Multiannual phytoplankton trends in relation to environmental changes across aquatic domains: A case study from Sardinia (Mediterranean Sea)

We investigated multiannual trends in phytoplankton in relation to several environmental drivers. We analyzed ecological data collected during the past two decades from three aquatic sites: an artificial lake, a coastal lagoon, and a marine coastal area. Hydrographic, nutrient, and phytoplankton data were statistically analyzed to detect trends and interactions. In all ecosystems, the chlorophyll a concentration decreased with increasing abundance of small-sized phytoplankton. Phytoplankton dynamics were related to decreased nutrient concentrations in the lake, to dynamics of phosphorus and decreased salinity in the lagoon, and probably to combined top-down and bottom-up processes in the marine gulf.     

The use of dialysis culture in a study of chlorophyll budget in a brackish lagoon,Japan

Growth rates of the entire phytoplankton community of a brackish lagoon in northeastern Japan were estimated by measuring increasing chlorophyll a content in dialysis bags during the summer and early autumn of 1986. The chlorophyll a contents of lagoon water fluctuated between 20 and 200 mg m^–3. At lower densities of phytoplankton (20–50 mg chl. a m^–3), growth rates (the rate of increase of chlorophyll a) exceeded 1 turnover per day, while at higher densities (more than 50 mg chl. a m^–3), the growth rate decreased rapidly. Tidal exchanges of chlorophyll a showed net exports of chlorophyll a from the lagoon to adjacent waters. The exchange rate of chlorophyll a was estimated to be 0.65 d^–1. At about 140 mg m^–3 of chlorophyll a concentration, the increase of chlorophyll in the lagoon water compensated for tidal export. Only a small proportion of primary production was consumed by zooplankton in the lagoon. There were also net exports of ammonium and phosphate from the lagoon. Nutrient flux from sediment exceeded the phytoplankton requirement and was the major source of the ammonium and phosphate exports from the lagoon. The low inorganic N/P atom supply ratio in the lagoon suggests that nitrogen is a major nutrient limiting phytoplankton growth.     

Long-term changes in the population of Eurytemora affinis (Copepoda,Calanoida) in the Gironde estuary (1978–1992)

Long-term changes in the population of Eurytemora affinis were investigated during 15 years (1978–1992) at a fixed station in the Gironde estuary (South West France). Total numbers, sex-ratio, % ovigerous females, proportion of copepodites and clutch-size were taken into account and their long-term distribution was related to temperature, river flow, salinity, suspended matter and chlorophyll a concentrations after the seasonal effect was removed for all variables.There was a great decrease of the river flow during the period of investigation, due to a general deficit in pluviosity. On the contrary, the general trend for salinity was an increase from 1978 to 1992. The same pattern was observed for chlorophyll, indicating an intrusion of marine phytoplankton. Water temperature increased significantly during the study period, following the general increase in air temperature observed in the area. For the suspended particulate matter (SPM), an increase was observed between 1978 and 1981–1982, then a very sharp decrease occurred from 1984 onwards.Numbers of E. affinis were inversely correlated with temperature and salinity, and positively correlated with the river flow. E. affinis was negatively correlated with chlorophyll concentration because of the covariation with salinity. No clear long-term trend was observed for the sex-ratio. A significant correlation was found between females carrying egg-sacs and SPM concentration, probably due to a decrease of the predation pressure in very turbid waters. The percentages of copepodites tended to decrease with time and were inversely correlated with temperature. Clutch-size significantly decreased during the 15-year period. This trend was mainly explained by temperature and salinity.It is concluded that, in the absence of strong human alteration in the Gironde estuary the long-term distribution of zooplankton can be explained by the natural environmental variability. The dominant factors are the river flow which governs the movement of the populations along the estuary and the temperature which influences the reproductive processes. Any change in climate will have consequences on the pelagic community.     

