Investigating the effects of environmental stress on coastal zooplankton populations: from mechanistic drivers to trophic impacts

Abstract

Environmental stressors, such as hypoxia and acidification, are increasing in intensity, duration, and extent in coastal waters and estuaries. Environmental stressors are known to affect a wide range of marine species, including zooplankton. Zooplankton are a critical link in marine food webs, connecting phytoplankton to higher trophic levels such as economically important fish, and are thought to be informative indicators of ecosystem change. For this reason, increased attention has been paid to understanding the mechanisms shaping zooplankton populations. Previous studies have shown that zooplankton exhibit both lethal and sublethal responses to changes in dissolved oxygen and pH. However, there is a range of species-specific responses to stressors. Different responses across species alter zooplankton community composition and spatial distributions, directly impacting predator-prey interactions and the trophic dynamics in coastal environments. This dissertation integrates laboratory experiments, in situ observations, and field work to understand how environmental stressors affect coastal zooplankton populations and nearshore food webs. In Chapter 1, I conducted laboratory experiments to investigate whether the copepod, Calanus pacificus, showed behavioral responses to stressors, and whether these responses lead to changes in vertical population distributions. Our laboratory experiments demonstrated significant effects of bottom water hypoxia and acidification on behavioral avoidance, swimming statistics, and apparent mortality rates in C. pacificus. In Chapter 2, I used a remote camera system to quantify in situ behavioral responses of zooplankton to stressors, using results from Chapter 1 to generate hypotheses about observations in the field. Our in situ videos revealed that copepods in stressful conditions exhibited significantly slower swimming speeds than copepods in non-stressful conditions, while amphipods showed significantly decreased abundances within stressful conditions. Finally, in Chapter 3, I collected zooplankton net tows in an intertidal estuary to investigate the transport of pelagic species into eelgrass beds and the role of eelgrass beds as potential sinks of pelagic zooplankton over the tidal cycle, potentially due to predation by juvenile fish. We found evidence of transport of pelagic species into intertidal habitats and measured large spatial and temporal variability, highlighting the need for sampling programs that can capture small-scale variability. This dissertation provides insight into the mechanisms that link the effects of environmental stressors across individual responses to population, community, and ecosystem level scales and suggests novel methodologies to help advance our understanding of changing zooplankton dynamics.