Transition from Planktonic to Benthic Algal Dominance Along a Salinity Gradient

Highly regulated salinity gradients in solar salt pond concentrating sequences provide an opportunity to investigate in situ salinity impacts on aquatic flora and fauna. The Shark Bay Salt solar ponds at Useless Inlet in Western Australia vary in salinity from seawater to four times seawater over the pond sequence. We observed a shift from planktonic to benthic primary productivity as salinity increased. Water column photosynthesis and biomass decreased markedly with increasing salinity, while benthic productivity increased as cyanobacterial mats developed. Correspondingly, productivity shifted from autotrophy to heterotrophy in the water column and from heterotrophy to autotrophy in the benthos. Both shifts occurred at intermediate salinity (S = 110 g kg⁻¹, ρ = 1.087 g cm⁻³) in the pond sequence, where there was little production by either. Within individual ponds, productivity, algal biomass and physico-chemical conditions were relatively constant over one year, with only water column photosynthesis significantly different between seasons, mostly due to greater winter production. Transitions between benthic and planktonic production and their relative magnitudes appear to be driven mostly by direct responses to salinity stress, but also by changes in nutrient availability and grazing, which are also influenced by salinity.     

Microbial mats and physicochemistry in a saltern in the Bretagne (France) and in a laboratory scale saltern model

Abstract A saltern near La Baule (Bretagne, France) was remodeled in a programmable temperature and humidity controlled walk-in environmental chamber resembling the characteristics of the original saltern. The saltern showed different types of microbial mats predominantly composed of algae, oxy- and anoxyphotobacteria, and associated chemoorganotrophic bacteria, fungi and animals. Well-developed microbial mats were found up to a salinity of 10% during the three or four months in summer when salinity gradients and NaCl precipitation were established. The main phototrophic organisms were diatoms, the cyanobacteria Aphanothece, Microcoleus, Spirulina , and Oscillatoria , and Chromatiaceae. At higher salinity, Halobacterium sp., diatoms, and Dunaliella were dominant. Typical microbial mats and saltern-typical invertebrate, algal and bacterial species also developed in the saltern model, building up a stable community. The ionic composition of the brines and physicochemical parameters were similar to those determined for the original saltern. Different photosynthetic organisms, e.g. a filamentous purple bacterium and a hypersaline Chloroflexus -like organism, could be enriched within the microbial mats by changing the light regime.     

Physico-chemical characteristics of a permanent Spanish hypersaline lake: La Salada de Chiprana (NE Spain)

La Salada de Chiprana Lake, located in the Ebro River basin, northeastern Spain, is the only permanent and deep water hypersaline ecosystem in all of western Europe. With a total surface of 31 ha and a maximum depth of 5.6 m, it has several basins bounded by elongated sandstone-bodies or ribbons which are paleochannels of Miocene age. Its salinity varied from 30 to 73 g 1^–1 during the 1989 hydrological cycle and the most abundant ions were magnesium and sulphate. Depth-time distributions of major physico-chemical variables demonstrated that the lake was stratified in two distinctive layers during most of the year. The chemocline disappeared only in October, with the complete overturn of the water column. In the deep water, three conditions occurred which allowed development of green sulphur bacteria populations: (1) oxygen depletion, (2) presence of hydrogen sulphide and (3) presence of light. Benthic microbial mats covered the sediments of shallow shores of moderate slope.     

