2022 13:26	15 July 2022 20:19	6.9	na	na
Site 11	13 July 2022 12:38	13 July 2022 20:18	7.68	4	1:20
Site 12	13 July 2022 14:51	14 July 2022 0:00	9.16	6	3:33
Site 13	13 July 2022 17:59	14 July 2022 4:51	10.89	7	5:41
Site 14	na	na	na	na	na
Spring Outflux	Time of Outflux Start	Time of Outflux End
Site 1	13 July 2022 19:18	14 July 2022 11:59	16.68	3	0:13
Site 2	13 July 2022 19:05	14 July 2022 11:48	16.72	2	0:00
Site 3	13 July 2022 18:50	14 July 2022 12:48	17.96	1	−0:15
Site 4	na	na	na	na	na
Site 5	na	na	na	na	na
Site 6	na	na	na	na	na
Site 7	13 July 2022 19:45	14 July 2022 13:18	17.55	4	0:40
Site 8	na	na	na	na	na
Site 9	na	na	na	na	na
Site 10	14 July 2022 20:16	14 July 2022 12:48	16.96	na	na
Site 11	13 July 2022 20:53	14 July 2022 13:12	16.22	5	1:48
Site 12	na	na	na	na	na
Site 13	14 July 2022 7:18	14 July 2022 17:25	10.13	6	12:13
Site 14	na	na	na	na	na
Neap Influx	Time of Influx Start	Time of Influx End
Site 1	19 July 2022 12:51	19 July 2022 21:08	8.28	2	0:10
Site 2	19 July 2022 12:41	19 July 2022 21:23	8.71	1	0:00
Site 3	19 July 2022 14:24	19 July 2022 21:17	6.88	6	1:43
Site 4	19 July 2022 13:58	20 July 2022 2:30	12.53	5	1:17
Site 5	na	na	na	na	na
Site 6	na	na	na	na	na
Site 7	19 July 2022 13:43	19 July 2022 21:50	8.12	4	1:02
Site 8	1 September 2022 7:34	1 September 2022 19:59	12.4	na	na
Site 9	na	na	na	na	na
Site 10	19 July 2022 13:22	19 July 2022 21:39	8.03	3	0:41
Site 11	1 September 2022 3:18	1 September 2022 16:40	10.38	na	na
Site 12	19 July 2022 18:39	20 July 2022 2:07	7.483	7	5:58
Site 13	na	na	na	na	na
Site 14	na	na	na	na	na
Spring Outflux	Time of Outflux Start	Time of Outflux End
Site 1	19 July 2022 22:18	20 July 2022 5:48	7.5	5	0:53
Site 2	19 July 2022 21:25	20 July 2022 6:53	9.48	1	0:00
Site 3	19 July 2022 21:37	20 July 2022 11:10	13.55	2	0:12
Site 4	no outflux	no outflux	no outflux	na	na
Site 5	na	na	na	na	na
Site 6	na	na	na	na	na
Site 7	19 July 2022 21:53	20 July 2022 7:42	9.83	3	0:28
Site 8	1 September 2022 21:39	2 September 2022 9:34	11.93	na	na
Site 9	na	na	na	na	na
Site 10	19 July 2022 22:09	20 July 2022 7:06	8.96	4	0:44
Site 11	1 September 2022 17:58	2 September 2022 3:28	9.5	na	na
Site 12	na	na	na	na	na
Site 13	20 July 2022 0:18	20 July 2022 13:22	13.08	6	2:53
Site 14	na	na	na	na	na

). Therefore, differences in tidal curve between the two deployment periods may cause slight alterations in the relative water volume flux quantifications for Sites 8 and 11, particularly for the neap tidal cycle.

To contextualize flow data with meteorological conditions, precipitation, wind direction, wind speed, and water and air temperature were obtained from a weather station Moku o Loʻe (21.4339° N, 157.7881° W), ~3 km from Nuʻupia Pond (Figure 1A). A sea level gauge equipped with a water temperature probe, positioned approximately 10 m offshore from the weather station at a depth of around 1 m, served as the source of tidal reference data.

2.3. Pond Bathymetries, Volumes, and Residence Times

Nuʻupia Ponds volumes were calculated using bathymetric depth measurements recorded by a Deeper Smart Sonar PRO+ 2 (Deeper, Vilnius, Lithuania) from 22 to 26 August 2022. Sonars were mounted to one kayak and one stand-up paddle (SUP) board that measured water depth along a grid that followed the ‘mowing the lawn’ strategy. Due to the large area that needed to be covered, cross lines were approximately 10–20 m apart, depending on the size of the area undergoing measurements. During the bathymetry mapping, HOBO^® water level loggers (Onset, Bourne, MA, USA) were deployed for reference pressure to record and adjust tidal fluctuations during the measurement period. To accomplish this, tidal changes during the measurement period were added or subtracted from the depth measurements to standardize the entire data set. Further, a second HOBO logger deployed on land was used to correct reference pressure data for atmospheric pressure fluctuations. To account for differences in tidal amplitude across the full tidal spectrum, bathymetry data were adjusted to four tidal stages: Spring High (SH); Spring Low (SL), Neap High (NH), Neap Low (NL) by adding/subtracting the change in water depth from the bathymetry data set. Bathymetry maps and pond volumes were calculated using the DrDepthPC Version 5.1.8. (Per Pelin) program with an interpolation limit of 250 m and an extrapolation limit of 25 m to fill in gaps between measured bathymetry points. The volumes and areas module calculated water volumes, area, as well as maximum, minimum, and average depths.

