Microphytobenthic responses to endobenthic bioturbator density, temperature and eutrophication in a global change mesocosm experiment

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The microphytobenthos (MPB) is a key determinant of ecological processes occurring in coastal sediments, where it functions as a trophic resource, sediment stabiliser and mediator of nutrient cycling and bentho-pelagic coupling. Endobenthic bioturbators are also important components of coastal sediments, wherein they influence ecosystem functioning principally through high sediment and pore-water manipulation rates. However, understanding of how endobenthic engineer activities influence MPB components, especially in the context of global change processes, is limited. We made use of a factorial mesocosm experiment and an in situ optical tool (BenthoTorch) to quantify responses of cyanobacteria, green-algae, diatoms and total MPB biomass to (1) eutrophic vs mesotrophic and (2) high vs low temperature conditions at increasing densities of sandprawns (Kraussillichirus kraussi). These crustaceans are highly influential endobenthic engineers in southern Africa, being widely distributed in estuaries, lagoons and embayments. Findings from our experiment indicated that total MPB biomass and that of cyanobacteria and diatoms (the dominant constituents of MPB assemblages in mesocosms) decreased from controls to maximum sandprawn-density treatments, but predominantly under low temperature. Sediment boundary roughness increased with sandprawn density, suggesting that sandprawn effects on MPB biomass and that of cyanobacteria and diatoms under low temperature was likely sediment mediated, with deposition of residual burrow sediment to the sediment-water interface reducing microalgal productivity. All MPB metrics declined with high temperature, suggesting a physiological intolerance of the MPB assemblages to upper thermal conditions in our experiment or nutrient limitation due to temperature-induced increases in metabolism. Our findings highlight the potential for rising temperature to reduce MPB biomass, as has been reported for open ocean and lake phytoplankton assemblages. Our results also highlight the functional significance of sandprawns in structuring MPB assemblages in coastal ecosystems, but their sediment-mediated limitation of benthic cyanobacterial biomass may confer a degree of resilience to coastal benthic ecosystems against harmful cyanobacterial blooms.