Impact of nutrient enrichment on productivity of coastal water along the SE Mediterranean shore of Israel - A bioassay approach

Nutrient concentration, loads and retention in a semiarid micro-estuary: The relative contribution of baseflow and flood events

Estuaries are a significant source of nutrients to the marine environment. The magnitude of this source is a function of nutrients load reaching the estuary and removal (attenuation) within estuaries. Most estuarine research is conducted in large estuaries, which do not reflect the processes in small estuaries in urban and semi-arid regions where flood water is a substantial portion of the annual discharge and the estuarine baseflow is often low and dominated by wastewater. To improve the understanding of nutrient attenuation and load into the Mediterranean, we conducted high-resolution nutrient sampling in the eutrophic Alexander micro-estuary as a test case. We sampled once per month during baseflows (years 2014-2019) and hourly during floods (years 2016-2018). The concentrations of inorganic nutrients (phosphorous (P) and nitrogen (N)) were extremely high during baseflows. Dissolved ammonium and particulate P were the only nutrients that were in the estuary (by 55 % and 30 %, respectively). Floods were rare, occurring ~4 % of the time, but contributed 62 % of the annual water discharge of the Alexander micro-estuary (14.7 ± 3.8 106 m3 y-1). The concentration of all dissolved nutrients decreased during floods but was higher than expected (DIN 584 ± 50 μmol L-1, phosphate 21 ± 2 μmol L-1), accounting for 42 % and 55 % of the overall annual DIN (123.5 ± 44.9-ton yr-1) and P (6.7 ± 1.9 ton yr-1) loads to sea, respectively. The N:P ratios were 16 and 34 during baseflow and flood events, respectively. Previously, nutrient loads were calculated by multiplying baseflow-measured concentrations by the total water volume of baseflow and floods. Our calculations, based on high-resolution sampling, revealed lower annual loads of P and N to the sea that were 56 % and 89 % of previous estimates, which is a considerable difference in an oligotrophic system such as the eastern Mediterranean.

Far-field effects of the Nile damming on the silica cycle in the Southeastern Mediterranean Sea

Silica plays a key role in the growth of silicifying primary producers (e.g., diatoms) and hence the ocean carbon pump. The Mediterranean Sea's eastern Levantine Basin (ELB) is a low silica (and low N and P) ultra-oligotrophic basin. Before 1965, Nile autumn floods were a major source of dissolved silica (DSi) and other nutrients to primary producers of the ELB continental shelf, also known as the Nilotic cell. The construction of the Aswan High Dam (AHD) in the mid-1960s, blocked these floods, drastically diminishing the autumn-diatom blooms offshore the Nile delta. However, the far-reaching and long-lasting effects of the Nile damming on the Si cycle in the ELB remain unclear. Here, we studied the changes in DSi in the surface water offshore Israel and the distribution of biogenic silica in deep-sea short sediment cores, collected hundreds of kilometers from the Nile outlet, at depths range of 1100-1900 m, offshore the ELB Israeli coast. We show post dam reduction and termination in flood related seasonality of DSi and a concurrent decrease (of up to 79 %) in biogenic silica (BSi) accumulation rates in surficial sediments relative to underlying sediments. These changes reflect the effects of Si (dissolved and particulate) retention by the AHD on diatoms production, export and burial in the ELB. This far-field effect was demonstrated in deep-sea areas subjected to intense lateral transport of resuspended sediments from the shelf via intermediate nepheloid layers and to coastal water intrusions, along the path of the pre-dam, flood plumes. Our core records show that the AHD worsened nutrient-diminished, exceptionally unfavorable conditions for diatoms that persisted in the deep ELB at least during the last four millennia.