To determine the minimum residence time in Nuʻupia Fishpond, we calculated the volume of water exchanged during the transition from ebb to flood tide for both neap and spring tides, employing methodologies similar to those utilized at Heʻeia fishpond [4,17]. This calculation was conducted using the following equations:

$$\begin{gathered} \tau \mathrm{N}\mathrm{P}\mathrm{S}=\mathrm{P}\mathrm{o}\mathrm{n}\mathrm{d} \mathrm{V}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{m}\mathrm{e} \mathrm{E}\mathrm{x}\mathrm{c}\mathrm{h}\mathrm{a}\mathrm{n}\mathrm{g}\mathrm{e}\mathrm{d}\left(\mathrm{s}\mathrm{p}\mathrm{r}\mathrm{i}\mathrm{n}\mathrm{g} \mathrm{h}\mathrm{i}\mathrm{g}\mathrm{h} \mathrm{t}\mathrm{i}\mathrm{d}\mathrm{e}-\mathrm{s}\mathrm{p}\mathrm{r}\mathrm{i}\mathrm{n}\mathrm{g} \mathrm{l}\mathrm{o}\mathrm{w} \mathrm{t}\mathrm{i}\mathrm{d}\mathrm{e}\right) \\ /\mathrm{P}\mathrm{o}\mathrm{n}\mathrm{d} \mathrm{V}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{m}\mathrm{e} (\mathrm{s}\mathrm{p}\mathrm{r}\mathrm{i}\mathrm{n}\mathrm{g} \mathrm{h}\mathrm{i}\mathrm{g}\mathrm{h} \mathrm{t}\mathrm{i}\mathrm{d}\mathrm{e}) \end{gathered}$$

(2)

$$\begin{gathered} \tau \mathrm{N}\mathrm{P}\mathrm{N}=\mathrm{P}\mathrm{o}\mathrm{n}\mathrm{d} \mathrm{V}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{m}\mathrm{e} \mathrm{E}\mathrm{x}\mathrm{c}\mathrm{h}\mathrm{a}\mathrm{n}\mathrm{g}\mathrm{e}\mathrm{d} (\mathrm{n}\mathrm{e}\mathrm{a}\mathrm{p} \mathrm{h}\mathrm{i}\mathrm{g}\mathrm{h} \mathrm{t}\mathrm{i}\mathrm{d}\mathrm{e}-\mathrm{n}\mathrm{e}\mathrm{a}\mathrm{p} \mathrm{l}\mathrm{o}\mathrm{w} \mathrm{t}\mathrm{i}\mathrm{d}\mathrm{e}) \\ /\mathrm{P}\mathrm{o}\mathrm{n}\mathrm{d} \mathrm{V}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{m}\mathrm{e} (\mathrm{n}\mathrm{e}\mathrm{a}\mathrm{p} \mathrm{h}\mathrm{i}\mathrm{g}\mathrm{h} \mathrm{t}\mathrm{i}\mathrm{d}\mathrm{e}) \end{gathered}$$

(3)

where τ_NPS is the minimum residence time during spring tide and τ_NPN is the minimum residence time during neap tide. In assessing residence time, we relied on the following assumptions: uniform mixing throughout the water column of Nuʻupia Ponds, consistent flushing cycles across all locations, exclusive water exchange occurring at designated exchange points (Sites 1–14) based on the following equation:

ϕ^x = 0.01

(4)

Here, ϕ^x represents the percentage of water that remains after one flushing cycle and x denotes the residence time in flushing cycles required to mix the initial water to a 1% dilution [4,17].

3. Results

3.1. Characterization of Meteorological Conditions

Meteorological conditions were typical for the Hawaiian dry season [16]. Rainfall was minimal during both deployment periods and ranged from 0.00 to 0.07 mm in July 2022 and 0.00 to 1.2 mm in September 2022 (Figure 3). As such, Nuʻupia Ponds was not affected by any of the tropical cyclones passing by Hawaiʻi during the summer. Wind direction primarily ranged from NE to E in both the July and September deployment with a mean wind direction of ~48° ± 10 s.d. in July and a mean wind direction of ~55° ± 22 s.d. in September. Wind magnitudes ranged from 10 to 23 knots in July with mean winds of 10 knots ± 1 s.d. and from 2 to 21 knots in September with mean winds of 10 knots ± 1.8 s.d. (Figure 3). Air temperature was very consistent during the deployment period with a mean temperature of 25 °C ± 0.44 s.d. in July and 26 °C ± 0.5 s.d. in September (Figure 3).

