Published by the University of Cape Town (UCT) under a non-exclusive license granted to UCT by the author. At the end of the experiment, the relative abundance of phytoplankton in the controls was 1.9 times higher than in the 100% treatment. Similarly, nitrite concentrations were 17.7 times higher in the controls compared to the 100% treatment at the end of the experiment.
Furthermore, increasing sand shrimp abundance induced a phytoplankton size-based shift from pico- to nano-dominance, with nanophytoplankton contributing 17.76% at the beginning of the experiment but shifting to 58.07% at the end of the experiment in the maximum sand shrimp density treatment. I would like to express my sincere appreciation to the National Research Fund (NRF) and the University of Cape Town, who funded this research by awarding me the DST-NRF Masters Innovation Scholarship and Masters Research Scholarship (respectively). Firstly, I would like to thank my supervisor Associate Professor Deena Pillay for his guidance and valuable feedback throughout the project.
Finally, but very importantly, I would like to say my deepest thanks to my family (especially Jody, Bianca, Aunt Henrietta and of course my parents Randall and Eleanor) for their endless care, support and encouragement.
INTRODUCTION 1
This results in the efficient transport of carbon into the ocean interior thus driving the marine carbon pump (Marañón, 2015; Ward et al., 2012). For example, communities dominated by large cells are typically associated with shorter, more direct trophic pathways that support large fish populations (Ward et al., 2012). In contrast, more productive regions support a greater proportion of larger species such as coccolithophores, diatoms, and dinoflagellates (Marañón, 2015; Ward et al., 2012).
These blooms can seriously degrade water quality by producing toxins such as anatoxin, nodularin and microcystins (Zhang et al., 2015). This can be partly attributed to the resuspension of sediment particles (through bioturbation) that clog these bivalve filtration apparatus (Pillay et al., 2007). However, the presence of sand shrimp did not affect suspended solids concentrations or increase ammonia concentrations to toxic levels (Venter et al., 2020).
Nevertheless, not much is known about the efficiency and general application of biofiltration on pathogen concentrations (Coen et al., 2007).
MATERIALS AND METHODS 16
- Sample collection site 16
- Experimental design 18
- Sample preparation 20
- Statistical analyses 23
All appropriate permissions (including animal ethics approval) were obtained before the start of the experiment. Each mesocosm was individually aerated (Pillay et al., 2012) and allowed to settle for 1 day before sand shrimp were added. Each biological response variable was measured by flow cytometry, which has proven to be a powerful tool for the analysis of aquatic microorganisms (Manti et al., 2012; Marie et al., 1999).
Since water depth was not found to affect nutrients or chlorophyll-a in an experiment with the same mesocosms (Venter et al., 2020), water samples were taken only from the surface. We used 5–5.9 μm fluorescent beads (AccuCount Fluorescent Particles, Spherotech, Lake Forest, IL, USA) with a standard concentration to determine absolute cell concentrations, which was achieved by comparing cell events with bead events (Gong et al. , 2017). ). In addition, the sensitivity of flow cytometry allows the discrimination of these two similar large groups (Marie et al., 1997).
Sampling days were considered a random effect, as measurements are expected to change over the duration of the experiment (Venter et al., 2020).
RESULTS 27
Abiotic variables 27
Nutrients 31
Results were obtained by analysis of variance (ANOVA) of mixed effects models and, in the case of suspended solids, a linear model.
Biological response variables 31
Proportion of variance explained by axes Dim1 and Dim2 is shown on each PCA. The fluorescence emitted by the cell populations gives an indication of the composition of the population. Inset (A) Proportion change (mean ± 1SE) in phytoplankton abundance among sand shrimp treatments at the end of the mesocosm experiment.
For cryptophytes, relative abundance peaked in the 50% sand shrimp treatment (Figure 10) and in the 100% sand shrimp treatment for E.coli (Figure 11). Inset (A) Proportion change (mean ± 1SE) in THB abundance among sand shrimp treatments at the end of the mesocosm experiment. Inset (A) Proportion change (mean ± 1SE) in cryptophyte abundance among sand shrimp treatments at the end of the mesocosm experiment.