Variability of nutrients and phytoplankton biomass in a shallow brackish water ecosystem (Chilika Lagoon, India)

Seasonal and spatial variations in water quality parameters, such as nutrients [NH₄ ⁺–N, NO₂⁻–N, NO₃⁻–N, PO₄³⁻–P, total nitrogen (TN) and total phosphorus (TP)], Secchi disc depth, salinity, dissolved oxygen, chlorophyll a, primary productivity and phytoplankton standing stock, were studied in Chilika Lagoon (from 27 sampling locations) during 2001–2003 to assess the present ecological status. The study was undertaken after a major hydrological intervention in September 2000, which connected the lagoon body and the Bay of Bengal via a manmade opening (new mouth). Current and old data on water quality were also compared to establish the changes that had occurred after the hydrological intervention. Multivariate techniques and gridding methods were used to investigate the spatial and seasonal variability of the data and to characterize the trophic evolution of the basin. Results of principal component analysis (PCA) indicated that the 27 stations can be classified into five groups based on similarities in the temporal variation of nutrients, chlorophyll a concentration, salinity, and other physicochemical parameters. The tributaries and the exchange of lagoon water with the Bay of Bengal most probably determine the water quality and the dynamics of the ecosystem. Hydrodynamics of the lagoon, weed coverage, input of urban sewage through tributaries and agricultural runoff are probably the key factors controlling the trophic conditions of the lagoon. An increase in salinity and total phosphorus was noted after the new mouth was opened, while the total suspended sediment load, the water column depth, and nitrogenous nutrients decreased. The new mouth opening also brought changes in the phytoplankton species composition.     

Spatial and temporal variations of hydrological conditions,nutrients and chlorophyll <Emphasis Type="Italic">a</Emphasis> in a Mediterranean coastal lagoon (Mar Menor,Spain)

The Mar Menor is a sheltered and hypersaline lagoon, with salinity ranges between 38 and 51 psu. The lagoon is threatened by several pressures and in the last decades detrimental impact on the natural community structure and dynamics have increased. In the watershed, agricultural practices are rapidly evolving from extensive dry crop farming to intensively irrigated crops, with increasing loads of nutrient and pollutants to the lagoon. Hydrological conditions, nutrients and chlorophyll a concentrations were analysed in 1997 and 2002–2003 in a grid of 20 stations in the lagoon. Different time scales, from daily to inter-annual, were considered. In the considered periods, the dissolved inorganic nitrogen (DIN) increased whilst phosphate decreased significantly. These contrasting patterns depended upon the increased agricultural loading for DIN and were due to the implementation of the wastewater works for phosphates. In 1997 and 2002, the highest nitrate concentrations were usually found on the west coast of the lagoon, close to the mouths of the main watercourses. In parallel, the lowest concentrations were detected at the inner coastline along “La Manga” sandy bar and “El Estacio” channel. Based on weekly data, correlations between chlorophyll a concentrations and environmental variables disagreed with traditional eutrophication models. Relationships between chlorophyll a and nutrients were negative, suggesting that in the short term phytoplankton controlled nutrient concentrations. Moreover, nitrate and phosphorous seemed to alternate as limiting factors. The relationships between chlorophyll a became positive when considering time lags and analysed at longer time scales (monthly or seasonal means), thus suggesting a very rapid response of primary producers to nutrient enrichment. A significant correlation between chlorophyll a concentration and fish larvae density was also found at all time scales analysed, suggesting a top-down control of the trophic web.     

The zooplankton and other limnological features of a humic coastal lagoon (Lagoa Comprida,Macé, R.J.) in Brazil

Comprida lagoon is a shallow lagoon separated from the sea by a sand barrier. It has a brown-coloured freshwater with a high concentration of humic compounds. Its zooplankton community and limnological features were studied from March 1992 to December 1995. The lagoon was characterized by low transparency, acid water and relative constant physical–chemical features, except during sporadic marine entrances. The zooplankton, composed of holoplanktonic and meroplanktonic forms, consisted of 60 taxa. Eleven were permanent elements of the community: e.g. Bosminopsis deitersi, nauplii and copepodites of `Diaptomus' azureus. B. deitersi correlated positively with the lowest pH values and with the highest total dissolved nitrogen. Only four taxa correlated with chlorophyll-a concentration. Pigmentation in `D.' azureus is suggested to be as an energy reserve in a system where phytoplankton is probably light-limited.     