3.2. Characterizing Water Volume Flux

Bidirectional flow was recorded at eight out of the ten exchange points, mediated by the semi-diurnal tidal cycle in Kāneʻohe Bay (Table 2). The change in water flow direction from influx to outflux and vice versa did not always correlate with the time of low and high slack water in Kāneʻohe Bay. For that reason, the onset of flood tide was defined as a switch in current direction to influx and, conversely, the ebb tide onset was defined as a switch in current direction to outflux. Thus, water volume flux is described here as spring influx, spring outflux, neap influx, and neap outflux.

At the beginning of the tidal cycle, after the switch in current direction, water volume flux slowly increases. Water volume flux is typically at its maximum mid-way through the tidal cycle and decreases again towards the end of the tidal cycle (Figure 4A). Since peak water volume flux often happens midway through the tidal cycle, the correlation between water level and water volume flux, represented by the rating curve, typically exhibits a “C” curve or vertical sine function shape. As an example, rating curves for spring and neap tide influx and outflux at Site 1 are shown in Figure 4A. A full set of rating curves as well as images of the exchange points can be found in Appendix A (Figure A3). Flushing in Nuʻupia Ponds is dominated by the western exchange points connected to Kāneʻohe Bay for all tidal stages: at all measured exchange points, mean and peak water volume flux were highest during spring tides (Table 2, Figure 4A, Figure A3). Site 1 and 2, respectively, recorded the fastest mean water volume flux (6.96 m³ s⁻¹; 4.01 m³ s⁻¹) and peak water volume flux (10.74 m³ s⁻¹; 6.43 m³ s⁻¹) during spring tide influx (Table 2). For spring tide outflux, the fastest mean water volume flux (4.18 m³ s⁻¹; 2.87 m³ s⁻¹) and peak water volume flux (6.07 m³ s⁻¹; 5.07 m³ s⁻¹) were recorded at Site 2 and Site 1, respectively. Site 1 and Site 2 also have the fastest mean and peak water volume fluxes for neap tide (Table 2). Medium water volume fluxes were recorded at Sites 7, 10, and 11 with mean water volume flux ranging from 0.80 m³ s⁻¹ to 3.8 m³ s⁻¹ and peak water volume flux ranging from 1.6 m³ s⁻¹ to 7.1 m³ s⁻¹ (Table 2). Water volume flux at the remaining sites is significantly lower with mean water volume flux ranging from 0.01 m³ s⁻¹ to 0.4 m³ s⁻¹ and peak water volume flux ranging from 0.04 m³ s⁻¹ to 0.56 m³ s⁻¹ across tidal stages (Table 2). At Site 4 and Site 9, no outflux was recorded during both spring and neap outflux (Table 2). Further, at Site 8, no outflux was recorded during spring outflux and at Site 13 no influx was observed during neap influx. Some sites displayed unidirectional flow, regardless of the tidal state: Site 4 and 12 did not measure any outflow during both spring and neap outflux and Site 8 did not indicate any spring outflux (Table 2). Further, Site 13 did not measure any neap influx (Table 2). At Site 10, pressure recordings were unreliable during the peak of spring tide. For that reason, water volume flux calculations and rating curves were measured for spring tide two tidal cycles later (on 15 July, see Table 3), which does not represent the highest spring tidal spectrum. As a result, we were not able to include Site 10 in the time lag and tidal onset order comparison (Table 3). We can also assume that water volume flux may be slightly higher during actual spring tide.

Sites 5, 6, 9, and 14 were too shallow for current meter measurements, and only pressure sensor data were recorded (Figure 4B). Site 5 was not submerged and showed only atmospheric pressure measurements during the measurement period (Figure 4B). Sites 6 and 9 show a normal tidal signal with water depth ranging from 3 to 25 cm across the tidal stages (Figure 4B). Pressure at Site 14 does not show a tidal signal but rather an increase in water level during spring tide and a drop in water level moving towards neap tide (Figure 4B).

