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Article

Sand Prawns Mitigate the Impact of Prolonged Drought on the Biology of a Temporary Open/Closed Estuary

by
Celiwe Yekani
1 and
William Pierre Froneman
1,2,*
1
Department of Zoology and Entomology, Rhodes University, P.O. Box 94, Grahamstown 6140, South Africa
2
Department of Biological Sciences, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
*
Author to whom correspondence should be addressed.
Diversity 2025, 17(4), 223; https://doi.org/10.3390/d17040223
Submission received: 20 January 2025 / Revised: 18 March 2025 / Accepted: 19 March 2025 / Published: 24 March 2025
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Abstract

:
The role of the sand prawn, Kraussillichirus kraussi (Stebbing, 1900), as an ecosystem engineer was evaluated through a two-and-a-half-month caging experiment conducted during a prolonged drought in the lower reaches of the temporarily open/closed Kasouga Estuary along South Africa’s eastern seaboard. Findings indicate that at intermediate densities, the burrowing activities of K. kraussi significantly enhanced microphytobenthic algal concentrations, leading to an increase in macrobenthic abundance and biomass (H(3) = 12.772, p < 0.0001; H(3) = 11.305, p = 0.001; H(3) = 13.787, p < 0.0001, respectively). This response was largely driven by elevated densities of the gastropod Nassarius kraussianus (Dunker, 1847), which benefited from the increased microphytobenthic biomass. These results highlight the critical role of K. kraussi as an ecosystem engineer, demonstrating its ability to locally enhance biological productivity even under environmental stress, such as prolonged drought conditions.

1. Introduction

Ecosystem engineers are organisms that change the state of matter by influencing both biotic and abiotic components through various mechanisms, including bioturbation, habitat construction, and resource modulation [1,2,34]. The mixing and alteration of sediment structure in coastal habitats are thought to be the primary drivers of biodiversity as they have a fundamental effect on the environment [5,6,7]. Bioturbation facilitates the dispersal of particles in sediments, modifies geochemical cycling, alters food sources, and alleviates the transport of water and gaseous exchange in soils [7]. Because bioturbators modify the availability of resources to other species, they are viewed as ecosystem engineers [6,7]. Bioturbation, therefore, affects the physical (by constructing burrows, mound construction, ploughing of the surface, and trampling) and chemical (the mixing of water and solutes) characteristics of soft-sediment habitats [8,9,10,11], which promotes food availability and species recruitment, increases oxygenation and mineralization, and modifies the distribution of microalgae and species composition of macrofauna [3,5,12,13,14].
The South African coastline is numerically dominated by temporarily open/closed estuaries (TOCEs) that, under natural conditions, are periodically separated from the marine environment by a sandbar at the mouth [15,16,17,18]. The duration of this separation is largely a function of rainfall within the catchment area and the extent of the sandbar [18]. During periods of drought, TOCEs may be separated from the marine environment for periods exceeding 12 months [14,17]. Mouth status plays an important role in determining the physical properties of the water column and, consequently, the biology within these systems [19,20]. Temporarily open/closed estuaries are generally characterized by low species richness [21] due to the lack of recruitment opportunities for marine breeding species into these systems and the fact that many marine species are incapable of completing their life cycles in these systems [14]. Moreover, extreme variability in physico-chemical factors such as temperature and salinity, typically recorded within these systems, can pose physiological constraints on many species, particularly freshwater and marine representatives [21,22].
The sand prawn, Kraussillichirus kraussi (Stebbing, 1900), has been identified as a key component of the macrobenthic fauna in soft-sediment nearshore marine environments and estuaries, including TOCEs along the southern African coastline [1,23]. Densities of sand prawn are highly variable and may attain levels of up to 100 ind m−2. Sand prawns create burrows extending over 1 m into the sediment, bringing the sediment to the surface where it is deposited in volcano-like mounds [3,24]. The burrowing activity of the sand prawn modifies the sediment’s biogeochemical properties, particularly pore-water, nutrients, as well as gaseous exchange between the sediment and water column and sediment granulometry and erodibility [3,5,13]. Sand prawns modify nutrient cycles in the sediments and water column [25,26] and contribute to the alteration of sediment texture (diagenesis), the mixing of water and solutes (bio-irrigation), and the redistribution of organic material and dead organisms [27]. Bioturbation fuels plankton primary production by releasing nutrients from sediments into the water column [10]. The direct release of nutrients as well as nutrient cycling are both enhanced by bioturbation through sediment oxygenation and an increase in the surface area available for microbial production and activity [8]. Therefore, bioturbation changes the dispersion deepness of organic matter and can raise the food stock and quality in sediments for deposit feeders. In addition to having a noticeable impact on bacteria, microalgae, meiofauna, macrofauna, and seagrasses in estuarine environments, sand prawns play a significant role in the structuring of soft-bottom communities through their sediment reworking activities [11,25,27,28]. Studies have shown that the presence of sand prawns can lead to a decrease in the abundance of macrofauna due to direct physical disturbances or competition for space and resources [29,30,31,32]. However, in other cases, they can promote biodiversity by creating habitats that are more favourable to certain species [24]. For instance, the presence of sand prawn burrows can provide shelter for small invertebrates or offer an oxygenated refuge in otherwise anoxic sediments [11].
Global climate change along the southern African coastline has been implicated in the increased incidence of extreme weather events including storm activity, increased wave height, and prolonged droughts [33,34]. These events are likely to have a far-reaching effect on the ecosystem functioning of TOCEs within the subregion [33,35]. Much of the Eastern Cape province of South Africa experienced a severe drought from 2015 to 2019 which contributed to the prolonged mouth closure of TOCEs within the region [34]. The current investigation was implemented to assess the role of the sand prawn, K. kraussi, as an ecosystem engineer under conditions of prolonged mouth closure in a temperate TOCE located along the southeastern coastline of South Africa. Given their important role in nutrient cycling and structuring benthic communities in shallow-water ecosystems, we hypothesized that the sand prawns could mitigate the adverse effects of the drought and that this effect would be density-dependent.