GROWTH AND ORGANIC OSMOLYTES OF GEOGRAPHICALLY DIFFERENT ISOLATES OF MICROCOLEUS CHTHONOPLASTES (CYANOBACTERIA) FROM BENTHIC MICROBIAL MATS:RESPONSE TO SALINITY CHANGE1

The comparative growth and osmotic acclimation often culture strains of the marine benthic cyanobacterium Microcoleus chthonoplastes Thuret isolated from microbial mats in Germany, Spain, Egypt, the United States, Mexico, Chile, and Australia were investigated in salinities ranging from freshwater to twice seawater. All isolates showed a broad growth versus salinity response consistent with the dominance of this species in intertidal and hypersaline microbial communities. Growth optima, salinity preferences, and maximum growth rates differed for each isolate and could be related to the habitat from which they were isolated. This is most obvious when comparing strains from brackish habitats with those from a hypersaline lake. While the former isolates exhibited sharply pronounced growth optima under hyposaline conditions, cultures from the hypersaline environment grew best in salinity more than double seawater. The major low-molecular weight organic compounds present in all M. chthonoplastes strains were the carbohydrates glycosylglycerol and trehalose. This was proven by using ¹³C-nuclear magnetic resonance spectroscopy. Glycosylglycerol was synthesized and accumulated with increasing salinities, indicating its role as an osmolyte. In contrast, trehalose was present in relatively high concentrations under hyposaline conditions only. Differences in the patterns of growth versus salinity, as well as in those of osmotic acclimation among the M. chthonoplastes isolates, point to the development of different physiological ecotypes within the species.     

Microphytobenthic Primary Production and Sedimentary Carbohydrates Along Salinity Gradients in the Lagoons of Grado and Marano (Northern Adriatic Sea)

Primary production of the microphytobenthic community and carbohydrates concentrations were studied in the lagoonal system of Grado and Marano, located in the Northern Adriatic coast. Sediment samples were collected along a salinity gradient. Abundance and species composition of the microphytobenthic communities were analysed and the benthic microalgal biomass was estimated as Chlorophyll a (Chl a). Primary production of benthic diatoms was estimated using ¹⁴C-tracer. Extracellular carbohydrates were extracted from the sediment and separated in two operationally defined fractions (colloidal and EDTA-extractable). Salinity was higher in the Grado lagoon, where the benthic microalgal community was mainly composed of marine diatoms. In the Marano lagoon, which has a lower salinity, freshwater species were also found. In both lagoons, photosynthetic efficiency showed an inverse relationship with salinity and a direct relationship with the main biological variables. Photosynthetic activity was directly related to Chl a and abundance of benthic microalgae, suggesting that in the benthic system microalgal community is responsible for primary production. Overall, salinity was also influent on the microphytobenthic primary production, which was greater in the more saline Grado lagoon.     

Development of ice biota in a temperate sea area (Gulf of Bothnia)

A study of sea ice biota was carried out in the Gulf of Bothnia (northern Baltic Sea) during the winter of 1989–1990. Samples (ice cores) were taken at a coastal station at regular time intervals during the ice season. Chlorophyll a concentration, algal species distribution, bacterial numbers, and primary and bacterial production were measured. Colonization of the ice began in January when daylight was low. As the available light increased, the algae started to grow exponentially. The vertical chlorophyll a distribution changed and algal species composition and biomass changed during the season. During the initial and middle phase of colonization, ice-specific diatoms, Nitzschia frigida and Navicula pelagica, dominated the algal biomass. Nutrients (PO₄ ^3– and NO_3j) were found to be depleted during the time of algal exponential growth. The maximum algal biomass exceeded 800 g C 1^–1. The primary production supplied food for heterotrophic organisms. The presence of heterotrophic organisms of different trophic levels (bacteria, flagellates, ciliates and rotifers) indicated an active microbial food web.     

A recently landlocked brackish-water lagoon of Lake Tanganyika: physical and chemical characteristics,and spatio-temporal distribution of phytoplankton