In order to compare the relative volumes of water exchanged at different sites, the relative water exchange contribution of each site in the pond system was assessed during spring influx, spring outflux, neap influx, and neap outflux (Table 2, Figure 5). Overall, we observed the majority of relative water volume flux at locations at the western end of the pond system directly connected Kāneʻohe Bay, while the eastern end of the pond system, including Nuʻupia ʻEhā, Kaluapuhi, and Paʻakai, experienced relatively low flushing. Together, Sites 1 and 2 accounted for the large majority of total water volume exchanged within the pond system across all tidal stages with a combined relative contribution of ~50–60% (Figure 5). Sites 1 and 2 are located at the western end of the pond system and directly connect to Kāneʻohe Bay through a dredged channel feeding water into Heleloa and Halekou ponds (Figure 5, Table 2). Sites 10 and 11, connecting Halekou to Nuʻupia ʻElua and Nuʻupia ʻEkolu, and Site 7, connecting Nuʻupia ʻElua to Nuʻupia ʻEkolu, together accounted for another ~35–45% of the total water volume exchanged within the pond system across tidal stages (Figure 5). The nine remaining sites account together for only ~3–7% of the total water volume exchanged within the pond system across tidal stages (Figure 5, Table 2). Sites 3 and 4, which directly connect Nuʻupia ʻEkahi to Kāneʻohe Bay, are small in size and contribute to 2–6% of the total volume exchanged (Figure 5, Table 1 and Table 2). Sites 6 and 8 connect Nuʻupia ʻEkahi to Nuʻupia ʻElua, and Site 9 connects Halekou to Nuʻupia ʻEkahi. Together, they account for <1% of the total volume exchanged (Figure 5, Table 1 and Table 2). The eastern side of the pond system experienced relatively minimal flushing: Sites 12, 13, and 14 exchanged <1% of total volume across all tidal stages (Figure 5, Table 1 and Table 2).

3.3. Water Flux Lags across Nuʻupia Ponds System

Tracking the times of the onset of inflow and outflow across sites, allowed us to arrive at a better understanding of circulation across the system: across all tides, we observed a trend of increasing time lag between the onset of influx/outflux among sites with from west to east: Site 2, which directly connects Kāneʻohe Bay through a dredged inflow channel to Nuʻupia Ponds, has a tidal signal that is close to identical with the reference tide at Moku o Loʻe tide gauge in Kāneʻohe Bay (Figure 6). For the purpose of visualizing time lags across the system, Site 2 was defined as time 00:00 in hours–minutes with all other sites showing a time lag in inflow/outflow (Figure 6, Table 3). Time lags became longer from the western to the eastern end of Nuʻupia Ponds system: while Sites 2–11 had an average time lag of ~55 min across all tidal stages, Sites 12–14 showed an average time lag of ~6 h (Table 3, Figure 6).

During flood tide, the Nuʻupia Pond system starts filling with Kāneʻohe Bay water at Site 2, closely followed by Site 1, as water starts flowing in through the dredged channel filling Heleloa and Halekou Ponds. Site 3 starts inflowing and filling Nuʻupia ʻEkahi early on during spring influx, but shows a longer lag during neap influx. Nuʻupia ʻElua and Helekou Ponds start filling Nuʻupia ʻEkolu Pond around the same time: Sites 10, 11, and 7 all show time lags similar in range between 40 and 80 min. Site 4 has a time lag of 1.3–2 h. Time lags for Sites 6 and 9 are unknown as these sites were too shallow to measure flux. Site 8 had to be redeployed and therefore could not be accounted for in the time lag comparison. Nuʻupia ʻEhā, Kaluapuhi, and Paʻakai Ponds fill are last to fill with time lags measured between 3.5 and 6 h (Table 3, Figure 6A). Slight variabilities in the order of onset of influx were observed between spring and neap influx (Figure 6A). Nuʻupia Ponds starts draining in a similar order. Sites 2 and 3 are the first to switch current direction to outflow, followed in order by Sites 1, 7, and 11 during spring tide and Sites 7, 10, and 1 for neap tide. Site 13 starts draining last with a particularly long-time lag of 12 h during spring tide and 3 h during neap tide (Table 3, Figure 6B).

The duration of tidal cycles was shorter during incoming tides (influx) compared to outgoing tides (outflux) at a majority of exchange points at both spring and neap tide: Mean tidal duration based on influx and outflux was 7.58 ± 1.67 s.d. and 9.20 ± 2.08 s.d. for spring influx and neap influx, respectively, while mean tidal duration was 16.03 ± 2.67 s.d. and 10.47 ± 2.13 s.d. for spring outflux and neap outflux, respectively (Table 3). Collectively, the shorter lag time during high water compared to low water, the prolonged duration of dropping tides—especially evident during spring tides—and the enhanced velocities of influx currents indicate that Nuʻupia Ponds primarily experiences flood-dominated dynamics.

3.4. Nuʻupia Volumes, Exchange Rates, and Residence Times

The bathymetry of Nuʻupia Ponds is characterized by a uniform and shallow bathymetry of ~0.2–0.3 m with some deeper portions (~0.9 m) in Halekou and Nuʻupia ʻEkolu Ponds (

Table 4. Pond volumes across tidal stages (SH, SL, NH, NL) as well maximum and average pond depth for the entire Nuʻupia Ponds system and individual ponds.