2. Study Site

This study took place in the lower reach of the medium-sized warm-temperate Kasouga Estuary (Figure 1) located on the southeastern coastline of South Africa which enters the Indian Ocean at 33°39′17″ S 26°44′08″ E [36,37]. Its surface area and width are estimated at 22 hectares and 150 m, respectively, and the estuary is navigable for about 2.5 km, with its main channel depth varying between 0.5 m and 2 m [38]. This system is considered to be pristine and in good ecological condition due to limited agricultural activities within its 39 km2 catchment area [39,40]. Due to limited freshwater inflow because of a small catchment area, the Kasouga Estuary receives limited freshwater which consequently results in low levels of macronutrients entering the system [41]. Macronutrients, such as silicon (Si), phosphorus (P), and nitrogen (N), are necessary nutrients needed by organisms in relatively large amounts for growth and metabolic processes and also play an important role in primary production and nutrient cycling [42]. Compared to permanently open estuaries in the region, Kasouga Estuary supports low levels of phytoplankton (<2.0 mg Chl-a m−3) and zooplankton biomass (<15 mg dwt m−3) [40]. The zooplankton community is numerically dominated by copepods such as the Pseudodiaptomus hessei (Mrazek, 1894) and Paracartia longipatella (Connell & Grindley, 1974) [43,44,45]. Additionally, benthic communities in TOCEs typically include diverse macrofaunal and meiofaunal assemblages such as polychaetes, amphipods, molluscs (e.g., gastropods like Nassarius kraussianus (Dunker, 1846), and crustaceans, including the burrowing sand prawn (Kraussillichirus kraussi) [11,12]. Microphytobenthic communities, such as benthic diatoms, also play a significant role in primary production nutrient cycling through bioturbation and organic matter decomposition [11].

