Qualitative and quantitative changes of plankton communities in a highly invaded estuary

Background/Question/Methods

Pervasive and accelerating changes to ecosystems due to human activities remain major sources of uncertainty in predicting the structure and dynamics of ecological communities. Understanding which biotic interactions within natural multitrophic communities are threatened or augmented by invasions of non-native species in the context of other environmental pressures is needed for effective management. We used multivariate autoregressive models with detailed time-series data from one freshwater and one brackish region of the upper San Francisco Estuary to assess the connectance, topology, direction and strength of trophic interactions following major invasions and establishment of non-native zooplankton in the early 1990s. We simultaneously compared the effects of fish and clam predation, environmental temperature, and salinity intrusion using time-series data from > 60 monitoring locations and spanning more than three decades. We assessed δ¹⁵N and δ¹³C stable isotopes, ratio of carbon to nitrogen nutrients and fatty acid profiles of zooplankton and seston to determine whether the shifts in zooplankton species composition due to invasions were associated with long-term changes in nutritional status of the zooplankton community, the major resource for pelagic fish.

Results/Conclusions

We found changes in the networks of biotic interactions in both regions after the major zooplankton invasions. Our results suggest intensified negative interactions between native herbivores and other zooplankton groups, increased bottom-up influence of larval copepods, but weaker influence of phytoplankton as a resource for higher trophic levels following the invasions. We identified salinity intrusion as a primary pressure but showed relatively stronger importance of biotic interactions. We show differences among zooplankton taxa in trophic position, elemental stoichiometry and concentrations of main fatty acids classes, confirming the taxon-specific differences in nutritional value within the zooplankton community. We found temporal declines in concentrations of monounsaturated and polyunsaturated fatty acids in the brackish region and decline of community-level monounsaturated acids in the freshwater region. Our results indicate that both quantitative and qualitative prey data in combination with taxonomic community composition better explained the precipitous decline of pelagic fishes in this estuary and potentially elsewhere. This work highlights the dynamic nature of biotic interactions and provides evidence of how simultaneous invasions of exotic species may modulate food web dynamics.