	Pond Volumes (m3)				Area (m2)	Maximum Depth (m)	Average Depth (m)	Maximum Depth (m)	Average Depth (m)	Maximum Depth (m)	Average Depth (m)	Maximum Depth (m)	Average Depth (m)
	Spring High	Spring Low	Neap High	Neap Low		Spring High		Spring Low		Neap High		Neap Low
All Ponds	311,900	160,700	235,400	168,100	996,500	0.9	0.3	0.7	0.2	0.8	0.2	0.7	0.2
Heleloa	4270	2459	3381	2541	11,950	0.6	0.4	0.4	0.2	0.5	0.3	0.4	0.2
Nuʻupia ʻEkahi	68,320	32,530	50,520	34,170	200,600	0.7	0.3	0.5	0.2	0.6	0.3	0.5	0.2
Halekou	79,380	50,080	64,310	52,100	166,600	0.9	0.5	0.7	0.3	0.8	0.4	0.8	0.3
Nuʻupia ʻElua	21,350	7770	14,600	8390	78,740	0.4	0.3	0.2	0.1	0.2	0.3	0.2	0.1
Nuʻupia ʻEkolu	110,900	46,580	78,510	49,460	381,700	0.8	0.3	0.6	0.1	0.7	0.2	0.6	0.1
Nuʻupia Ehā	5375	4201	4983	4496	20,070	0.5	0.3	0.4	0.2	0.5	0.2	0.4	0.2
Kaluapuhi	12,570	8692	11,190	9590	67,140	0.4	0.2	0.3	0.1	0.4	0.2	0.4	0.1
Paʻakai	9730	8652	8118	7583	69,650	0.3	0.1	0.1	0.2	0.2	0.1	0.2	0.1

, Figure 7). Nuʻupia Ponds is the deepest during SH tide (

Table 5. Exchange rates for spring and neap tides as well as residences times in flushing cycles, hours, and days for the entire Nuʻupia Ponds system as well as individual ponds.

	Exchange Rates		Residence Time			Residence Time
	Spring	Neap	Minimal	Maximal	Minimal		Maximal
	Tide	Tide	Flushing Cycles	Flushing Cycles	Hours	Days	Hours	Days
All Ponds	48%	29%	7.0	13.5	169	7.0	269	11.2
Heleloa	42%	25%	8.5	16.0	203	8.5	320	13.3
Nuʻupia ʻEkahi	52%	32%	6.3	11.9	150	6.3	239	10.0
Halekou	37%	19%	10.0	21.9	239	10.0	437	18.2
Nuʻupia ʻElua	64%	43%	4.5	8.2	108	4.5	164	6.8
Nuʻupia ʻEkolu	58%	37%	5.3	10.0	127	5.3	199	8.3
Nuʻupia Ehā	22%	10%	18.5	43.7	445	18.5	874	36.4
Kaluapuhi	31%	14%	12.4	30.5	298	12.4	611	25.4
Paʻakai	11%	7%	39.5	63,5	948	39.5	1269	52.9

, Figure 7A), averaging 0.3 m with a maximal water depth of 0.9 m resulting in a maximal volume of ~311,900 m³ for all ponds during SH tide (Table 4, Figure 7). During SL tide, the ponds retain a minimum water volume of approximately 160,700 m³, which corresponds to roughly 52% of the SH volume (Table 4, Figure 7), and water depth averages 0.2 m with a maximal depth of 0.7 m. NF tidal volume is 235,400 m³, ~75% of the SH tidal volume, with a mean depth of 0.20 m and a maximal depth of 0.8 m. NL tidal volume is 168,100 m³ and 54% of the SH volume, and has a mean depth of 0.20 m and a maximal depth of 0.7 m (Table 4, Figure 7). Volumes and depth ranges for all eight individual ponds are listed in Table 4.

We determined that around 48% of the water in the entire pond system undergoes exchange during the ebb–flood transition at spring tide. In contrast, during the neap tide ebb–flood transition, only 29% of the water is exchanged (see Table 5). One flushing cycle was defined as the duration required to flush out 48% of total pond water during spring ebb tide and 29% during neap ebb tide, and to replenish that water again with new Kāneʻohe Bay water during the subsequent spring/neap flood tide. Using the average tidal duration for a full tidal cycle (flood and ebb) for spring and neap tides (Table 3) as a baseline, we defined one flushing cycle as 24 h for spring tide and 20 h for neap tide. Assuming that the incoming water would uniformly mix with the water already present in the ponds during the initial flushing cycle (52%), we estimated that approximately seven flushing cycles are needed to dilute the initial 52% of water to a concentration of less than 1%. Hence, the minimum residence time of Nuʻupia Ponds is seven flushing cycles approximately equivalent to 7 days, and occurs during spring tide when water exchange is at its peak (Table 5). In contrast, during periods of minimal water exchange (such as neap tides), it takes approximately 13 flushing cycles, roughly equivalent to 11 days, to dilute the 71% of retained water down to less than a 1% concentration (Table 5). An overview of individual exchange rates and residence times can be found in Table 5.

