Nitrate-Enrichment Structures Phytoplankton Communities in the Shallow Eastern Mediterranean Coastal Waters

Eyal Rahav,Meor Maayani,Peleg Astrahan,Dror L Angel,Ofrat Raveh,Barak Herut,Natalia Belkin,Keren Yanuka-Golub,Noam Tsumi,Yael Kiro,Jacob Silverman

doi:10.3389/fmars.2020.611497

Abstract

Submarine groundwater discharge (SGD) has been shown to be an important source of nutrients in coastal environments, especially nitrogen and silica, and thereby relive nutrient limitation to phytoplankton. Here, we followed autotrophic microbial biomass, activity, and community composition at a site strongly influenced by SGD and a nearby nutrients-poor reference site at the oligotrophic Israeli shallow rocky coast [southeastern Mediterranean Sea (SEMS)] between 2011 and 2019. The surface water at the SGD-affected area had significantly higher NO3 + NO2 (∼10-fold) and Si(OH)4 (∼2-fold) levels compared to the reference site, while no significant differences were observed for PO4 or NH4. This resulted in a significant increase in algae biomass (∼3.5-fold), which was attributed to elevated Synechococcus (∼3.5-fold) and picoeukaryotes (∼2-fold) at the SGD-affected site, and in elevated primary production rates (∼2.5-fold). Contrary to most SGD-affected coastal areas, diatoms biomass remained unchanged between sites, despite the elevated N and Si, suggesting the dominance of picophytoplankton over microphytoplankton at the SEMS. DNA sequencing of the 16S and 18S rDNA supported these findings. These results highlight the influence of SGD on shallow-water microbial populations. Our observations are consistent with recent studies showing that phytoplankton along the Israeli coast are likely nitrogen + silica limited, and may have important ecological and regulatory implications for environmental policy and management of coastal aquifers.

Highlights

The southeastern Mediterranean Sea (SEMS) is an impoverished environment (Siokou-Frangou et al, 2010; Berman-Frank and Rahav, 2012)
A previous study demonstrated that the yearly N, P, and Si inputs from submarine groundwater discharge (SGD) to the Mediterranean Sea is comparable to riverine and atmospheric inputs combined (Rodellas et al, 2015), suggesting this process may have a significant impact on primary production in coastal waters
The highest water temperature was measured in the fall (∼27◦C, October 2015), whereas the lowest temperature was measured at the end of winter (∼18◦C, March 2015 and March 2016) (Table 1)

Summary

Introduction

The southeastern Mediterranean Sea (SEMS) is an impoverished environment (Siokou-Frangou et al, 2010; Berman-Frank and Rahav, 2012). Nutrient enrichment of coastal waters occurs through inputs from estuaries (Herut et al, 2000b; Kress and Herut, 2001) and other coastal streams (Krom et al, 2014), sporadic terrestrial runoff (Rahav and Bar-Zeev, 2017), aerosol deposition (Rahav et al, 2016b, 2018a), and other anthropogenic inputs (Belkin et al, 2017; Frank et al, 2019; Raveh et al, 2019), as well as submarine groundwater discharge (SGD) (Garcés et al, 2011; Weinstein et al, 2011; Rodellas et al, 2015) Among these external nutrient inputs, the effect of SGD on phytoplankton biomass/activity has been far less studied in Israel coastal waters (SEMS), despite being a potentially important source of nutrients (Weinstein et al, 2011). A previous study demonstrated that the yearly N, P, and Si inputs from SGD to the Mediterranean Sea is comparable to riverine and atmospheric inputs combined (Rodellas et al, 2015), suggesting this process may have a significant impact on primary production in coastal waters

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