Interplay among anthropogenic impact, climate change, and internal dynamics in driving nutrient and phytoplankton biomass in the Gulf of Naples

Abstract

Due to an ever-increasing demographic pressure, coastal areas are hotspots of anthropogenic impact on marine ecosystems. Understanding the extent and nature of these impacts is critical for developing effective conservation and management strategies to protect and restore coastal marine ecosystems and the services they provide. The Gulf of Naples is a coastal embayment in the Tyrrhenian Sea, Western Mediterranean Sea, which is severely impacted by human activities. Here, we discuss the intertwining of anthropogenic pressures and climate variations in regulating phytoplankton biomass dynamics in the Gulf and the presence of possible long-term changes. We analysed three decades of T-S, nutrient, and chlorophyll a data collected at a fixed station, the LTER-MC site. Despite some spikes in nutrient concentrations (up to 9.92 mmol m−3 for NO3−, 13.12 mmol m−3 for NH4+, 0.75 mmol m−3 for PO43−, and 13.59 mmol m−3 for Si(OH)4), median surface and depth-integrated concentrations are relatively low (0.73 mmol m−3 and 0.45 mmol m−3 for NO3−, 0.73 mmol m−3 and 0.47 mmol m−3 for NH4+, 0.07 mmol m−3 and 0.04 mmol m−3 for PO43−, 1.61 mmol m−3 and 1.44 mmol m−3 for Si(OH)4, respectively). Chlorophyll a concentrations, here taken as a proxy of phytoplankton biomass, occasionally display high values (up to 17.33 mg m−3) and fluctuating highs and lows lasting several months, with an overall median value of 1.37 mg m−3 at the surface, and 0.55 mg m−3 as integrated mean. These values are nine and three times higher than the offshore concentrations. The plankton food web at the site is mostly driven by terrestrial inputs. The effect of large-scale–long-term trends in nutrient inputs (e.g., phosphate load reduction) is comparable to that of local drivers, also because of the relatively shallow depth (75 m) of the station and its vicinity to the coastline. Importantly, the physical dynamics, despite the more closed morphology of the Gulf with respect to other bays in the Tyrrhenian Sea, efficiently removes excess nutrients and biomass, preventing dystrophic phenomena.