Monthly measurements of physical and chemical characteristics were made at two localities in the eastern part of a recently landlocked lagoon of Lake Tanganyika. Variables analysed were: temperature, pH, conductivity, sodium, potassium, magnesium, calcium, carbonate, bicarbonate, chloride and sulphate. Large seasonal fluctuations of salinity were recorded (1.68–8.21 g l^–1). The seasonal water input controlled algal seasonality mainly through its effect on salinity and indirectly through its influence on nutrient concentration by dissolution and dilution of the excrements of the numerous cattle and other organic matter. Phytoplankton was mainly composed of Cyanophyta and Euglenophyta. Euglenophyta dominated during the dry periods with high salinity and probably very high nutrient levels, while Cyanophyta preferred moderate salt and nutrient concentrations during the rainy periods. The phytoplanktonic community was composed of a large number of perennial and a reduced quantity of annual organisms.A spatial study of the recently landlocked lagoon revealed an ascending salinity gradient, principally due to a sodium bicarbonate/carbonate enrichment, between locations near the lake and more inland situated stations. These facts point to a lake water supply and a salt concentration by evaporation in the swamps. Proportionally lower magnesium, calcium and potassium values were recorded at high salinities, due to chemical precipitation and biotic factors.A blue-green algal bloom was observed in the eastern water-body (salinity: 4.64 g l^–1); simultaneously an important development of diatoms dominated the western water-body (salinity: 2.18 g l^–1). No significant differences in morphometry, exposure, water temperature or nutrient levels (nitrate, nitrite, ammonia, orthophosphate) were observed. The relatively low salinity and high nutrient concentration in the western water-body probably favoured diatom development during the rainy season. The relatively higher salinity in the eastern water-body during the rainy season was probably responsible for the dominance of blue-green algae through its negative influence on silica concentration and notwithstanding the high inorganic nitrogen concentration.     

The biology and evolution of Antarctic saline lakes in relation to salinity and trophy

. Five brackish to hypersaline lakes (Highway, Ace, Pendent, Williams and Rookery) in the Vestfold Hills, eastern Antarctica were investigated during the austral summer of 1999/2000. The aims were to characterise the functional dynamics of the plankton and gain an understanding of how the different environments in the lakes have led to the evolution of different communities. The plankton was dominated by microorganisms and differed across the salinity spectrum in relation to trophy, age and the presence of meromixis. However, some elements of the plankton were common to all of the lakes, e.g. the mixtrophic ciliate, Mesodinium rubrum, which reached abundances of 2.7᎒⁵ l^-1 and spanned a salinity gradient of 4-63‰. Marine dinoflagellate species also occurred in all of the lakes, often at high abundances in Highway Lake, Pendent Lake and Lake Williams. During December (mid-summer), primary production showed an increase along the salinity gradient from Highway Lake to Lake Williams; however, it was low in hyper-nutrified Rookery Lake because of the turbidity of the waters. Bacterial production followed the same trend and was extremely high in Rookery Lake (327 µg l^-1 h^-1 in January). The lakes possessed a marine microbial plankton that has become very simplified through time, and now contains a small number of highly successful species, which were pre-adapted to surviving in extreme Antarctic lakes.     

Dunaliella salina from saline environments of the central coast of Peru

An evaluation of the algal flora of the Salinas de Huacho on the coast of Peru between 1984 and 1990 was carried out by collection of algal mats and natural waters from a variety of habitats and by microscopic examination. D. salina occurred in lagoons and pools with a salinity range of 165\% to 350\% and formed planktonic and benthic communities. The benthic palmelloid stage of D. salina was found at higher salinities. Aplanospore formation was also observed. Associated halophilic species included D. viridis, Oscillatoria tenuis and Pleurocapsa entophysaloides. D. salina was also found at two other salinas on the central Peruvian coast (Chilca and Otuma).     