4. Discussion

4.1. Site Specific Details and Technical Limitations

Peak water volume flux is a combination of the area and the flow velocity (c.f., Equation (1)). Sites 1 and 2 have the combination of the biggest channels and fastest velocities rendering the highest average and peak water volume fluxes as well as the largest proportion of relative water volume flux across the system with 50–60% of the relative flux (Table 2, Figure 5). Site 2 is also with a maximum of 1.33 m during spring influx, the deepest site compared to other exchange points. This contributes to higher water volume flux rates compared to sites that have similar width and velocities (Table 2). Site 3 has the highest water velocities with up to 0.68 m s⁻¹; however, it is comparably small in size with a width of only 3.45 m leading to low relative water volume fluxes despite the high flow velocity.

Most sites show clearly bidirectional flow (see Appendix A, Figure A3 and Figure A4). Site 7, connecting Nuʻupia ʻElua to Nuʻupia ʻEkolu, shows more variability in flow direction, likely because the nature of the exchange point presents a large gap that allows for more angled flow direction than a culvert does (Figure A4). Looking at the Kāneʻohe Bay site of Site 4, we see dense overgrowth by mangroves (Figure A5). Mangroves are known to inhibit flux [4,13,14]. It is possible that while Site 4 shows influx during both spring and neap tide (Table 2, Figure A3) due to the pressure gradient building up on the Kāneʻohe Bay site during flood tide, the barrier presented by mangroves is sufficient for the water flow seeking easier pathways during outflux. As such, Site 2 recorded a higher percentage of relative water volume flux during outflux compared to influx, indicating that it might be compensating for the outflux inhibited at Site 4 (Table 2, Figure 5).

Site 8, which connects Nuʻupia ʻEkahi and Nuʻupia Elua, recorded an influx during the spring tidal cycle only (Table 2, Figure A3). We observed no change in flow direction during spring outflux; however, the flow velocities drop. It is possible that during spring outflux, the pressure gradient forces water drainage through pathways with larger exchange points such as Site 7, Site 10, and Site 11 (Table 1, Figure 6 and Figure A3). Throughout the entire measurement period we observed only brief periods of outflux that become more frequent with neap tide at Site 8. For Site 12, we observed largely unidirectional flow in the form of influx across tidal cycles and no outflux during the selected spring or neap outflow timeframes (Table 2, Figure A3). However, brief periods of outflux were recorded occasionally in between spring and neap tides at Site 12 and coincided with extremely low velocities, suggesting that the flow direction switches to wind driven westward flow due to a lack of pressure gradient from the west. Further, the observation that flow switches more frequently towards neap tide suggests that the pressure gradient pushing inward flow subsides from spring to neap tide. Another possible explanation for the lack of outflux at Site 12 may be the long time lag: by the time the pressure gradient switches to outflow in the eastern part of the system, an incoming new tidal cycle from the west “pushes” against the comparably smaller pressure gradient in the east. The long time lags weaken the tidally driven pressure gradients and flow velocities continuously from west to east (Table 2, Figure 6). While we recorded influx and outflux at Site 13 during spring tide, solely outflux was recorded during neap tide and flow velocities are higher during outflow periods (Table 2, Figure A3). It is likely that the strong westward wind is accelerating outflow, which is aligned with the westward direction, and that the combined wind and outflow pressure gradient cause the acceleration in velocity. The low velocities at spring influx are an indication that the pressure gradient is competing against the prevailing westward force caused by the trade winds (Figure 3). During neap tide, the pressure gradient decreases even further and it is likely that the wind-driven force does not allow for any influx.

Current meter data were collected for ten out of fourteen sites: Sites 5, 6, 9, and 14 were too shallow (<0.3 m in depth) to measure flow with the CM-4 Shallow Water Tilt Current Meter and only pressure sensor data were recorded (Figure 4B) leading to gaps in flow data for these locations when comparing water volume fluxes across the pond system. Site 5 presented a culvert with a small accumulation of water that was shallow and disconnected from the remaining pond system (Figure 4B). The pressure sensor was not submerged as the water was too shallow, thus the measured signal is solely the atmospheric pressure measured. As there is no significant increase in pressure measured over the course of 14 days, we conclude that Site 5 is disconnected and did not facilitate any water exchange with the remaining system during the measurement period (Figure 4B). However, it could be possible that this site drains stormwater runoff during heavy rain events into Nuʻupia Ekahi Pond. It is likely that Sites 6 and 9 facilitate minimal exchange between Nuʻupia ʻEkahi and Nuʻupia ʻElua Pond and Nuʻupia ʻEkahi and Helekou Pond, respectively, as they consist of culverts similar in size to Sites 12 and 13 (Table 1) and empirical observations confirm minimal flow. Based on water volume flux at Sites 12 and 13, which was measured to be 0.6% at its maximum (Table 2, Figure A3), and given the comparably shallower water depth, we inferred a minimal exchange of <1% for Sites 6 and 9. Site 9 seeps into marshland on the Nuʻupia ʻEkahi side of the exchange point (see Figure 4B and Figure A5), suggesting that, if there is any exchange, it is likely to be diffusive flow. Site 14 is a gap between Kaluapuhi and Paʻakai (Figure 4B). Although larger in size, empirical observations showed very slow flow compared to other sites like Sites 3 and 4, which are similar in size but have much higher water velocities. For these reasons we feel confident that the relative water volume flux of <1% lies within a realistic accuracy range for Site 14.

