We investigated seawater microbial abundance, activity and diversity throughthree laboratory-controlled bottle incubations mimicking different mixing scenarios between SGD (either ambient or filtered through 0.1 µm/0.22 µm) and seawater to determine the contribution of SGD to the coastal microbial community. The experiments were conducted with five different treatments (including ambient seawater not exposed to SGD) in triplicates. The first experiment (Exp. 1) was designed to test the relative contribution of brackish discharged groundwater (salinity = 7.9 ppt vs. the ambient salinity of the SEMS of ~39.5 ppt) on the microbial productivity and abundance of reference coastal seawater by mixing different ratios (1, 5, 10 and 20% v:v) of discharged groundwater. Discharged groundwater was collected into acid-cleaned containers on the day the experiment was initiated near Achziv Nature Reserve (33° 3′52 N, 35° 6′14.94 E). The second and third experiments (Exp. 2; Exp.3) were designed to extend Exp. 1 and aimed to specifically investigate how groundwater-derived microorganisms affect the activity and abundance of marine organisms once discharged into the sea. For these experiments, fresh groundwater (FGW) was collected from drilling wells and pumped into 20 L acid-cleaned sample-rinsed carboys the same day the experiment was initiated. At the laboratory, fresh groundwater was either filtered through a 0.1 μm polycarbonate filter (Exp. 2) or serially filtered through 0.22 and 0.1 μm polycarbonate filter (Exp. 3) and the filtrate was added to seawater in different mixing scenarios. Ambient coastal seawater was collected by pumping at the Israel Oceanographic and Limnological Research Institute (IOLR) into acid-cleaned carboys, and mixed with either brackish groundwater (Exp.1) or fresh groundwater (Exp. 2, Exp. 3) at the desired ratios and filtration size. The duration of the experiments was 3-5 days, and samples were taken for the following analyses: chlorophyll a (Exp. 1 & 2, every 24Hr.), dissolved nutrient concentrations (Exp. 2 & 3 T zero and T final), flow cytometry (bacterial and phytoplankton abundance, every 24Hr.), primary and heterotrophic production rates (Exp. 1 & 2, every 24Hr.; Exp. 3 T zero and T final). Currently, little is known about the interactions between groundwater-borne and coastal seawater microbial populations, and groundwater microbes' role upon introduction to coastal seawater populations. Here, we investigated seawater microbial abundance, activity and diversity through laboratory-controlled bottle incubations mimicking different mixing scenarios between SGD (either ambient or filtered through 0.1 µm/0.22 µm) and seawater.