Food sources and lipid retention of zooplankton in subarctic ponds

1. Subarctic ponds are seasonal aquatic habitats subject to short summers but often have surprisingly numerous planktonic consumers relative to phytoplankton productivity. Because subarctic ponds have low pelagic productivity but a high biomass of benthic algae, we hypothesised that benthic mats provide a complementary and important food source for the zooplankton. To test this, we used a combination of fatty acid and stable isotope analyses to evaluate the nutritional content of benthic and pelagic food and their contributions to the diets of crustacean zooplankton in 10 Finnish subarctic ponds. 2. Benthic mats and seston differed significantly in total lipids, with seston (62.5 μg mg^?1) having approximately eight times higher total lipid concentrations than benthic mats (7.0 μg mg^?1). Moreover, the two potential food sources differed in their lipid quality, with benthic organic matter completely lacking some nutritionally important polyunsaturated fatty acids (PUFA), most notably docosahexaenoic acid and arachidonic acid. 3. Zooplankton had higher PUFA concentrations (27–67 μg mg^?1) than either of the food sources (mean benthic mats: 1.2 μg mg^?1; mean seston: 9.9 μg mg^?1), indicating that zooplankton metabolically regulate their accumulation of PUFA. In addition, when each pond was evaluated independently, the zooplankton was consistently more ¹³C‐depleted (δ¹³C ?20 to ?33‰) than seston (?23 to ?29‰) or benthic (?15 to ?27‰) food sources. In three ponds, a subset of the zooplankton (Eudiaptomus graciloides, Bosmina sp., Daphnia sp. and Branchinecta paludosa) showed evidence of feeding on both benthic and planktonic resources, whereas in most (seven out of 10) ponds the zooplankton appeared to feed primarily on plankton. 4. Our results indicate that pelagic primary production was consistently the principal food resource of most metazoans. While benthic mats were highly productive, they did not appear to be a major food source for zooplankton. The pond zooplankton, faced by strong seasonal food limitation, acquires particular dietary elements selectively.     

Effects of a larval mosquito biopesticide and Culex larvae on a freshwater nanophytoplankton (Selenastrum capricornatum) under axenic conditions

The effects of microbial biopesticides used for mosquito control on autotrophic microorganisms such as nanophytoplankton are equivocal. We examined impacts of mosquito biopesticides and mosquito larvae on primary producers in two independent experiments. In the first experiment, we examined the effects of a commonly used microbial biopesticide formulation (VectoMax^® CG) on a unicellular microalga, Selenastrum capricornatum Printz, under axenic laboratory conditions. The biopesticide treatments included two concentrations (0.008 and 0.016 g liter^?1) of VectoMax^® CG and two controls (one untreated and another with autoclaved 0.016 g VectoMax^® CG liter^?1) in replicated axenic experimental microcosms. Spectrophotometric analysis of chlorophyll a (proxy for algal biomass) and direct enumeration of algal cells following the treatments revealed no significant effects of the microbial biopesticide on algal population growth during the four‐week study. In the second experiment, we tested the effects of different densities of Culex larvae on the population of S. capricornatum. Effects of mosquito larvae feeding on S. capricornatum were significant with a curvilinear relationship between larval density and algal abundance in the water column. Together, these studies demonstrated a lack of direct cytological/toxicological effects of Bacillus‐based microbial pesticides on freshwater primary production and support the hypothesis that the reduction in algal primary production previously reported when Bti products were applied to aquatic environments was likely independent of the Bacillus‐based larvicidal toxins. Instead, it was likely mediated by microbial interactions in the water column and the trophic cascade effects that resulted from the removal of larval mosquitoes. These studies suggest that mosquito larvae independent of pesticide application can influence primary production. Our method of evaluating biopesticides against small photoautotrophs can be very useful for studying the unintended effects on autotrophic microorganisms of other pesticides, including herbicides and pesticides applied to aquatic environments.     