Tar patties are hotspots of hydrocarbon turnover and nitrogen fixation during a nearshore pollution event in the oligotrophic southeastern Mediterranean Sea

Weathered oil, that is, tar, forms hotspots of hydrocarbon degradation by complex biota in marine environment. Here, we used marker gene sequencing and metagenomics to characterize the communities of bacteria, archaea and eukaryotes that colonized tar patties and control samples (wood, plastic), collected in the littoral following an offshore spill in the warm, oligotrophic southeastern Mediterranean Sea (SEMS). We show potential aerobic and anaerobic hydrocarbon catabolism niches on tar interior and exterior, linking carbon, sulfur and nitrogen cycles. Alongside aromatics and larger alkanes, short-chain alkanes appear to fuel dominant populations, both the aerobic clade UBA5335 (Macondimonas), anaerobic Syntropharchaeales, and facultative Mycobacteriales. Most key organisms, including the hydrocarbon degraders and cyanobacteria, have the potential to fix dinitrogen, potentially alleviating the nitrogen limitation of hydrocarbon degradation in the SEMS. We highlight the complexity of these tar-associated communities, where bacteria, archaea and eukaryotes co-exist, likely exchanging metabolites and competing for resources and space.

Jellyfish swarm impair the pretreatment efficiency and membrane performance of seawater reverse osmosis desalination

Circumstantial evidence has suggested that jellyfish swarms impair the operation of seawater reverse osmosis desalination facilities. However, only limited information is currently available on the pretreatment efficiency of jellyfish and their effects on reverse osmosis (RO) membrane performance. Here, we have comprehensively tested the pretreatment efficiency of a dual-media gravity filter and cartridge micro-filtration following the addition of jellyfish into the feedwater. Concurrently, the fouling propensity and performance of the RO membranes were examined. We show that jellyfish demise resulted in seawater eutrophication that triggered a significant increase in bacterial biomass (∼50-fold), activity (∼7-fold), and release of transparent exopolymer particles (∼5-fold), peaking three days after the addition of jellyfish into the feedwater. In parallel, a significant reduction in permeate water flux was recorded (∼10%) while trans-membrane pressure sharply increased (15%), reaching the operation pressure limit of our system (75 bar) after five days. At the conclusion of the experiments, the membrane surface was heavily covered by large chunks of organic-rich material and multilayered biofilms. Our results provide a holistic view on the operational challenges of seawater reverse osmosis (SWRO) desalination triggered by jellyfish swarms in coastal areas. Following the above, it can be inferred that freshwater production will likely be halted three days after drawing the jellyfish into the pretreatment system. Outcomes from these results may lead to the development of science-based operational protocols to cope with growing occurrence of jellyfish swarms around the intake of SWRO desalination facilities worldwide.

Colimitation assessment of phytoplankton growth using a resource use efficiency approach in the Bay of Seine (French-English Channel)

Eutrophication and dystrophy are two of the main problems affecting coastal ecosystems. In the Bay of Seine, phosphorus (P) inputs from the Seine estuary have been largely reduced in the last decade, in contrast to nitrogen (N), which leads to high N/P ratio inputs. To study the effect of dystrophy, an enrichment bioassay using water sampled from the Bay of Seine was repeated 19 times over a period of 18 months with six different enrichments. After a few days, chlorophyll a (chl a), alkaline phosphatase activity (APA), transparent exopolymeric particles (TEPs), cytometric size structure, and maximum quantum yield of photosystem II were measured. The data provide strong evidence for an N & P colimitation system in the vast majority of the incubations, as only the N + P and N + P + Si enrichments supported phytoplankton growth, and Si only appeared to play a secondary role in our incubations. A N/P ratio of 16 equal to the Redfield ratio was identified as the optimum for balanced growth, as chl a was the highest and TEP and APA production was the lowest at this ratio. To fit the requirements of the colimited system, a new resource use efficiency (RUE_NP) calculation was developed to account for N and P colimitation instead of only one nutrient, as is usually the case. This calculation allows better representation of RUE in dystrophic conditions, as found in many highly anthropized ecosystems. The relationships between RUE_NP and the size structure of the phytoplankton community were explored, and a significant positive correlation between RUE_NP and larger cells (>2 μm) and a negative correlation with smaller cells (<2 μm) were noted, showing a better use of nutrients by larger cells. This study highlights an increase of RUE_NP with the phytoplankton cell size in a colimited system.

