Abstract
Ecological communities are increasingly exposed to multiple interacting stressors. For example, warming directly affects the physiology of organisms, eutrophication stimulates the base of the food web, and harvesting larger organisms for human consumption dampens top-down control. These stressors often combine in the natural environment with unpredictable results. Bacterial communities in coastal ecosystems underpin marine food webs and provide many important ecosystem services (e.g. nutrient cycling and carbon fixation). Yet, how microbial communities will respond to a changing climate remains uncertain. Thus, we used marine mesocosms to examine the impacts of warming, nutrient enrichment, and altered top-predator population size structure (common shore crab) on coastal microbial biofilm communities in a crossed experimental design. Warming increased bacterial α-diversity (18% increase in species richness and 67% increase in evenness), but this was countered by a decrease in α-diversity with nutrient enrichment (14% and 21% decrease for species richness and evenness, respectively). Thus, we show some effects of these stressors could cancel each other out under climate change scenarios. Warming and top-predator population size structure both affected bacterial biofilm community composition, with warming increasing the abundance of bacteria capable of increased mineralization of dissolved and particulate organic matter, such as Flavobacteriia, Sphingobacteriia, and Cytophagia. However, the community shifts observed with warming depended on top-predator population size structure, with Sphingobacteriia increasing with smaller crabs and Cytophagia increasing with larger crabs. These changes could alter the balance between mineralization and carbon sequestration in coastal ecosystems, leading to a positive feedback loop between warming and CO2 production. Our results highlight the potential for warming to disrupt microbial communities and biogeochemical cycling in coastal ecosystems, and the importance of studying these effects in combination with other environmental stressors.
Highlights
Coastal marine ecosystems are among the most important providers of ecosystem services, but are some of the most heavily exploited (Barbier et al, 2011)
The community shifts observed with warming depended on top-predator population size structure, with Sphingobacteriia increasing with smaller crabs and Cytophagia increasing with larger crabs
Using marine tidal mesocosms that mimic naturally occurring temperate rock pools, we investigate the effect of multiple stressors on bacterial biofilm communities to test the following hypotheses: (H1) Warming will lead to more diverse bacterial biofilms, with more degraders of complex biomolecules; (H2) Nutrient enrichment will lead to less diverse bacterial biofilms with a less even distribution of diversity; and (H3) Decreasing top-predator population size structure will lead to a reduction in bacteria associated with arthropods and an increase in the relative abundance of bacteria associated with algae
Summary
Coastal marine ecosystems are among the most important providers of ecosystem services, but are some of the most heavily exploited (Barbier et al, 2011). With more nitrogen input into the oceans coming from anthropogenic sources, natural denitrification is overwhelmed, destabilizing coastal food webs, biogeochemical cycles, and associated ecosystem services (Gruber & Galloway, 2008) This can have a dramatic effect on microbial communities in coastal environments, including decreased diversity, increased abundance of denitrifiers, and coastal eutrophication (Graves et al, 2016; Wang et al, 2019). Even just changes in their body size, can cause trophic cascades that alter the biomass of lower trophic level consumers and subsequently their resources at the base of the food web (Jochum et al, 2012; Myers et al, 2007; O’Connor & Bruno, 2007; O’Connor et al, 2013) These effects may propagate down to bacterial biofilm communities. Using marine tidal mesocosms that mimic naturally occurring temperate rock pools, we investigate the effect of multiple stressors (warming, nutrient enrichment, and top-predator population size structure) on bacterial biofilm communities to test the following hypotheses: (H1) Warming will lead to more diverse bacterial biofilms, with more degraders of complex biomolecules; (H2) Nutrient enrichment will lead to less diverse bacterial biofilms with a less even distribution of diversity; and (H3) Decreasing top-predator population size structure will lead to a reduction in bacteria associated with arthropods (e.g. chitin degraders) and an increase in the relative abundance of bacteria associated with algae
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