LB 2-12 - CANCELLED - The influence of phytoplankton and wetland detrital resources on copepod survival and growth in a low-productivity estuary

Background/Question/Methods:

Resource availability affects consumer growth and survival, with the potential to impact consumer populations and food web dynamics. In the Sacramento-San Joaquin River Delta, primary consumers, such as zooplankton, are generally considered food-limited due to declines in pelagic primary production. Detrital inputs from wetland vegetation within this delta may provide alternate food sources for zooplankton. We investigated how resource abundance and composition, in terms of phytoplankton and vascular plant detritus, affect copepod survival and growth. In a set of four-day experiments repeated across three seasons, we exposed copepods (Eurytemora affinis and Pseudodiaptomus forbesi) to field water from sites varying in phytoplankton abundance and detrital inputs and measured survival and growth. We assessed the prevalence of phytoplankton and vascular plant detritus in the field water with chlorophyll-a and lignin phenol concentrations, respectively. As an additional assessment of resource composition and use, we are comparing the taxonomic composition of phytoplankton and vascular plants in field water and copepod guts using amplicon metagenomic analysis. In a separate set of experiments, we examined E. affinis survival and growth across a gradient of phytoplankton (Chlorella sp.) availability that was crossed with two levels of detrital resources (addition of tule detritus, Schoenoplectus sp., or no addition).

Results/Conclusions:

Survivorship was relatively high in our field water exposure experiments; however, copepod growth varied across sites and season in which the experiment was conducted. Variation in growth appeared linked to resource composition and availability. For example, the highest growth rates were associated with high lignin and at least moderate chlorophyll-a concentrations. In contrast, low growth was observed when both lignin and chlorophyll-a concentrations were low. In our other set of diet experiments, Chlorella quantity had a positive effect on the duration and probability of copepod survival, but the addition of tule detritus did not affect survival. Copepod body sizes significantly increased with Chlorella quantity, and the addition of tule also had a small, positive effect on body size. Our study illustrates that both phytoplankton and wetland detritus support copepod growth, highlighting the joint importance of autochthonous and allochthonous resources in this system. In accordance, our results suggest that restoration efforts promoting the establishment and preservation of wetland habitat may positively affect food webs in the Sacramento-San Joaquin River Delta and other estuarine systems with low primary productivity.