Decadal acidification in a subtropical coastal area under chronic eutrophication

Coastal acidification is often much more intense than ocean acidification due to eutrophication. To better understand the relationship between long-term coastal acidification (CA) and coastal eutrophication (CE), in-situ monthly data over the past three decades (1986-2017) were analyzed from Hong Kong Coast (HKC). The coastwide annual mean pH change (ΔpH_mean) was estimated at -0.0085 ± 0.0069 unit·yr^-1 in last decades, which was over four times stronger than current estimation on open ocean acidification rate (∼-0.0019 unit·yr^-1). According to the CA spatial pattern, greater pH decline (ΔpH_mean = -0.017 ± 0.009 unit·yr^-1) occurred in northwest, central south and central east HKC areas, much higher than the less acidified (ΔpH_mean = -0.004 ± 0.002 unit·yr^-1) southwest and northeast HKC areas. The spatiotemporal CA variations were associated with water discharges, atmospheric CO₂ increase and respiration/production that was indicated by DIN:DIP structure changes. The annual mean DIN:DIP ratio increased progressively from initial ∼16 in 1986 to ∼37 in 2017, revealing excess nitrogen load from rapid urbanization in this region. Such discharge-induced acidification was estimated as the major contributor for the total CA in HKC over the last three decades. In addition, our simulation results indicated that a potential CA rate at ∼0.0035 unit·yr^-1 could be reached if reducing mean DIN:DIP from discharged water to ∼23 from HKC. This study revealed a previously not recognized relationship between coastal acidification and changing coastal nutrient stoichiometry, and proposed possible management approaches.

OUP accepted manuscript

Nutrient pollution is linked to coral disease susceptibility and severity, but the mechanism behind this effect remains underexplored. A recently identified bacterial species, ‘Ca. Aquarickettsia rohweri,’ is hypothesized to parasitize the Caribbean staghorn coral, Acropora cervicornis, leading to reduced coral growth and increased disease susceptibility. Aquarickettsia rohweri is hypothesized to assimilate host metabolites and ATP and was previously demonstrated to be highly nutrient-responsive. As nutrient enrichment is a pervasive issue in the Caribbean, this study examined the effects of common nutrient pollutants (nitrate, ammonium, and phosphate) on a disease-susceptible genotype of A. cervicornis. Microbial diversity was found to decline over the course of the experiment in phosphate-, nitrate-, and combined-treated samples, and quantitative PCR indicated that Aquarickettsia abundance increased significantly across all treatments. Only treatments amended with phosphate, however, exhibited a significant shift in Aquarickettsia abundance relative to other taxa. Furthermore, corals exposed to phosphate had significantly lower linear extension than untreated or nitrate-treated corals after 3 weeks of nutrient exposure. Together these data suggest that while experimental tank conditions, with an elevated nutrient regime associated with coastal waters, increased total bacterial abundance, only the addition of phosphate significantly altered the ratios of Aquarickettsia compared to other members of the microbiome.

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

Planktonic heterotrophic diazotrophs (N₂-fixers) are widely distributed in marine and freshwater systems, yet limited information is available on their activity, especially in environments with adverse conditions for diazotrophy (e.g., N-rich and oxygenated). Here, we followed the localization and activity of heterotrophic diazotrophs in the hyper-eutrophic N-rich Qishon River—an environment previously considered to be unfavorable for diazotrophy. Our results indicate high heterotrophic N₂ fixation rates (up to 6.9 nmol N L^–1 d^–1), which were approximately three fold higher at an upstream location (freshwater) compared to an estuary (brackish) site. Further, active heterotrophic diazotrophs were capture associated with free-floating aggregates by a newly developed immunolocalization approach. These findings provide new insights on the activity of heterotrophic diazotrophs on aggregates in environments previously considered with adverse conditions for diazotrophy. Moreover, these new insights may be applicable to other aquatic regimes worldwide with similar N-rich/oxygenated conditions that should potentially inhibit N₂ fixation.