3. Materials and Methods

The estuary remained closed throughout the duration of the caging experiment because of the protracted drought within the region. Due to the presence of an extensive sandbank at the mouth of the system, no overtopping events were recorded over the duration of the caging experiment. Caging experiments were executed in the lower reach of the temporarily open/closed Kasouga Estuary (Figure 1). Manipulations were performed over a period of two and a half months from 1 January to 15 March 2020. The densities of the sand prawn in the lower reach of the system where the experiment was conducted ranged from 5 to 36 ind m−2 (determined by counting the number of active burrows [5]). Stainless steel open-topped cages with dimensions of 50 cm × 50 cm × 30 cm, with 1 mm mosquito netting on the bottom and sides, were buried in the sediment, leaving ≈ 10 cm protruding above the sediment, in the lower reaches of the estuary and were filled with sun-dried sand collected from the edge of the estuary 48 h before the study to ensure faunal absence at the onset of the study. The cages were not roofed, since this would affect their colonization from the water column and would alter the light environment. Three weeks after installing the cages, sand prawns of between 43 and 54 mm standard length, tail not included (mean = 44.3 ± 5.35 mm), were collected from the lower reach of the estuary using a prawn pump. The three-week delay before adding sand prawns ensured that the sediment stabilized, and natural conditions were restored. Four treatments were prepared. For the Control, prawns were absent from the cages. For Treatment 1, 5 prawns were added; Treatment 2 involved 10 prawns, and for Treatment 3, 20 prawns were added to each of the cages. Three replicates were prepared for each treatment. Cages were monitored weekly to ensure that prawn densities in the various treatments remained constant. Densities of the prawn within the cages were assumed to correspond to the number of burrows [11]. At the end of the experiment, bioturbation by the sand prawn, microphytobenthic algal concentrations, macrobenthic (epifauna and infauna) abundance, biomass, and community composition were determined as described below.

3.1. Bioturbation

Centrifuge tubes, 10 cm long with a mouth of 0.78 cm2, were employed as sediment traps to estimate bioturbation by K. kraussi. The aspect ratio (length/diameter) of each tube was 10:1, which was considered the optimum value to prevent re-suspension of trapped sediments [5]. Tubes were buried vertically, leaving ~2 cm protruding above the sediment surface. Tubes were sealed after 1 h and the sediment in each was extracted and dried at 60 °C for 24 h and weighed using a Sartorius microbalance. The amount of sediment within each tube was employed as an index of sediment erodibility and expressed as g cm−2 h−1 [44].

3.2. Microphytobenthic Algal Concentrations

Microphytobenthic algal concentrations were determined by collecting a sediment surface sample in the upper 1 cm of the sediment from each cage using a polycarbonate tube. The tubes were sealed and kept in the dark in a cooler box until their return to the laboratory and were stored at −40 °C in a freezer in the dark with 30 mL of 90% acetone until processing. Total microphytobenthic chlorophyll a (Chl-a) concentrations were then determined fluorometrically using a Turner 10-AU fluorometer before and after acidification [44]. Microphytobenthic Chl-a concentrations were expressed as μg Chl-a cm−2.

3.3. Macrobenthic Community Structure

The macrobenthic community structure in the various treatments was compared by collecting the sediment in the cages at the end of the experiment and sifting it through a 1 mm mesh on site. The organisms collected were preserved in 70% alcohol and transported to the laboratory. Species composition was identified to the lowest taxon using identification keys [46]. The number of individuals per m3 and the milligrams of wet weight per m3 were used to represent biomass and abundance, respectively. Wet weight was determined by removing excess water from the animals using blotting paper and weighing to the nearest 0.1 g using a Sartorius electronic balance.

3.4. Statistical Analysis

To assess differences in macrobenthic abundance, biomass, microphytobenthic algal concentration, and bioturbation rates among treatments, a Kruskal–Wallis test was performed, as data did not meet the assumptions of normality and homogeneity of variances required for parametric tests. When significant differences were detected, Dunn’s post hoc test with Bonferroni correction was applied to identify specific treatment differences. To analyze community composition differences among treatments, a hierarchical cluster analysis was performed using group-average linkage based on Bray–Curtis similarity. This method groups samples according to their similarity, with lower values indicating greater differences in community structure. One-way ANOSIM (Analysis of Similarities) was then used to test for significant differences among predefined treatment groups (Control, Treatment 1, Treatment 2, Treatment 3). Finally, Similarity Percentage (SIMPER) analysis was conducted to determine which taxa contributed most to dissimilarities between treatments. All statistical analyses were performed using Statistica version 8, PAST version 4, Minitab version 19, and Microsoft Excel’s XLSTAT 2022. 1.