All data were collected during Hawaiʻi’s dry season [16] with minimal precipitation (Figure 3). While this study can be considered representative for the dry season, we do not expect significant alterations in flux dynamics during the Hawaiian winter/wet season due to the absence of direct freshwater stream input.

4.2. Management Implications

Water circulation is crucial for maintaining healthy water quality dynamics in fishponds, preventing stagnation and maintaining stable dissolved oxygen levels for aquatic biota to thrive [4,16]. Adequate levels of dissolved oxygen (DO) are crucial for the functioning of biological processes in aquatic environments. Decreases in DO can trigger significant shifts in productivity, biodiversity, and biogeochemical cycles, potentially resulting in notable alterations to food webs [18,19,20]. Oxygen depletion is often linked to excessive nutrient availability, causing eutrophication. This process can lead to oxygen deficiencies, ultimately resulting in large-scale fish mortality [19]. Further, the capability of water to hold dissolved oxygen decreases with increasing temperature and salinity. As such, there have been raised concerns regarding fish stress associated with the warming sea surface trends [16]. Thus, the combination of Nuʻupia Ponds’ shallow water environment, high water temperatures, high water column, and sediment oxygen demand (SOD), due to the decomposition of organic matter, renders particular importance for maintaining a well-circulated environment with high flushing rates and low residence times. Increased exchange with well-mixed ocean water from Kāneʻohe or Kailua Bay would be beneficial to ensure sufficient dissolved oxygen in a contained environment such as Nuʻupia Ponds.

Generally, the ponds on the western side of the pond system have higher flushing rates and lower residence times compared to the eastern side of the system: Heleloa, Nuʻupia ʻEkahi, Halekou, Nuʻupia ʻElua, and Nuʻupia ʻEkolu have an average exchange rate of 51 ± 0.11% during spring tide and 31 ± 0.09% during neap tide, while the ponds at the eastern end (Nuʻupia Ehā, Kaluapuhi, Paʻakai) have a much lower average flushing rate of 21 ± 0.1% during spring tide and 10 ± 0.04% during neap tide (Table 5). Minimal residence time for the western ponds is on average 7 ± 2.25 flushing cycles equaling to about 7 days and maximal residence time is 13.6 ± 5.45 flushing cycles equaling to just above 11 days. In contrast, residence times for ponds in the east are significantly higher: Minimal residence time for Nuʻupia Ehā, Kaluapuhi, Paʻakai is on average 23.5 ± 14.21 flushing cycles equaling to about 23.5 days and maximal residence time is 45.9 ± 16.57 flushing cycles equaling to just under 39 days (Table 5). Paʻakai, the most eastern pond, has the lowest exchange rates and the longest residence times with a minimal residence time of ~40 flushing cycles or 40 days and a maximal residence time of 63 flushing cycles or 53 days. Both qualitative observations as well as Google Earth Imaging (Figure 1) suggest heavier sedimentation of the eastern ponds: water with a high suspended sediment load is flushed towards the eastern ponds and starts to settle here due to low flow velocities and stagnant water, ultimately leading to heavy sedimentation. Re-establishing the former ocean connection with Kailua Bay could improve circulation and flushing in the fishpond system as well as decreasing the residence times of Nuʻupia Ehā, Kaluapuhi, and Paʻakai (Figure 2B).

Further, lowering culverts at Kāneʻohe Bay sites, Sites 3 and 4, could increase water volume flux and exchange between Kāneʻohe Bay and Nuʻupia ʻEkahi: influx at Site 3 (during neap tide only) and Site 4 (during both spring and neap tide) takes longer compared to nearby sites (Table 3, Figure 6), suggesting that a certain sea level on the Kāneʻohe Bay site needs to be reached before water can start flowing in through the culverts at Sites 3 and 4. Thus, the height of culverts relative to the sea level affects the timing of influx/outflux at different sites and impairs efficient exchange at these sites. Further, the culvert at Site 5 could be cleared and reconnected to provide additional exchange between Kāneʻohe Bay and Nuʻupia ʻEkahi. Generally, the enlargement of existing culverts or strategic placement of additional culverts connecting Nuʻupia ʻEkahi to Kāneʻohe Bay would increase the exchange of Nuʻupia Pond and ocean water and decrease residence times for the Nuʻupia Pond system as a whole.