Long-term phytoplankton changes in an artificially divided,top-down manipulated humic lake

Lake Große Fuchskuhle (Brandenburg, Germany) is a naturally acidic bog lake that was artificially divided into four basins by large plastic curtains for biomanipulation experiments in 1990. Different numbers of perch were added to each compartment beginning in the spring of 1993. The species composition and abundance of phytoplankton, pH, nutrient concentrations, dissolved organic carbon (DOC) and chlorophyll a content were analyzed at regular intervals during 1991 and 1998. The division of the lake resulted in divergent developments in the physical and chemical environment of the compartments. This study compared the phytoplankton assemblages of the Northeast- (NE) and Southwest- (SW) basins which differed strongly in chemistry during the investigation period. Divergent developments in phytoplankton species composition in both basins can be explained by changes in physical and chemical conditions (bottom-up effects). Increased pH values and DOC concentrations probably favoured mass developments of the dinoflagellate Gymnodinium uberrimum since 1993, while increased nutrients (dissolved inorganic carbon, total nitrogen and especially total phosphorus) as well as further changes in pH and DOC led to the dominance of the raphidophyte Gonyostomum semen in 1998. This bloom was characterized by extreme biomasses of up to 143 mg l^–1 wet weight, corresponding with high chlorophyll a concentrations of up to 413 g l^–1 at the same time. In contrast, no significant relationship between experimental manipulations by piscivorous fish stocking (top-down effects) and phytoplankton biomass were observed.     

Seasonal variations in copepod body length: a comparison between different species in the Lagoon of Venice

The seasonal variation in length of the most abundant copepod species in the lagoon of Venice were compared and the relative influence of temperature, chlorophyll concentration and salinity examined. Temperature seems to be the primary factor influencing copepod body length in the lagoon of Venice, but the different species vary markedly in their response. Calanoid copepods (Acartia clausi, A. tonsa, Paracalanus parvus and Centropages ponticus) showed a more definite trend of size variation with temperature than the cyclopoids (Oithona nana and O. similis) and the harpacticoid Euterpina acutifrons. The size of the poecilostomatoids Oncaea media and O. subtilis was not affected by temperature, and was almost constant over time. Differences in the metabolic rates, longevity and specialization of calanoid, cyclopoid and harpacticoid copepods could explain their different responses to temperature.     

Factors Controlling Phytoplankton Blooms in a Temperate Estuary: Nutrient Limitation and Physical Forcing

A combination of enclosure nutrient enrichment experiments and historical data analysis was used to identify the factors controlling seasonal dynamics and competition of the phytoplankton community in the Curonian lagoon (Southeast Baltic Sea). Experiments using different nutrient (N, P and Si) manipulations were performed in 10-l enclosures for 48 h. Changes in chlorophyll a concentrations, inorganic nutrient concentrations, and plankton cell density were monitored. Results revealed that phytoplankton development in the lagoon is strongly affected by ambient physical factors (wind, temperature). Nutrient limitation, however, also plays an important role in seasonal succession mechanisms showing quite distinct seasonal development patterns. Based on the data, available phytoplankton seasonal succession in the Curonian lagoon could be described as composed by three phases corresponding to different domination and regulatory mechanisms.     

Distribution and production of macrophytes and phytoplankton in the lagoon of Venice: comparison of actual and past situation

This paper aims at comparing inedited maps of macrophytes and phytoplankton distribution in the lagoon of Venice in 1980 and 2003. The macrophyte distribution is displayed with reference to different biomass intervals which allow the calculation of the occupied surface, standing crop (SC), net (NPP) and gross (GPP) production. In 1980 the total macroalgal SC was ca. 841 ktonnes whereas the annual NPP and GPP were estimated to be ca. 2912 and 18498 ktonnes, respectively. In 2003 macroalgae displayed a marked regression and the SC, NPP and GPP decreased to ca. 89, 471 and 2336 ktonnes. Maps of the seagrass distribution date back to 1990 but their biomass and production have been quantified only in 2003. On the whole, in 2003, on a surface of ca. 56 km2 the three species living in the lagoon accounted for a SC and a NPP of ca. 209 and 821 ktonnes, whereas the GPP estimated according to literature was about twice as high as the NPP. Cymodocea nodosa was the most abundant phanerogam in the lagoon. Its SC was higher than the total of the macroalgae, although the latter were more productive. Zostera marina covered the highest surface in the lagoon but its biomass and production were a little lower than that of C. nodosa. Nanozostera noltii, which was common and widespread in the past, displayed an evident regression trend which was mainly due to the increase of the water turbidity and the disruptions of its habitat. Similarly, phytoplankton, underwent a descending trend, although data quoted in the present paper display its distribution only in the summer period.     