Seawater quality at the brine discharge site from two mega size seawater reverse osmosis desalination plants in Israel (Eastern Mediterranean)

Two mega-size seawater desalination plants, producing 240 Mm³/y freshwater, discharge brine into the Mediterranean coast of Israel through two marine outfalls, located 0.8 km apart. Six years monitoring brine discharge have shown almost no impact on seawater quality. The brine dispersed near the bottom following its initial mixing, and was not detected near the surface. Maximal excess salinity at the salty layer ranged from 4.3 to 9.1% over the reference and the affected area was highly variable (2 km² - >13 km²), with maximal plume size from 1.75 to more than 4.4 km. Brine increased seawater temperature by up to 0.7 °C near the outfalls. It had no impact on oxygen saturation, turbidity, pH, nutrients (except for total organic phosphorus (TOP)), chlorophyll-a and metal concentrations. TOP, from the polyphosphonate-based antiscalant discharged with the brine, was correlated with excess salinity. It is unknown if the results of this short term study represent a steady state, with temporal variability, or the beginning of a slow incremental impact. Israel is planning to more than double desalination along its 190 km Mediterranean coast by 2050. A long term, adaptable, program, in conjunction with specific research and modeling, should be able to assess and predict the impact of large scale brine discharge on the marine environment.

DOP Stimulates Heterotrophic Bacterial Production in the Oligotrophic Southeastern Mediterranean Coastal Waters

Phytoplankton and heterotrophic bacteria rely on a suite of inorganic and organic macronutrients to satisfy their cellular needs. Here, we explored the effect of dissolved inorganic phosphate (PO₄) and several dissolved organic molecules containing phosphorus [ATP, glucose-6-phosphate, 2-aminoethylphosphonic acid, collectively referred to as dissolved organic phosphorus (DOP)], on the activity and biomass of autotrophic and heterotrophic microbial populations in the coastal water of the southeastern Mediterranean Sea (SEMS) during summertime. To this end, surface waters were supplemented with PO₄, one of the different organic molecules, or PO₄ + ATP, and measured the PO₄ turnover time (Tt), alkaline phosphatase activity (APA), heterotrophic bacterial production (BP), primary production (PP), and the abundance of the different microbial components. Our results show that PO₄ alone does not stimulate any significant change in most of the autotrophic or heterotrophic bacterial variables tested. ATP addition (alone or with PO₄) triggers the strongest increase in primary and bacterial productivity or biomass. Heterotrophic bacterial abundance and BP respond faster than phytoplankton (24 h post addition) to the various additions of DOP or PO₄ + ATP, followed by a recovery of primary productivity (48 h post addition). These observations suggest that both autotrophic and heterotrophic microbial communities compete for labile organic molecules containing P, such as ATP, to satisfy their cellular needs. It also suggests that SEMS coastal water heterotrophic bacteria are likely C and P co-limited.

Phytoplankton response to N-rich well amelioration brines: A mesocosm study from the southeastern Mediterranean Sea

Human-induced eutrophication of coastal water may be a major threat to aquatic life. Here, we investigated the effects of N-rich well amelioration brines (WAB) on coastal phytoplankton population's habitat in the surface oligotrophic waters of the southeastern Mediterranean Sea (SEM). To this end, we added WAB (2 concentrations) to mesocosms (1-m³ bags) to surface SEM water during summer and winter, where changes in phytoplankton biomass, activity and diversity was monitored daily for 8 days. Our results demonstrate that WAB addition triggered a phytoplankton bloom, resulting in elevated algal biomass (maximal +780%), increased primary production rates (maximal +675%) and a decrease in eukaryotic algal α-diversity (ca. -20%). Among the species that bloomed following WAB amendments, we found the potentially toxic dinoflagellate Karlodinium venificum. This study adds valuable perspective to the effect of nutrients discharged into nutrient limited SEM coastal waters, and in particular of N-derived WAB.

Direct Detection of Heterotrophic Diazotrophs Associated with Planktonic Aggregates

N₂ fixation by planktonic heterotrophic diazotrophs is more wide spread than previously thought, including environments considered “unfavorable” for diazotrophy. These environments include a substantial fraction of the aquatic biosphere such as eutrophic estuaries with high ambient nitrogen concentrations and oxidized aphotic water. Different studies suggested that heterotrophic diazotrophs associated with aggregates may promote N₂ fixation in such environments. However, this association was never validated directly and relies mainly on indirect relationships and different statistical approaches. Here, we identified, for the first time, a direct link between active heterotrophic diazotrophs and aggregates that comprise polysaccharides. Our new staining method combines fluorescent tagging of active diazotrophs by nitrogenase-immunolabeling, polysaccharides staining by Alcian blue or concanavalin-A, and total bacteria via nucleic-acid staining. Concomitant to N₂ fixation rates and bacterial activity, this new method provided specific localization of heterotrophic diazotrophs on artificial and natural aggregates. We postulate that the insights gained by this new visualization approach will have a broad significance for future research on the aquatic nitrogen cycle, including environments in which diazotrophy has traditionally been overlooked.