The dynamics of benthic microbial communities at Davies Reef,central Great Barrier Reef

The dynamics of benthic microbial communities were examined within different functional zones (reef crest, reef flat, lagoon) of Davies Reef, central Great Barrier Reef, in winter. Bacterial numbers did not change significantly across the reef with a mean abundance of 1.3 (±0.6) x 10⁹ cells g^-1 DW of sediment. Bacterial production, measured as thymidine incorporation into DNA, ranged from 1.2 (±0.2) to 11.6 (±1.5) mg C m^-2h^-1 across the reef and was significantly lower in a reef crest basin than in the other zones. Bacterial growth rates () across the reef (0.05 to 0.33 g^-1) correlated only with sediment organic carbon and nitrogen. Protozoan and meiofaunal densities varied by an order of magnitude across the reef and correlated with one or more sediment variables but not with bacterial numbers or growth rates. Nutrient flux rates were similar to those found at other reefs in the central and southern Great Barrier Reef and are significantly lower than rates measured in temperate sand communities. In the front lagoon, bioturbation and feeding acitivity by thalassinid shrimps (Callianassa spp.) negatively influenced microbial and meiofaunal communities with a net import of organic matter necessary to support the estimated rates of bacterial productivity. In lagoonal areas not colonized by shrimps, primary productivity (400–1100 mg C m^-2d^-1) from algal mats was sufficient to support bacterial growth. It is suggested that deposit-feeding macrobenthos such as thalassinid crustaceans play a major role in the tructuring and functioning of lower trophic groups (bacteria, microalgae, protozoa, meiofauna) in coral reef sedments, particularly in laggons.     

A case of unusually high oxygen demand in a eutrophic lake

An unusually high hypolimnetic water column BOD (WCBOD), roughly 40 times higher than the sediment oxygen demand (SOD), was observed in a small eutrophic lake and an adjoining lagoon. The mean 5-day WCBOD during thermal stratification in the lake was 29 and 49 g/m² at 10 and 20 °C, respectively, while in the lagoon it was even higher (47 and 87 g/m² at 10 and 20 °C, respectively). The soluble fraction comprised about two-thirds of the WCBOD. WCBOD in the lake was much less during the unstratified period (5-day = 5 g/m²). The SOD rates at two depths in both the lake (0.31 and 0.2 g/m²-d) and lagoon (0.41 and 0.28 g/m²-d) were not unusually high. The ultimate whole BOD (UWCBOD + USOD) was approximately 96 g/m² in the lake and 136 g/m² in the lagoon and UWCBOD formed over 90% of the ultimate whole BOD in both water bodies. A possible cause for these abnormally high WCBODs, in addition to the normal autochthonous production, is an allochthonous source from loosely aggregated and flocculant mats of the bog moss, Sphagnum, which surrounds the lake-lagoon system. Storm water per se was clearly insignificant, but would have contributed indirectly through nutrients for autochthonous production. Such high short-term BOD rates may greatly over-estimate the demand to be satisfied by continuous aeration.     

Shifts in methanogen community structure and function associated with long-term manipulation of sulfate and salinity in a hypersaline microbial mat

Methanogenesis was characterized in hypersaline microbial mats from Guerrero Negro, Baja California Sur, Mexico both in situ and after long-term manipulation in a greenhouse environment. Substrate addition experiments indicate methanogenesis to occur primarily through the catabolic demethylation of non-competitive substrates, under field conditions. However, evidence for the coexistence of other metabolic guilds of methanogens was obtained during a previous manipulation of sulfate concentrations. To fully characterize methanogenesis in these mats, in the absence of competition for reducing equivalents with sulfate-reducing microorganisms, we maintained microbial mats for longer than 1 year under conditions of lowered sulfate and salinity levels. The goal of this study was to assess whether observed differences in methane production during sulfate and salinity manipulation were accompanied by shifts in the composition of methanogen communities. Culture-independent techniques targeting methyl coenzyme M reductase genes ( mcrA ) were used to assess the dynamics of methanogen assemblages. Clone libraries from mats sampled in situ or maintained at field-like conditions in the greenhouse were exclusively composed of sequences related to methylotrophic members of the Methanosarcinales . Increases in pore water methane concentrations under conditions of low sulfate correlated with an observed increase in the abundance of putatively hydrogenotrophic mcrA , related to Methanomicrobiales . Geochemical and molecular data provide evidence of a significant shift in the metabolic pathway of methanogenesis from a methylotroph-dominated system in high-sulfate environments to a mixed community of methylotrophic and hydrogenotrophic methanogens under low sulfate conditions.     

