The effect of groundwater discharge on microbial biomass and activity in the ultra-oligotrophic eastern Mediterranean coast

Abstract:

Submarine groundwater discharge (SGD) is a globally important process supplying nutrients, trace elements and microorganisms to the oceans, thereby affecting different biogeochemical cycles. The zone where terrestrial groundwater and seawater mix (subterranean estuary, STE) is an important hotspot for biogeochemical reactions, and yet only a handful of studies have investigated its microbial diversity and functioning. This is especially true for ultra-oligotrophic coastal regions such as the Eastern Mediterranean where external nutrient/ trace-metals/ bacterial inputs through SGD greatly affect this sensitive system. Our objective is to fundamentally understand which microbes are driving biogeochemical cycles in the STE at the Northern part of a carbonate-rich coastal aquifer of Israel (Achziv). At this site, we found that SGD contributes significantly higher concentrations of dissolved nitrate and orthophosphate to the coastal seawater, resulting in elevated in-situ phytoplankton biomass and primary productivity. A complimentary microcosm experiment, in which groundwater (Achziv) was mixed with unaffected seawater at different ratios (0-20% volumetric), showed unexpected results (Figure 1). Lower mixing ratio of freshwater to seawater (5%) triggered a stronger increase in phytoplankton biomass (specifically of Synechococcus) and primary productivity relative to higher mixing scenarios (10% and 20%). Alternatively, bacterial production exhibited a dose-dependent response (i.e., the higher groundwater addition the stronger the increase in activity). This suggest that phytoplankton were outcompeted by bacteria for the SGD-derived nutrients, highlighting the complex and delicate interactions between autotrophic and heterotrophic microbes in oligotrophic realms such as the Mediterranean coast.