4. Results

4.1. Bioturbation

The Kruskal–Wallis test indicated that treatment had a significant effect on bioturbation (H(3) = 12.995; p < 0.001) (Figure 2a; Table S1). Following Bonferroni correction (α = 0.0083), Dunn’s pairwise comparisons showed that Treatment 3 was significantly different from the Control (p = 0.001) and Treatment 2 (p = 0.007).

4.2. Microphytobenthic Algal Concentration

The Kruskal–Wallis test indicated a significant difference between treatments (H(3) = 12.772; p < 0.001) (Figure 2b, Table S2). Significant differences were seen between Treatment 1 (p = 0.001) and Treatment 3 compared to the Control (p = 0.017).

4.3. Macrobenthic Abundance

The results of the Kruskal–Wallis test showed a significant difference between treatments (H(3) = 11.305; p < 0.001) (Figure 2c, Table S3). Significant differences in total macrobenthic abundances were observed between Treatment 3 and the Control (p = 0.023) and between Treatment 3 and Treatment 1 (p = 0.003).

4.4. Macrobenthic Biomass

The Kruskal–Wallis test revealed that treatment had a significant effect on the total macrobenthic abundances (H(3) = 13.79; p < 0.05) (Figure 2d, Table S4). Following Bonferroni correction, Dunn’s pairwise comparisons showed that the total macrobenthic biomass was significantly different between Treatment 3, the Control (p = 0.001) and Treatment 1 (p = 0.007).

4.5. Bray–Curtis Cluster Analysis

At the 60% similarity level, hierarchical cluster analysis identified two distinct groupings, designated Groups 1 and 2. Group 1 largely comprised the Control and Treatment 1 samples, while Group 2 comprised samples from the Control, Treatment 1, Treatment 2, and Treatment 3 (Figure 3). One of the replicates from Treatment 2 was identified as an outlier. ANOSIM indicated that the differences between the treatments was significant (R = 0.37; p = 0.008).
SIMPER analysis indicated that the tick shell, Nassarius kraussianus, was the most influential species, contributing 27.7% to the dissimilarity between treatments. The difference between the treatments could be explained by in the abundances of the amphipods Potegeloides laticeps (Barnard, 1914), Exosphaeroma esturium (Barnard, 1940), and Grandiriella lignorum (Barnard, 1935), which contributed 19.9%, 18.6%, and 17.3%of the differences, respectively (Table 1).