The Relationship between Air-Mass Trajectories and the Abundance of Dust-Borne Prokaryotes at the SE Mediterranean Sea

Airborne prokaryotes are transported along with dust/aerosols, yet very little attention is given to their temporal variability above the oceans and the factors that govern their abundance. We analyzed the abundance of autotrophic (cyanobacteria) and heterotopic airborne microbes in 34 sampling events between 2015–2018 at a coastal site in the SE Mediterranean Sea. We show that airborne autotrophic (0.2–7.6 cells × 103 m−3) and heterotrophic (0.2–30.6 cells × 103 m−3) abundances were affected by the origin and air mass trajectory, and the concentration of dust/aerosols in the air, while seasonality was not coherent. The averaged ratio between heterotrophic and autotrophic prokaryotes in marine-dominated trajectories was ~1.7 ± 0.6, significantly lower than for terrestrial routes (6.8 ± 6.1). Airborne prokaryotic abundances were linearly and positively correlated to the concentrations of total aerosol, while negatively correlated with the aerosol’s anthropogenic fraction (using Pb/Al or Cu/Al ratios as proxies). While aerosols may play a major role in dispersing terrestrial and marine airborne microbes in the SE Mediterranean Sea, the mechanisms involved in the dispersal and diversity of airborne microorganisms remain to be studied and should include standardization in collection and analysis protocols.

Environmental status of Israel's Mediterranean coastal waters: Setting reference conditions and thresholds for nutrients, chlorophyll-a and suspended particulate matter

Criteria for eutrophication related parameters to achieve and preserve good environmental status (GES) of the oligotrophic Israeli Mediterranean coast were proposed for nutrients, chlorophyll-a (Chl-a) and suspended particulate matter (SPM) concentrations. The criteria were derived from current conditions, the best choice for the area that has undergone large and irreversible ecological changes compared to the pristine background. A five-year data set (2010-2014, ca. 800 data points) was analyzed using statistical methods and best professional judgement. The coastal waters were divided into four provinces, data gaps were identified, and seasonal reference and threshold values for each province determined as the median and 1.5 times the median, respectively. Application of the derived criteria to data up to 2016 showed the coastal waters to be mainly in GES, with a few exceptions. Simplification of the proposed criteria for environmental management was addressed as well.

Chronic effects of brine discharge form large-scale seawater reverse osmosis desalination facilities on benthic bacteria

Seawater desalination facilities continuously discharge hyper-saline brine into the coastal environment which often flows as a concentrated plume over the seafloor, hence possibly impacting benthic microorganisms. Yet, the effects of brine discharge from desalination plants on benthic bacteria, key players in biodegradation of organic material and nutrient recycling is unknown. In this study, we tested the chronic (years) effects of brine discharge from three large-scale desalination facilities on the abundance, metabolic activity and community composition of benthic bacteria. To this end, four sampling campaigns were carried at the outfall areas of the Ashkelon, Sorek and Hadera desalination facilities. The effects of the brine were compared to corresponding reference stations which were not influenced by the brine (i.e., water temperature and salinity). Our sampling data indicate that bacterial abundance and activity that includes bacterial growth efficiency were 1.3-2.6-fold higher at the outfall area than the reference station. Concomitant analysis pointed out that the bacterial community structure at the brine discharge area was also different than the reference station, yet varied between each desalination facility. Our results demonstrate that the impact of brine effluent from desalination facilities on benthic bacteria are site-specific and localized (<1.4 Km²) around the discharge point. Namely, that the effects on benthic bacteria are prominent at the brine mixing zone and change according to the discharge method used to disperse the brine as well as local stressors (e.g., eutrophication and elevated water temperature). Our results contribute new insights on the effects of desalination-brine to benthic microbes, while providing scientifically-based aspects on the ecological impacts of brine dispersion for decision makers.

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

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.