Factors Controlling Blooms of Microalgae and Macroalgae (Ulva rigida) in a Eutrophic, Urban Estuary: Jamaica Bay, NY, USA

Eutrophic estuaries are increasingly common features along global coastlines and are typically prone to micro- and macroalgal blooms. Yet, studies concurrently assessing the factors controlling these distinct algal populations have been rare. During 2010–2012, the temporal and spatial dynamics of macro- and microalgal communities in the eutrophic estuary, Jamaica Bay, NY, USA, were investigated in parallel with the factors that control their growth. Phytoplankton blooms dominated by centric diatoms of the genera Thalassiosira spp. were intense within poorly flushed regions during the spring and summer (>135 μg L⁻¹ chlorophyll a; >55,000 algal cells mL⁻¹) and, in some cases, were genetically confirmed to be caused by Thalassiosira angulata. Dissolved inorganic nitrogen and phosphorus levels were high throughout the year, while silicate concentrations were sometimes reduced to <1 μM and limited the growth of Thalassiosira spp. during the late spring and early summer. This limitation facilitated a shift in phytoplankton community dominance toward autotrophic nanoflagellates. While silicate influenced phytoplankton community composition, the magnitude of phytoplankton biomass across Jamaica Bay was largely controlled by bay residence time. Sequencing ITS regions of ribosomal DNA confirmed that macroalgal communities were dominated by Ulva rigida with the densest populations (>90 % bottom coverage) present in the shallow, central portion of the Bay and significantly lower coverage within deeper regions. The δ¹⁵N signature of U. rigida tissue (13–17 ‰) indicated that wastewater was the primary nitrogen (N) source of this alga, and the N content of their tissues indicated that this alga was N replete. Accordingly, while nutrients almost never restricted the growth of U. rigida, multiple lines of evidence indicated that these populations were light-limited by phytoplankton blooms within the deeper regions of the Bay. Collectively, this study reveals the interactive roles that residence time, silicate, light, and competition play in controlling micro- and macroalgal populations in eutrophic estuaries such as Jamaica Bay.