Characterization of phytoplankton assemblages in a tropical coastal environment using Kohonen self‐organizing map

This study was aimed at understanding the main abiotic environmental factors controlling the distribution patterns of abundance and composition of phytoplankton (size less than 10 μm) assemblages in the coastal waters of south‐eastern Côte d'Ivoire. Data were collected during two cruises, in January (low‐water period) and October (high‐water period) of 2014. A total of 67 species were identified and assigned to Bacillariophyceae (49%), Cyanophyceae (21%), Chlorophyceae (13%), Euglenophyceae (10%), Dinophyceae (4%) and Chrysophyceae (3%). Three biotic zones (I, IIA and IIB) were distinguishable on a Kohonen self‐organizing map after an unsupervised learning process. The diatom genera Eunotia sp., Navicula sp. and Actinoptychus senarius are significantly associated with I, IIA and IIB biotic zones, respectively. A clear seasonal cum salinity trend was apparent in phytoplankton distribution patterns. Turbidity and nitrate levels were the main abiotic factors controlling phytoplankton distribution in I, the upland tidal regions of the lagoon. In regions along the lagoon–sea continuum, phosphate and turbidity exert the most control during the low‐water season (IIA), while total dissolved solids control phytoplankton distribution during the high‐water season (IIB). These are climate‐sensitive parameters whose concentrations depend on prevailing hydroclimatic processes. Therefore, seasonality can have important consequences on phytoplankton community and inadvertently the productivity of these systems.     

The Relationship between Phytoplankton Diversity and Community Function in a Coastal Lagoon

The decrease of biodiversity related to the phenomena of global climate change is stimulating the scientific community towards a better understanding of the relationships between biodiversity and ecosystem functioning. In ecosystems where marked biodiversity changes occur at seasonal time scales, it is easier to relate them with ecosystem functioning. The objective of this work is to analyse the relationship between phytoplankton diversity and primary production in St. André coastal lagoon – SW Portugal. This lagoon is artificially opened to the sea every year in early spring, exhibiting a shift from a marine dominated to a low salinity ecosystem in winter. Data on salinity, temperature, nutrients, phytoplankton species composition, chlorophyll a (Chl a) concentration and primary production were analysed over a year. Modelling studies based on production-irradiance curves were also conducted. A total of 19 taxa were identified among diatoms, dinoflagellates and euglenophyceans, the less abundant group. Lowest diversities (Shannon–Wiener index) were observed just before the opening to the sea. Results show a negative correlation (p<0.05) between diversity and chlorophyll a (Chl a) concentration (0.2–40.3 mg Chl a m⁻³). Higher Chl a values corresponded to periods when the community was dominated by the dinoflagellate Prorocentrum minimum (>90% of cell abundance) and production was maximal (up to 234.8 mg C m⁻³ h⁻¹). Maximal photosynthetic rates (P_max) (2.0–22.5 mg C mg Chl a⁻¹ h⁻¹) were higher under lower Chl a concentrations. The results of this work suggest that decreases in diversity are associated with increases in biomass and production, whereas increases correspond to opposite trends. It is suggested that these trends, contrary to those observed in terrestrial and in some benthic ecosystems, may be a result of low habitat diversity in the water column and resulting competitive pressure. The occurrence of the highest photosynthetic rates when Chl a is low, under some of the highest diversities, suggests a more efficient use of irradiance under low biomass–high diversity conditions. Results suggest that this increased efficiency is not explained by potential reductions in nutrient limitation and intraspecific competition under lower biomasses and may be a result of niche complementarity.     

Zooplankton variability related to environmental changes in a eutrophic coastal lagoon in the Po Delta

Zooplankton variability in a lagoon of the Po Delta, the Sacca di Goro, was studied in relation to changes in its hydrodynamic and hydrological features. From 1987 to 1992 the lagoon was affected each summer by severe anoxia due to the decomposition of dense and widespread beds of Ulva rigida. In August 1992 a canal was opened through the sand bank closing off the lagoon from the sea in order to improve the water circulation. This hydraulic intervention led to significant environmental changes in the lagoon: during the two subsequent years the summer dystrophic crises were brief and less severe, due to a sharp decrease in the Ulva cover. At the same time a clear increase in phytoplankton biomass values was observed. Following the opening of the channel, the most remarkable change in zooplankton was a significant density increase of calanoids, particularly of Acartia tonsa.     

