Phytoplankton and Bacterial Response to Desert Dust Deposition in the Coastal Waters of the Southeastern Mediterranean Sea: A Four-Year In Situ Survey

Abstract

Atmospheric dust/aerosol deposition is an important source of external nutrients for the surface of the ocean. This study shows high-resolution observational data gathered in situ over a period of four years on bacterial and phytoplankton abundance and activity during typical background atmospheric conditions and during intense dust storm events in the low-nutrient, low-chlorophyll (LNLC) coastal waters of the southeastern Mediterranean Sea (SEMS). Chlorophyll a (an estimate for phytoplankton biomass) and bacterial abundance show moderate changes in response to dust deposition/events (−10% and +20%, respectively), while primary production, bacterial production, and N2 fixation rates were all significantly and positively affected by deposition (+25 to +40%; p < 0.05). The rapid changes in bacterial and/or phytoplankton rate parameters suggest that the released micro-/macronutrients from atmospheric deposition are tunneled directly in metabolic processes and, to a lesser extent, for biomass accumulation. The predicted expansion of LNLC areas in oceans in the future, and the projected increase in dust emission due to desertification, may affect the production of marine microbial communities in the surface of the ocean, yet only moderately affect their biomass or standing stock. Such alterations may impact carbon sequestration to the deep ocean.

Direct Detection of Heterotrophic Diazotrophs Associated with Planktonic Aggregates

“…In addition, heterotrophic N 2 fixation rates were 0.96 nmol N L −1 d −1 following dark incubation with DCMU which have likely impaired the activity of phototrophs 18,34 . These N 2 fixation rates were ~2–3 fold higher than previous reports from the southeastern Mediterranean Sea, despite the high concentrations of ambient TN 58,77–79 that should suppress diazotrophy 13 .…”

Heterotrophic Nitrogen Fixation at the Hyper-Eutrophic Qishon River and Estuary System

“…Despite the unfavorable chemical conditions that prevailed along the river, heterotrophic N 2 fixation rates (up to 6.9 nmol N L −1 d −1 , Figure 2A) were several folds higher than the typical values reported in the nearby oligotrophic southeastern Mediterranean Sea (usually <0.05 nmol N L −1 d −1 , Raveh et al, 2015;Rahav et al, 2016Rahav et al, , 2018aRahav and Bar-Zeev, 2017). Corresponding heterotrophic N 2 fixation rates were seven fold higher at the hyper-eutrophic stream (median ∼3.30 nmol N L −1 d −1 ) compared to the estuary (median ∼0.45 nmol N L −1 d −1 ) (t-test, P = 0.01, Figure 2A).…”

The effects of decomposing invasive jellyfish on biogeochemical fluxes and microbial dynamics in an ultra-oligotrophic sea

“…Using mesocosm experiments, West et al (2009) found that excretion of jellyfish C. mosaicus led to a 10-fold increase in diatom abundance. In the EMS, R. nomadica typically peaks in the summer months and collapses at the end of July (Edelist et al, 2020), whereas peak chlorophyll a concentrations in the water column are measured during wintertime (Ignatiades et al, 2009;Rahav et al, 2018a. This may result from the competitive exclusion of phytoplankton by heterotrophic bacteria (Sisma-Ventura and Rahav, 2019).…”