Benthic and pelagic responses to endobenthic bioturbator (Kraussillichirus kraussi) density, temperature and eutrophication in a global change mesocosm experiment

Thomas, Cheryl

Benthic and pelagic responses to endobenthic bioturbator (Kraussillichirus kraussi) density, temperature and eutrophication in a global change mesocosm experiment

Thesis / Dissertation

2025

Publisher

University of Cape Town

Department

Department of Biological Sciences

Faculty

Faculty of Science

Abstract

Coastal ecosystems are increasingly being threatened by global change stressors such as eutrophication and warming, which can impair ecosystem functioning and ultimately affect societal well-being. Addressing this challenge, in part, requires identifying and understanding mechanisms that may enhance ecosystem resilience to global change. The axiid crustacean Kraussillichirus kraussi (sandprawn) plays a critical role as an endobenthic bioturbator in southern African estuaries and coastal ecosystems, with previous research highlighting its role in mediating bentho-pelagic coupling via top-down reductions in phytoplankton biomass, likely through phytoplankton adsorption onto burrow walls during bi-directional water- pumping. This novel finding has the potential to be an important resilience-enhancing mechanism against eutrophication in estuarine and coastal ecosystems locally. However, the robustness of this process, along with other benthic functions provided by sandprawns, is unclear given that they are nested within global change processes such as warming and eutrophication, which have the potential to alter fundamental benthic and pelagic processes mediated by sandprawns. In this context, this thesis addressed critical knowledge gaps in understanding the individual and combined impacts of warming, eutrophication and sandprawn density on ecosystem functions and community dynamics in both pelagic and benthic compartments, using an indoor mesocosm experiment. Results from Chapter 3 of the thesis revealed that sandprawn water filtration (reduction in phytoplankton biomass) remained robust under simulated warming to levels predicted by the year 2100, with elevated temperatures accelerating reductions in phytoplankton biomass. Sandprawn biofiltration also remained robust under eutrophic conditions, reducing phytoplankton biomass by approximately 74%. Importantly, under eutrophic conditions, sandprawn filtration prevented the development of extreme eutrophy, and prevented a switch to nanoplankton dominance, resulting in even contributions of pico- and nanophytoplankton. Findings thus highlight the potential for sandprawn water filtration to be an important natural process that can mitigate eutrophication symptoms under future global change conditions. The observed declines in the abundance of subsurface microphytobenthic algal biomass and abundance generally declined with warming, likely due to physiological intolerance of cool- temperate assemblages to elevated temperatures. Sandprawn induced reductions of cyanobacteria may likely enhance ecosystem resilience to coastal eutrophication by limiting harmful cyanobacterial blooms. However, warming-induced declines in microphytobenthic biomass may have important implications for sediment stability and trophic resource availability for benthic feeders under future global change scenarios. Findings from Chapter 5 highlighted the importance of temperature and sandprawn density rather than eutrophication in determining sandprawn bioturbation rates. Sediment boundary roughness (a proxy for bioturbation) increased by 43% at maximum sandprawn density under high temperatures (29.5℃), suggesting robustness of sandprawn physiology to future warming scenarios, particularly within populations inhabiting cool-temperate distribution ranges. Organic matter degradation rates increased under mesotrophic conditions and warming, particularly at high sandprawn densities, but was suppressed under eutrophic conditions, emphasising the importance of minimising nutrient loading to estuaries to prevent impairment of benthic carbon degradation. Lastly, Chapter 6 explored meiofaunal responses to predictor variables, and highlighted exceptionally complex and varied outcomes and the importance of contextual settings in determining responses to global change. Notably however, findings showed that higher sandprawn densities were associated with increased meiofaunal biomass dominance relative to abundance, even under high temperatures and eutrophic conditions (measured using W-statistics and ABC curves), suggesting that sandprawn bioturbation and associated benthic ecosystem engineering contributes to less perturbed states that facilitate meiobenthic assemblages. Overall, findings of this thesis demonstrate the potential for sandprawns to facilitate the development of less disturbed benthic ecosystems, resulting in meiobenthic attributes indicative of stable communities. Findings also indicate the ability of sandprawns to enhance ecosystem resilience by preventing phytoplankton and cyanobacterial blooms and shifts to nanophytoplankton dominance under eutrophic conditions, including under warming scenarios. These findings highlight the potential of sandprawns to be important nature-based processes that can combat global change challenges, while also emphasising the need to conserve and manage sandprawn populations so that they are included in resilience-based ecosystem management.