4.1. Organic Matter Export
Precipitation-driven runoff into Pellicer Creek is the largest driver of nutrient concentrations within the waterway (
Figures S1 and S2). However, lag times for the export of organic matter from terrestrial to aquatic environments are extremely variable and the monthly sampling times were inadequate to determine a timeline between runoff and export of organic matter into Pellicer Creek [
36,
37]. A previous study conducted a meta-analysis on thirty forested watersheds that determined that mean annual stream yield from precipitation for the Eastern United States was 100 cm [
15]. Therefore, it could be assumed that a significantly larger annual stream yield from precipitation could occur from runoff during years of hurricane influence, but again, this could not be directly calculated for this study. Precipitation drives organic matter into waterways, but river-mediated transport of carbon and nutrients is critical in exporting OM from high-order streams to the ocean.
Rivers, globally, transport 0.45 peta-grams OC yr
−1 or 1.2 × 10
9 kg OC day
−1 to the ocean [
38]. Pellicer Creek discharges less water and DOC when compared to larger Florida systems, with an average discharge of 1.16 m
3s
−1 for spring, 1.78 m
3s
−1 for summer, and 2.09 m
3s
−1 for fall, and exports an average of 38.9 kg DOC day
−1 (spring), 62.6 kg DOC day
−1 (summer), and 87.7 kg day
−1 (fall). In comparison, Apalachicola Bay seasonally discharges 757 m
3s
−1 for spring, 441 m
3s
−1 for summer, and 272 m
3s
−1 for fall and exports 177,120 kg DOC day
−1 (spring), 97,632 kg DOC day
−1 (summer), and 56,160 kg DOC day
−1 (fall) [
39]. However, Pellicer Creek represents blackwater rivers, which are organic-rich waters and prolific feeders of larger coastal systems in the Southeastern United States [
40,
41]. Additionally, Pellicer Creek is only 5% developed and has been continuously monitored for a variety of parameters in the past 3–15 years, making it a model for a predominantly natural coastal Florida system.
Hurricanes, a relatively common occurrence in Florida, have the ability to drive the export of DOC and nutrients in natural and non-natural systems alike, rapidly pulsing a sizable percentage of annual export into aquatic systems [
16,
42]. A study on Hurricane Irene determined that 25–45% of annual carbon and 11–35% of annual nitrogen were exported from a forested watershed into the Esopus Creek in New York [
42]. Another study of Hurricane Gustav determined that 24% of DOC, 1.7% of dissolved inorganic nitrogen, and 6% of phosphate yearly exports were transported downstream in the Pearl River in only 9 days [
43]. A long-term (20-year) study discovered that wet hurricanes (hurricanes with high precipitation) export 21% DOC, 26% soluble reactive phosphorus (SRP), and 11% total nitrogen (TN) of long-term loads in the Neuse River Estuary in North Carolina [
18]. Additionally, hurricanes in Puerto Rico were shown to increase riverine nitrogen by 297-times and riverine phosphorus by 306-times. This study, in addition to many other hurricane studies, indicated that the primary driver of these inputs is surface runoff from heavy precipitation [
44,
45]. In this study, Pellicer Creek exported 39% of annual average DOC, 180% annual average ammonia-N, 54% annual average ortho-phosphate, 48% annual average TKN, and 33% annual average nitrate during the month of Hurricane Irma (
Table 2 and
Table 3). Although differences in sampling methods make it difficult to compare studies directly, the high percentages of annual nitrogen species and SRP export are similar or higher than estimates from examples for other systems discussed above. Most hurricane nutrient transport is driven by “wet” hurricanes with high precipitation, whereas windy, dry hurricanes may not drive nutrient loading in coastal systems to the same extent [
18]. In this study, September 2019 (Hurricane Dorian) only exported 3% yearly average DOC, 7% yearly average NH
3-N, 4% yearly average phosphate, 5% yearly average TKN, and 1% yearly average NO
3-N. These values are much lower than for other non-hurricane months and at least a degree of magnitude less than the month of Hurricane Irma, indicating substantial variability in hurricane impact, depending on intensity, proximity, antecedent conditions, etc.
Antecedent conditions pre-hurricane are predominately dependent on landscape saturation from summer storms in the months preceding hurricane impact. In the summer of 2016, before Hurricane Matthew, St. Augustine experienced a period of drought with fewer summer thunderstorms than average, creating dry terrestrial conditions and higher salinities in local waterways (
Figure 4). On the contrary, the months preceding Hurricane Irma were rainy, saturating the upland systems, lowering salinity levels in the waterway. Since soil saturation will increase sheet-flow runoff from landscapes into streams and rivers, rainy summers, pre-hurricane Irma, aided in the spikes of carbon and nutrients in Pellicer Creek [
46]. Additionally, Hurricane Irma’s storm surge pushed saline water into the previously fresh upper reaches of the creek, leaching additional ions from the soil in the rapid transition from fresh to saline water [
47,
48]. Since salinity in Pellicer Creek was already elevated pre-Matthew, it is less likely that this mechanism of ion release had an equivalent impact. Additionally, it is possible that Matthew’s approach of St. Augustine from the Atlantic led to a longer storm surge period, which can be seen by the heightened salinities and possibly by the lower nutrient values in comparison to Irma (
Figure 2;
Table 3). Irma approached from the western side of Florida and might have created a flashier storm surge, which led to the quick spike in salinity and higher nutrient exports than Matthew (
Figure 2).
