Phytoplankton Community Structure in a Seasonal Low-Inflow Estuary Adjacent to Coastal Upwelling (Drakes Estero, CA, USA)

Low-inflow estuaries (LIEs) in Mediterranean climates located within regions of wind-driven coastal upwelling have seasonally minimal freshwater inflow, are supplied by nutrient-rich upwelled water, and often used for sustainable shellfish mariculture supported by a rich pelagic food web. They likely support endemic estuarine phytoplankton supplemented during the upwelling season with phytoplankton supplied from the ocean. There are few descriptions of LIE algal assemblages. This study characterizes the spatio-temporal phytoplankton community dynamics in a low-inflow estuary located along the northern California coast with high frequency sampling to assess seasonal influences on functional groups, blooms, and endemic populations. Phytoplankton biomass and community composition were measured in Drakes Estero monthly from June 2016 to November 2017. We predicted blooms with diatoms in summer and dinoflagellates in fall but had no prediction for non-bloom situations and what phytoplankton would occur year-round. Flagellates were ubiquitous throughout the Estero at all times. Two summer blooms occurred with diatoms characteristic of upwelling (Chaetoceros,Skeletonema,Asterionellopsis,Thalassiosira) and two blooms in fall with dinoflagellates dominated byProrocentrumand high densities of the raphidophyteHeterosigma.Harmful algal bloom (HAB) taxa were observed, some at levels reported to have negative effects. Non-metric multi-dimensional scaling analyses showed the diatoms to be associated with upwelling, cooler temperatures and high nitrate, and the dinoflagellates and flagellates associated with warmer temperatures and reduced upwelling. Dinoflagellate and flagellate communities, especially HAB taxa, might be promoted with warmer temperatures or longer warm seasons accompanying climate change; an important consideration for managers of LIEs, especially those containing mariculture facilities.

Automated summary

The study focused on the phytoplankton community dynamics in low-inflow estuaries, specifically looking at diatoms associated with upwelling and dinoflagellates with warm temperatures. Results showed that diatoms were linked to upwelling, cooler temperatures, and high nitrate levels, while dinoflagellates and flagellates were associated with warmer temperatures and reduced upwelling. The presence of harmful algal bloom (HAB) taxa, especially with warmer temperatures due to climate change, is a concern for estuaries containing mariculture facilities.