5. Discussion

Historically, studies on the estuarine ecosystem functioning of temporarily open/closed South African estuaries have largely focused on the importance of freshwater inflow, sediment characteristics, and mouth status in determining biological community structure within these systems [45,47]. There is now, however, a growing appreciation of the importance of non-trophic interactions in driving community dynamics within these systems [48]. Ecosystem engineers have emerged as one of the forms of non-trophic interactions responsible for shaping communities and ecosystems within shallow water marine ecosystems [27,49,50,51,52,53]. The current study was designed to assess the role of the sand prawn, Kraussillichirus kraussi, as an ecosystem engineer in a TOCE during a prolonged drought which contributed to the Kasouga Estuary along the southeast coastline of South Africa being separated from the marine environment for a period exceeding 12 months.
Studies conducted in several southern African TOCEs have shown that peak biological productivity, including microphytobenthic biomass and macrobenthic abundances, are typically reached during the closed phase of the system following freshwater inflow [27,54,55,56]. However, the estimates of total microphytobenthic algal biomass and macrobenthic abundance and biomass recorded in the current study were lower compared to previous studies in TOCEs within the same region [27,54]. The lower estimates can likely be attributed to the prolonged mouth closure which limited the recruitment of marine breeding species into the system and low macronutrient availability conferred by the low freshwater inflow resulting from the prolonged drought within the region [56,57]. The macrobenthic community structure observed during the present study agrees with previous investigations, both within the same estuary and, indeed, in TOCEs within the same biogeographic region [4,49,51,54,56].
The increased rates of bioturbation recorded with the increase in the density of K. kraussi observed during the caging experiment (Figure 2a) are not unexpected and likely contributed to the decline in the total microphytobenthic algal concentrations within the cages (Figure 2b). The reduced microphytobenthic algal concentrations observed in Treatments 2 and 3 can be linked to the higher prawn densities which, through their burrowing and feeding activities, contribute to increased sediment turnover and re-suspension of sediment which, on settlement, buries the microphytobenthos [5,6,9,55]. The burrowing activities of the sand prawn have been demonstrated to modify nutrient cycles in the sediments and water column of shallow-water ecosystems [25,58]. The observed increase in microphytobenthic algal concentrations in Treatment 1 can possibly be attributed to the increased availability of macronutrients due to the burrowing activities of the sand prawn.
Total macrobenthic abundances and biomass within the different treatments generally decreased with increased sand prawn density (Figure 2c,d). Again, a notable exception was recorded in Treatment 1, where the values increased. The observed patterns in Treatments 2 and 3 can possibly be ascribed to reduced food availability (microphytobenthic algae) or an attempt of macrobenthos to avoid being buried by settling sediment due to the bioturbating activities of sand prawns [3,12]. Indeed, it is worth noting that differences in the groupings identified with the numerical analyses could largely be attributed to changes in the abundances of mobile components of the macrobenthos including the tick shell, Nassarius kraussianus, and the isopod, Exospheroma hylocoetes (Table 1). [4,26,29,55]. For instance, ref. [10] proposed that the reduction in surface microalgal development brought about by K. kraussi’s reworking of sediments resulted in enhanced food intake by N. kraussianus and decreased food uptake by E. paupercula. The observed pattern could be attributed to disturbances which altered food availability for the different species [59].
The elevated concentrations of microphytobenthic algae in Treatment 1 coincided with elevated abundances and biomass of the macrobenthos (Figure 2a–d). Bioturbation generated by sand prawns at low densities has been demonstrated to enhance macrobenthic abundances and species richness [60,61]. The increase in abundances and biomass of macrobenthos observed in Treatment 1 could mainly be attributed to an increase in the abundance of N. kraussianus (Table 1) which were likely sustained by the elevated biomass of microphytobenthos [4,11]. In contrast, under elevated densities of sand prawn, the abundance and biomass of the macrobenthos were reduced. This result suggests that the effect of K. kraussi burrowing activities in shaping macrobenthos within the study estuary was density dependent [15].

6. Conclusions

The results of this study confirm that the sand prawn, Kraussillichirus kraussi, acts as a significant ecosystem engineer in the Kasouga Estuary along South Africa’s Eastern Cape coastline. This aligns with findings from both local and international studies. Importantly, the results indicate that at low to intermediate densities (5 ind m−2), the burrowing activities of K. kraussi significantly enhanced microphytobenthic algal concentrations, which in turn supported higher macrobenthic biomass and abundance (H(3) = 12.772, p < 0.0001; H(3) = 11.305, p = 0.001; H(3) = 13.787, p < 0.0001, respectively). However, at higher densities (10 and 20 ind m−2), excessive sediment turnover led to lower microphytobenthic biomass, likely limiting macrobenthic abundance due to increased burial and disturbance.
Statistical analyses revealed significant differences in macrobenthic abundance, biomass, and community composition between the low- and high-density treatments, reinforcing the ecological importance of K. kraussi at moderate densities. These findings suggest that at low to medium densities, burrowing activities may mitigate the negative effects of prolonged mouth closure in temporarily open/closed estuaries (TOCEs) by enhancing microphytobenthic production, thereby supporting higher macrobenthic biomass and diversity. This study highlights the critical role of bioturbation by K. kraussi in shaping benthic community structure under varying environmental conditions.

