Effects of ocean circulation on the eutrophication of a Mediterranean gulf with river inlets: The Northern Thermaikos Gulf

Abstract

Thermaikos Gulf is a typical, river-fed, microtidal, semi-enclosed, coastal inlet of the east-central Mediterranean Sea. It is an important coastal ecosystem susceptible to several anthropogenic pressures, strong river discharges and variable meteorological and ocean (met-ocean) conditions. One of the most significant environmental problems of the region is the occasional formation of extended eutrophication phenomena (red tides, mucilaginous aggregates), especially over the Northern Thermaikos Gulf (NTG). Herein, we investigate the contribution of hydrodynamic processes on the formation of such events, under the effects of different meteorological and river discharge conditions during the annual cycle. We conducted field observations (physical-chemical measurements), microscopy analysis of phytoplankton samples, satellite ocean color image analysis, and implemented high-resolution numerical hydrodynamic simulations with updated river discharge outflows to detect eutrophication events and correlate them with the prevailing physical processes and ocean circulation patterns. The eutrophication events were mainly associated with the dominance of southerly winds, which affect the ocean circulation over the NTG in three ways: i) they confined the surface waters in the northern parts of the NTG separating the waters masses between the northern and southern regions, ii) they contribute on the northward spreading of nutrient-rich brackish waters towards the northern parts of the Gulf, and iii) they impose an anticyclonic circulation, especially in the inner- and central-Gulf weakening its renewal process. Northerly winds contribute on the renewal of the Gulf imposing a two-layer flow, especially along the eastern coasts. The seasonal observational campaigns of the 2017–2018 period captured three eutrophication events (June–July 2017, December 2017, and May 2018) and two renewal periods (late-July 2017 and October 2017) that were favoured by the two aforementioned types of met-ocean synergy. New insight is provided on the mesoscale ocean circulation and sub-mesoscale local effects on eutrophication events in a typical microtidal coastal system of the east-central Mediterranean, where freshwater discharges by a multi-river inlet.