Pelagic contribution to gross primary production dynamics in shallow areas of York River, VA, U.S.A.

The pelagic contribution to total gross primary production (GPP) is often used to evaluate the assemblages of primary producers in shallow estuarine and coastal ecosystems that shift between benthic‐dominated and pelagic‐dominated status as a response to nutrient loading change. This study investigated the spatial and temporal variability of the pelagic contribution to ecosystem GPP for the years 2003–2015 in shallow areas of the York River. The monthly average values of ecosystem GPP and pelagic GPP were estimated by using 15‐min observational data of dissolved oxygen and chlorophyll a, respectively. A larger pelagic contribution, generally, corresponded to a higher pelagic GPP, while pelagic GPP showed significant positive correlations to riverine discharge and total nitrogen loading at mesohaline and polyhaline stations. The interannual variability in benthic GPP (ecosystem GPP – pelagic GPP), however, exhibited a non‐monotonic relationship to the increase in pelagic GPP. A conceptual model with four ecosystem stages in response to nutrient loading changes is proposed based on changes in the relationship between benthic GPP and pelagic GPP, and that between pelagic contribution and pelagic GPP. The model can explain the different responses of an ecosystem to nutrient loading observed in various estuaries and can be used to evaluate the status of an ecosystem undergoing the eutrophication/oligotrophication process. The stages of York River ecosystems during the study period were quantitatively examined. Results suggest that benthic GPP shows negative correlations to pelagic GPP in the downstream York River over most months of the year, indicating a gradual degradation of the benthic community.