Inset (A) Proportion change (mean ± 1SE) in E.coli abundance among sand shrimp treatments at the end of the mesocosm experiment. However, at the end of the experimental period, the 100% treatment had the greatest decrease in total phytoplankton relative to controls. Consequently, after the end of the experiment, the control contained the largest proportion of phytoplankton cells (8.1%) compared to the initial conditions (Figure 12).
This was followed by the 50% treatment, which retained 5.8%, and the 100% treatment, which recorded 4.3% phytoplankton cells relative to initial conditions. The 100% sand shrimp treatment had the greatest decline, resulting in a final abundance of 13.59% of initial conditions, followed by the 50% treatment, which resulted in an abundance of 15.67% (Figure 13). Letters a and b, generated by Tukey's pairwise comparisons, represent significant differences (p<0.05) between treatments; Inset A: Proportion change (mean ± 1SE) in nannoplankton abundance between sand shrimp treatments at the end of the mesocosm experiment.
Trends for picoplankton followed those for phytoplankton and nanoplankton, with the control having the highest relative abundance of picoplankton at the end of the experiment (5.57%, Figure 14), followed by the and 100% treatments (2.13%). Letters a, b and c created by Tukey pairwise comparisons, represent significant differences (p<0.05) between treatments; Inset A: Proportion change (mean ± 1SE) in picoplankton abundance among sand shrimp treatments at the end of the mesocosm experiment.
DISCUSSION 41
Nitrification (reduction of nitrogen compounds to nitrate and nitrite) and denitrification (reduction of nitrite to gaseous nitrogen) are bacterially mediated processes (Howe et al., 2004). Therefore, any effect on bacterial activity is likely to affect nitrification and denitrification and ultimately the nitrogen cycle (Howe et al., 2004). This is consistent with Howe et al. 2004), who found that the presence of burrowing mud shrimp Upogebia deltaura significantly increased both denitrification rates (i.e. nitrite utilization) and coupled nitrification-denitrification rates.
Interestingly, the finding of nitrite concentration being reduced in the presence of sand contrasts with the findings of Venter et al. 2020), who found no significant influence of sand on nitrate concentrations in the water column. While Venter et al. 2020) and my experiments were based on the same design (except for the experimental duration) and with ecological materials for the experiment originating from the Zandvlei estuary, both experiments were carried out at two different times (2019 vs. 2020). The findings regarding the decline in total phytoplankton are consistent with Venter et al. 2020), who reported a decrease in pelagic microalgal biomass by approximately 50% in the presence of sand.
Moreover, Rosa et al., (2017) showed that several surface properties of microalgal cells drive selection by two bivalve species. Food was then carried to the mouth with the help of the 3rd jaw bone (Coelho et al., 2000). This has been demonstrated in other callianassid species (Callianassa subterrenea, Biffarius arenosus and Trypea austaliensis) which have also been described as optimal foragers (Stamhuis et al., 1998; Stapleton et al., 2001).
Furthermore, in marine waters they reach fairly high divisions in the order of 1 division per day (Partensky et al., 1999). Mesocosm experiments can additionally accelerate the understanding of environmental problems and ultimately facilitate the development of practical solutions. (Benton et al., 2007). Finally, some of the results obtained in my experiment are consistent with Venter et al., (2020), whose mesocosm experiment results indicated a decrease in chlorophyll-a in response to the presence of sand shrimp.
However, the idea that sand shrimp could increase the amount of carbon sequestered from the atmosphere must also be considered in the context of the reduction in phytoplankton caused by sand shrimp recorded in my experiment and that of Venter et al. The findings of my experiment are based on the work of Venter et al. 2020), which added new perspectives to existing benthocentric paradigms.
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APPENDICES 78
Appendix A 78
Linear mixed effects models were fitted to logit transformed variables (cryptophytes and Prochlococcus-like algae), and do not generate p-values.
Appendix B 79