Hurricane Irma created a 1.3 m storm surge, causing saltwater to creep into the previously freshwater portion of the river [
48]. The increased ionic strength of saltwater may have displaced or obstructed ions from ion exchange sites in soil, causing ammonium, phosphate, and other ions to desorb [
49]. Desorbed ammonium, phosphate, and other ions were then added to the bioavailable nutrient pool within the water column. Due to higher exchangeable ammonium present in freshwater wetlands and waterways, salinization can release adsorbed ammonium from soils rapidly and increase water column ammonium concentrations within hours [
47,
50,
51]. Due to the bioavailability of ammonium, ammonium desorption and diffusion into the water column can increase microbial processing, increasing biological oxygen demand and driving down DO concentrations [
52]. Furthermore, microbial sulfate reduction becomes the dominant degradation pathway as salinity increases, in situ organic matter mineralization doubles, and additional nutrients are released into the water column, further increasing DOM concentrations [
49].
In forested watersheds, it is estimated that 86% of DOC is exported during storm and snowmelt events [
15]. A watershed in Maryland contributed 53% annual carbon export from storm events, totaling 1052 mm of precipitation in 2008, and 60% annual carbon export from storm events, yielding 1238 mm of precipitation in 2009. The same study produced similar estimates for hurricane years and calculated 972 mm of precipitation in 2010 (Hurricane Nicole) contributed 57% of yearly carbon export, and 1462 mm in 2011 (Hurricane Irene) contributed 76% annual export [
11]. In Juneau, Alaska, storms between September 6–9 and 9–14 July produced 48 mm and 72 mm of precipitation, which exported 22–28% annual DOC and 31–37% annual DOC [
53]. In comparison, the two largest storm/precipitation months from this study contributed 42% annual DOC export (May 2018) and 47% annual DOC export (August 2018). Although coarse estimates, these values are in line with other estimates and display the contributions of non-hurricane storm events to DOC export.
4.2. Evaluation of Ecosystem Metabolism
The export of DOC and other nutrients is vital to biogeochemical processing that is coupled with metabolic processes in streams and rivers [
14]. A transition occurs between heterotrophic and autotrophic conditions moving downstream from headwaters to estuaries. Heterotrophic portions of waterways are dominated by allochthonous inputs and organisms that gain energy from organic matter consumption, whereas autotrophic zones are dominated by autochthonous organic matter added to the waterway by primary production [
19,
54]. Pellicer Creek is a 3–4 order predominantly heterotrophic stream, indicating high allochthonous inputs in that area of the river. A study on the Ogeechee River (another blackwater river in Georgia, USA) revealed that allochthonous inputs were the largest driver of stream metabolism, regardless of stream order [
55]. Due to the coloration and high-DOM concentrations indicative of blackwater rives, it is understandable that heterotrophy tends to be prevalent in these systems. Rapid additions of organic matter during hurricane disturbance further decrease net ecosystem metabolism in Pellicer Creek (increasing the heterotrophic condition) (
Figure 10). Net ecosystem metabolism was significantly lower in hurricane months than other time points in Pellicer Creek during the study interval, even though no significant differences were seen across export values (
Figure 10b).
On a monthly timescale, no significant differences are seen in overall export, but the rapid pace of runoff into Pellicer Creek during a hurricane is much quicker than another storm event. A study in Cape Fear found that hurricanes multiplied organic matter inputs by three times, adding considerably more labile organic matter than runoff from other events and increasing biological oxygen demand [
23]. Pellicer Creek experienced a similar phenomenon, and the rapid addition of TKN and ammonia-N (during Irma) into this nitrogen-limited system catalyzed microbial processing and further increased respiration rates [
56]. A spike in community respiration resulted in a drop in net ecosystem metabolism that persisted in Pellicer Creek for up to 3–4 months (
Figure 10a).
One additional factor in the initial decrease in NEM is rapidly increased turbidity that occurs during a hurricane event [
48]. As turbidity increases, photo-synthetic organisms are out-shaded or potentially exported with increased discharge, as seen by the decrease in chl-a downstream after Hurricane Irma. Although high turbidity (up to 70 NTU) was only seen to persist for a couple of days after Hurricane Irma’s passage, it is possible that the turbidity spike aided in decreasing primary production initially [
48].