Seasonal variations in water quality and phytoplankton–bacteria interactions mediated through dissolved organic matter in New Jersey coastal waters

Abstract

New Jersey coastal areas are experiencing eutrophication due to human-induced nutrient overloading. Algal blooms occur frequently in New Jersey coastal waters, and excessive blooms shift water quality. However, phytoplankton–bacteria interactions mediated through dissolved organic matter (DOM) have not been extensively studied in New Jersey coastal waters, especially near overburdened communities. We targeted a traditionally underrepresented township area, Keyport Harbor, as a model site to investigate seasonal variabilities of phytoplankton biomass, DOM, and bacteria biomass. Chlorophyll-a concentrations were significantly higher in spring–summer (bloom) than in fall–winter (nonbloom). Nitrate + nitrite and ammonium were negatively correlated with chlorophyll-a, and the water was nitrogen-limited during bloom time while phosphorus-limited during nonbloom time, implying that regulating nitrogen loading was key to controlling algal blooms, especially during bloom seasons. Phytoplankton–bacteria interactions were assessed by monitoring dissolved organic carbon (DOC) and bacterial abundance between bloom and nonbloom time from field and incubation studies. A significantly higher DOC, but not dissolved organic nitrogen, occurred in the bloom than nonbloom period, suggesting that phytoplankton contributed to the production of more carbon-rich than nitrogen-rich compounds. DOC fueled threefold bacterial growth in the bloom period, exceeding the temperature effect and indicating strong phytoplankton–DOM–bacteria connections. Microbial remineralization incubations showed rapid phytoplankton–DOC drawdown, and more ambient DOC drawdown and bacterial growth in the bloom than nonbloom time, further supporting the important role of phytoplankton–DOC in shaping bacteria. With water quality monitoring via chemical and biological indicators, the study aimed to understand carbon cycling better, assess anthropogenic impacts on coastal environments, and help facilitate coastal management.