Shifts of phytoplankton community in Long Island Sound over the recent decade of nitrogen reduction

Tangcheng Li¹, Huan Zhang², Senjie Lin^2*, Matthew Lyman³, Katie OBrien-Clayton³ and Mary Becker^3
1Shantou University, Guangdong Province, China
²Department of Marine Sciences, University of Connecticut, USA
³Connecticut Department of Energy and Environmental Protection, USA

Climate change and eutrophication are causing alteration of phytoplankton community structure and ecosystem service globally and locally such as in Long Island Sound (LIS). While nutrient management is expected to reverse the alteration, ecological responses are often nonlinear and complex. This study investigated the interplay between phytoplankton communities and environmental factors in Long Island Sound, drawing from a decade of continuous monitoring along with nitrogen reduction effort that has been in place for three decades. Data reveal an upward trajectory in sea temperature, Chl a, SiO₂, and total dissolved phosphorus content, juxtaposed with a decline in nitrogen concentration, dissolved oxygen, salinity, and biological silicon. The pronounced reduction in total soluble nitrogen and concurrent rise in total soluble phosphorus contribute to a decreased N/P ratio, signifying a heightened nitrogen limitation for phytoplankton in the region. In accordance, the observed negative correlation between Chl a and dissolved inorganic nitrogen (DIN) suggests stimulation of phytoplankton by other factors leading to depletion of DIN. Furthermore, the correlation analysis of environmental factors reveals a significant negative relationship between surface dissolved oxygen and temperature, suggesting a plausible link between rising temperatures and hypoxia events in the marine environment. Concurrently, shifts in total phytoplankton abundance (Chl a increase) and community composition are noted, with individual species exhibiting differential responses to the changing conditions. While diatom abundance exhibited a significant decline (P < 0.05), dinoflagellate abundance showed no significant change (P > 0.05), resulting in a declining diatoms/dinoflagellates ratio over the past decade. The dwindling diatom abundance corresponded with a marked increase in seawater silicate concentration (P < 0.05), suggesting that the diatom decline was not due to Si limitation. Additionally, blooms of Eutreptiella sp., the rhodophytes Chondrus crispus and Grateloupia turuturu, and the cryptophyte Teleaulax amphioxeia were observed within this period. This research sheds light on the complexity of phytoplankton response to N reduction coupled with ocean warming, with important implications in single-nutrient management.
Keywords: Phytoplankton, nutrient reduction, global warming, diatom decline
 

Phytoplankton, nutrient reduction, global warming, diatom decline,machine learning