Cryptic freshwater ciliates in a hypersaline lagoon

Ubiquitous dispersal of free-living microbial species implies that each and every ecosystem supports a ‘seedbank’ of microbial species that are imported by random dispersal. However, many of the microbial species present in any particular ecosystem will probably never thrive there because the local environment is unsuitable for their population growth. To test this, we investigated the ciliated protozoa in a hypersaline lagoon in Almería, Spain, using selective enrichment to reveal typical freshwater species, as the ‘signature’ of random dispersal. Twenty-four ciliate species, 14 of them not previously recorded from hypersaline waters, were identified in the undiluted waters of the lagoon. But when the salinity was gradually diluted, further species typical of fresh- and brackish waters emerged, indicating that they had persisted in a viable state at the previously high salinity. These additional species increased the recorded ciliate species total for the lagoon to 36. The species found in the lagoon appeared to be adapted to either high, or variable, or low salinity, implying that they may have originated in a variety of habitats that differed greatly with respect to salinity regime.     

The distribution and abundance of algae in saline lakes of Saskatchewan,Canada

Collections of algae, mainly planktonic, were made from 41 saline lakes in southern Saskatchewan ranging in salinity from 3.2 to 428 g l^-1. Algae in 7 phyla, 8 classes, 42 families, 91 genera and 212 species and varieties were identified. Fourteen species were restricted to hypersaline (50 g l^-1) waters and eleven of these were diatoms. In general, species diversity was inversely related to lake salinity. Algae that were important community constituents over a broad spectrum of salinities were the green algae Ctenocladus circinnatus, Dunaliella salina and Rhizoclonium hieroglyphicum, the blue-green Lyngbya Birgei, Microcystis aeruginosa, Oscillatoria tenuis, O. Utermoehli and Nodularia spumigena and the diatoms Melosira granulata, Stephanodiscus niagarae and Chaetoceros Elmorei. In general green algae were dominant when lake salinity exceeded 100 g l^-1 although diatoms played important roles in most of these highly saline lakes except for Patience Lake.     

Benthic response to ammonium pulses in a tropical lagoon: implications for coastal environmental processes

In New Caledonia, the benthic communities living in the coral reef lagoon around Noumea city are subjected to regular shifts from oligotrophic conditions typical of lagoon waters to nutrient enrichment due to waste water inputs. The influence of ammonium pulses on microphytobenthos production was experimentally tested under varying light intensities in the vicinity of Noumea. Benthic oxygen, ammonium and silicon fluxes at the sediment-water interface were measured in situ using benthic enclosures. Three ammonium concentrations were tested. Gross primary production was doubled with a 13.8 μmol l⁻¹ ammonium concentration increase. Fitted PI curves showed that maximum production (F_max) was linearly related to ammonium concentration, but not the optimal irradiance (I_k). Silicon fluxes were characterized by dissolution in the absence of light, a process that declined with increasing illumination. These results were attributed to microphytobenthos activity, mainly diatoms that are nutrient-limited and strongly reactive to ammonium inputs. Production may result from a multiplication of cells, but migration up to the water sediment interface may also be involved. Oxygen consumption was also significantly influenced by ammonium concentration as a positive linear relationship with added ammonium concentration was established. Even during short-term experiments, ammonium enrichment stimulated photoautotrophic production, increasing the energy available to heterotrophs. Furthermore, microbenthic activities as well as nitrate production were increased by ammonia-oxidizing bacteria able to grow chemolithotrophically at the expense of oxygen. Therefore, in the study area, pulses of urban waste waters resulted in a decrease of plant-related autotrophy in benthic communities.     