Supplementary Materials

The supporting information can be downloaded at https://www.mdpi.com/article/10.3390/d17040223/s1. Table S1: Minimum and maximum values for Bioturbation rates for all treatments as well as the mean, std error, std deviation and variance within each treatment. Table S2: Minimum and maximum values for Microphytobenthic algal concentrations for all treatments as well as the mean, std error, std deviation and variance within each treatment. Table S3: Minimum and maximum values for Macrobenthic abundances for all treatments as well as the mean, std error, std deviation and variance within each treatment. Table S4: Minimum and maximum values for Macrobenthic biomass for all treatments as well as the mean, std error, std deviation and variance within each treatment. Table S5: Showing the original data for the sand prawn experiment. Table S6: Macrobenthic species recorded during the experiment conducted in the lower reach of the temporarily open/closed Kasouga Estuary located on the Eastern Cape coastline of South Africa. Data shown are ind/m3.

Author Contributions

Conceptualization, C.Y. and W.P.F.; formal analysis, C.Y. and W.P.F.; resources, W.P.F.; data curation, C.Y.; writing—original draft preparation, C.Y. and W.P.F.; writing—review and editing, C.Y. and W.P.F.; funding acquisition, W.P.F. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by Rhodes University and the South African Research Chairs Initiative (SARChI) chair in marine ecosystems, funded by the Department of Science and Technology and the National Research Foundation (NRF) of South Africa (grant number 64801).

Institutional Review Board Statement

Ethical review and approval were waived for this study due to its focus on invertebrates.

Data Availability Statement

Data are available on request from the corresponding author.

Conflicts of Interest

There are no conflicts of interest to declare.

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Figure 1. Geographic location of Kasouga Estuary along the southern African coast [5].
Figure 1. Geographic location of Kasouga Estuary along the southern African coast [5].
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Figure 2. (a) Sediment bioturbation rate, (b) microphytobenthic algal concentrations, (c) macrobenthic abundance, and (d) macrobenthic biomass in various treatments in Kasouga Estuary. Error bars denote standard error. Different letters indicate significant differences detected by Dunn’s post hoc test (p < 0.05).
Figure 2. (a) Sediment bioturbation rate, (b) microphytobenthic algal concentrations, (c) macrobenthic abundance, and (d) macrobenthic biomass in various treatments in Kasouga Estuary. Error bars denote standard error. Different letters indicate significant differences detected by Dunn’s post hoc test (p < 0.05).
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Figure 3. Hierarchical cluster plot, showing Bray–Curtis similarity for the different treatments. Blue lines on the dendrogram indicate different groups and the outlier is circled.
Figure 3. Hierarchical cluster plot, showing Bray–Curtis similarity for the different treatments. Blue lines on the dendrogram indicate different groups and the outlier is circled.
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Table 1. Showing SIMPER analysis for the most abundant macrobenthic species with their contributions to the Bray–Curtis cluster analysis.
Table 1. Showing SIMPER analysis for the most abundant macrobenthic species with their contributions to the Bray–Curtis cluster analysis.
Species Names Average DissimilarityContribution %Cumulative %Mean
Abundance
Group 1		Mean
Abundance
Group 2
Nassarius kraussianus12.627.727.73513.67
Pontegekloides laticeps9.06919.9447.6445.525.33
Exosphaeroma esturium8.47518.6366.272820.67
Grandiriella lignorum7.87617.3283.5930.6719.67
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Yekani, C.; Froneman, W.P. Sand Prawns Mitigate the Impact of Prolonged Drought on the Biology of a Temporary Open/Closed Estuary. Diversity 2025, 17, 223. https://doi.org/10.3390/d17040223

AMA Style

Yekani C, Froneman WP. Sand Prawns Mitigate the Impact of Prolonged Drought on the Biology of a Temporary Open/Closed Estuary. Diversity. 2025; 17(4):223. https://doi.org/10.3390/d17040223

Chicago/Turabian Style

Yekani, Celiwe, and William Pierre Froneman. 2025. "Sand Prawns Mitigate the Impact of Prolonged Drought on the Biology of a Temporary Open/Closed Estuary" Diversity 17, no. 4: 223. https://doi.org/10.3390/d17040223

APA Style

Yekani, C., & Froneman, W. P. (2025). Sand Prawns Mitigate the Impact of Prolonged Drought on the Biology of a Temporary Open/Closed Estuary. Diversity, 17(4), 223. https://doi.org/10.3390/d17040223

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