In addition to increasing exchange of Nuʻupia Ponds and ocean water, it is important to maintain circulation and flushing throughout the fishpond system. To ensure regular flushing between the individual ponds within Nuʻupia Pond system, enlargement of existing culverts at exchange points with low relative water volume flux (<1%) such as Sites 6, 8, 9, 12, 13, and 14 (see Figure 5) or strategic placement of additional culverts would increase and more equally distribute water circulation across the system. Recurring observations of dead fish at Site 13 suggest that one cause of fish mortality may be a lack of oxygen or impaired water quality, which can be caused by the limited flushing and long residence times measured here (Table 5). In addition, clearing existing exchange points from sediment, coral material, and vegetation that might be clogging or blocking the drainage area, could enhance water exchange among ponds and improve circulation dynamics.

Mangroves fulfill important ecosystem functions in their native habitats such as protecting shorelines, stabilizing sediment, litterfall subsidy, and serving as nursery areas. Nevertheless, in the coastal ecosystems of Hawaiʻi, mangroves have resulted in a range of adverse ecological and economic consequences [21,22]: Mangroves tend to thrive in holotypic ecotones, which leads to their proliferation in estuarine environments, where their root systems can impede the flushing and circulation of fishponds [4,23,24]. Further, areas vegetated with mangroves have high sedimentation rates changing sandy habitats into muddy anoxic sediments as a result of bacterial decomposition of mangrove leaf detritus [23,25,26]. The drawdown of nitrogen and phosphate in areas with mangroves can lead to a decline of dissolved oxygen that can inhibit primary production rates in fishponds [25]. Therefore, by altering their environment, mangroves can trigger cascading adverse consequences for resident ecosystems in Hawaiʻi, which has motivated their removal as a management action at fishponds [4,13,24]. At Nuʻupia Ponds Wildlife Management Area, mangroves have been documented to overgrow mudflats, causing heavy sedimentation and inhibiting flux as well as threatening the physical integrity and function of fishpond walls and channels [11]. Thus, the removal of non-native mangroves (Rhizophora mangle) has been part of management plans in the past [11,24]. Further, pickleweed (Batis maritima) is an introduced colonizer of mudflats and fishponds forming a monotypic salt marsh vegetation diminishing habitat for native seabirds [11,27]. Between 1994 and 1995, approximately 10 acres of mangrove were removed from shorelines of Nuʻupia ʻEkahi, Nuʻupia ʻElua, and Heleloa Ponds. These removal efforts resulted in a documented increase in stilt forging and nesting in areas cleared of mangrove and other alien vegetation such as pickleweed [27,28]. As such, regular control of invasive species such as mangroves and pickleweed can help maintain important mudflat habitat for endangered and protected waterbird species, minimize sedimentation, and maximize water circulation across the Nuʻupia fishpond system. Further, it is advisable to clear the dense mangrove overgrowth in Kāneʻohe Bay at Site 4 (Figure A5) to increase water volume flux and overall exchange between Kāneʻohe Bay and Nuʻupia ʻEkahi.

Qualitative observations during field measurements suggest heavy sedimentation in all eight ponds with a thick anoxic sediment layer of Nuʻupia Ponds system. When anoxic sediment conditions lead to the buildup of reducing agents like sulfides and ferrous iron, these compounds can react with oxygen, effectively consuming it and creating a feedback loop that can further deplete oxygen [18,29]. This process is known as oxygen demand. In shallow aquatic ecosystem, the oxygen levels are determined by the balance between oxygen generation and consumption within the water column, as well as by sediment oxygen demand (SOD) [18]. Anoxic conditions can be detrimental to benthic organisms like worms, mollusks, and other bottom-dwelling species that rely on oxygen for respiration and leading to reduced biodiversity and changes in benthic community composition [30]. Dredging the upper sediment layer could prevent further sediment build up and deepen the water column, which could increase available dissolved oxygen in the water column. However, negative consequences, such as resuspension of pollutants from sediments and disturbance of benthic and aquatic biota as well as bird populations, should be carefully considered [31,32,33]. Conducting an environmental impact assessment prior to dredging operations to identify and address potential impacts and risks of dredging on water quality and ecosystems, is essential. Implementation of best management practices, which may include measures like sediment containment, water quality monitoring, and proper disposal or treatment of dredged sediment can help mitigate potential adverse effects of dredging operations.

Overall, this study outlays the physical components of the Nuʻupia Ponds ecosystem and provides an important baseline that can guide further research and allow for evaluation of future ecosystem management regimes. Our findings suggest that there is considerable potential for strategic ecosystem management to enhance water circulation, thereby benefiting ecosystem health. Integrating traditional Hawaiian ecosystem management practices with contemporary estuarine management methods can safeguard this culturally and economically important area, ensuring the sustainability of coastal ecosystems for generations to come.