Counting Viruses and Bacteria in Photosynthetic Microbial Mats

Viral abundances in benthic environments are the highest found in aquatic systems. Photosynthetic microbial mats represent benthic environments with high microbial activity and possibly high viral densities, yet viral abundances have not been examined in such systems. Existing extraction procedures typically used in benthic viral ecology were applied to the complex matrix of microbial mats but were found to inefficiently extract viruses. Here, we present a method for extraction and quantification of viruses from photosynthetic microbial mats using epifluorescence microscopy (EFM) and flow cytometry (FCM). A combination of EDTA addition, probe sonication, and enzyme treatment applied to a glutaraldehyde-fixed sample resulted in a substantially higher viral (5- to 33-fold) extraction efficiency and reduced background noise compared to previously published methods. Using this method, it was found that in general, intertidal photosynthetic microbial mats harbor very high viral abundances (2.8 × 10¹⁰ ± 0.3 × 10¹⁰ g⁻¹) compared with benthic habitats (10⁷ to 10⁹ g⁻¹). This procedure also showed 4.5- and 4-fold-increased efficacies of extraction of viruses and bacteria, respectively, from intertidal sediments, allowing a single method to be used for the microbial mat and underlying sediment.     

Spatial distribution of diatom and cyanobacterial mats in the Dead Sea is determined by response to rapid salinity fluctuations

Cyanobacteria and diatom mats are ubiquitous in hypersaline environments but have never been observed in the Dead Sea, one of the most hypersaline lakes on Earth. Here we report the discovery of phototrophic microbial mats at underwater freshwater seeps in the Dead Sea. These mats are either dominated by diatoms or unicellular cyanobacteria and are spatially separated. Using in situ and ex situ O₂ microsensor measurements we show that these organisms are photosynthetically active in their natural habitat. The diatoms, which are phylogenetically associated to the Navicula genus, grew in culture at salinities up to 40 % Dead Sea water (DSW) (14 % total dissolved salts, TDS). The unicellular cyanobacteria belong to the extremely halotolerant Euhalothece genus and grew at salinities up to 70 % DSW (24.5 % TDS). As suggested by a variable O₂ penetration depth measured in situ, the organisms are exposed to drastic salinity fluctuations ranging from brackish to DSW salinity within minutes to hours. We could demonstrate that both phototrophs are able to withstand such extreme short-term fluctuations. Nevertheless, while the diatoms recover better from rapid fluctuations, the cyanobacteria cope better with long-term exposure to DSW. We conclude that the main reason for the development of these microbial mats is a local dilution of the hypersaline Dead Sea to levels allowing growth. Their spatial distribution in the seeping areas is a result of different recovery rates from short or long-term fluctuation in salinity.     

Triggers of phytoplankton bloom dynamics in permanently eutrophic waters of a South African estuary

The permanently eutrophic Sundays Estuary experiences recurrent harmful algal blooms (HABs) of Heterosigma akashiwo (Raphidophyceae). This study aimed to identify the environmental variables shaping phytoplankton community composition and succession patterns during a typical spring/summer harmful algal bloom (HAB) period. Monitoring of abiotic and phytoplankton variables was undertaken over the period of a month in 2016. Surface water salinity corresponding to mesohaline conditions (9 to 12) was a prerequisite for site selection. During the study, two HABs (>550 µg Chl a l^?1) of H. akashiwo occurred, each lasting for approximately a week in duration. Analyses highlighted nutrient depletion (i.e. nitrate and phosphate concentrations) as the key constraint on bloom duration. When the density of H. akashiwo decreased, the community composition became more diverse with species belonging to Bacillariophyceae and Dinophyceae becoming more abundant; albeit to a lesser degree (<180 µg Chl a l^?1). Dissolved oxygen shifted from super-saturated conditions (>14 mg l^?1) during peak HAB conditions, to instances of bottom water oxygen depletion (2–4 mg l^?1) during the decay phase. These findings highlight the potential severity of transforming a catchment from natural to one that is highly regulated by agricultural practices, while also emphasising the need for management intervention.