Feeding behaviour and migrations in a natural population of the copepod Acartia tonsa

This paper deals with the variations on feeding activities and diel migrations of Acartia tonsa Dana, the dominant copepod species in Berre lagoon (west Mediterranean French coasts).A 27 hour in situ study was carried out during June 1989, at a station located in the south west of the lagoon. Vertical profiles of salinity, temperature and dissolved oxygen taken each 12 hours showed a stratification of the water column in two distinct layers: (1) a superficial layer with higher temperature, moderate salinity, and high oxygen concentrations; (2) a colder, more saline and almost anoxic deep layer. In situ chlorophyll a measurements were made at –1, –3 and –5 m; concentrations were relatively homogeneous through the water column during the whole sampling period.Zooplankton samples were collected every 3 hours with a 200 µm mesh net, in three strata (0–2 m, 2–4 m, 4–6 m). A complete dominance of A. tonsa was observed in the zooplankton community. Our results point out clearly a nocturnal migration, with individuals concentrating in both superficial layers; thus an unimodal migratory pattern can be inferred. Gut flourescence measured following the Mackas & Bohrer technique (1976), showed higher values during night time, and values for females were the highest with wider day—night variations. Similar results were found in laboratory experiments with copepods fed with a culture of Dunaliella tertiolecta.Gut evacuation rate was measured in two laboratory experiments either mixing or separating males and females. Evacuation rate was 17.92 and 27.25 min for males and females respectively.Phytoplankton daily ration for A. tonsa calculated by gut flourescence and gut evacuation rate was particularly low, for it represents only 10% of the individual carbon weight.Moreover, grazing impact on phytoplankton is very restricted, it represents less than 1% of the daily phytoplankton stock.     

Decadal weakening of the wind-induced upwelling reduces the impact of nutrient pollution in the Bay of Málaga (western Mediterranean Sea)

The hypothesis that nitrate versus phosphate regulates the coastal primary production has been assessed at different time scales in the northwest Alboran Sea. Time series of temperature, salinity, nutrients and chlorophyll a obtained at three stations located off Málaga city (the greatest coastal urban core along the Alboran Sea shoreline) from 1992 to 2006 have been analysed. At the decadal scale, temperature increased linearly while salinity decreased. These changes were related to a shift in the wind regime suggesting that coastal upwelling became steadily weaker. In contrast to phosphate, nitrate was positively correlated with salinity at the seasonal scale and decreased linearly from 1992 to 2006. Seasonal and decadal changes in chlorophyll a were correlated with nitrate (and uncorrelated with phosphate). However, non-regular variability in chlorophyll a was correlated with phosphate. Consequently, the results demonstrate that nitrate controls the phytoplankton biomass at the inter-annual scale while both nitrate and phosphate do so at a shorter time scale. The Bay of Málaga receives elevated entries of domestic waste waters that release high loads of phosphate compared to nitrate. Our analysis indicates that the expected impact of this pollution on chlorophyll a at the inter-annual scale is reduced in comparison with the effects of atmospheric forcing.     

Space and time distribution of zooplankton in a Mediterranean lagoon (Etang de Berre)

In the Berre lagoon, a large brackish and swallow area near Marseille, the environmental factors (temperature, salinity, oxygen, suspended particulate matter and chlorophyll) generally display strong space and time variations. The rotifer Brachionus plicatilis and the copepod Acartia tonsa constitute the bulk of the zooplankton population during all the year. Their space and time distributions were studied in 23 stations distributed all over the lagoon, during four seasonal cruises (February, June, October, November), at surface and bottom layers. There is no marked difference in the horizontal and vertical distribution of the two species, (except in November when rotifers were prevailing in surface and copepods at depth) and in their time occurence. When the four series of data are pooled, correlation analysis show that A.tonsa is positively correlated with temperature, salinity and seston and negatively to oxygen and chlorophyll. B. plicatilis is positively correlated with temperature and seston, but also with chlorophyll, while salinity has a negative effect. The specific eggs number of both species is chlorophyll dependent. Considering seasonal cruises separately, some differences appear in the sense or the significance of these different correlations. The respective distribution of the two species is only partly dependent on the variation of the environmental factors: most of the variance remains unexplained, as indicated by the result of a stepwise multiple regression analysis using the most significant factors (temperature, salinity and oxygen explain 33 to 42% of the variance in Acartia, while temperature and salinity explain 27 to 28% of the variance in Brachionus). Thus, internal behavioral factors could also play a role in the distribution of organisms, particularly in some cases of aggregations of organisms observed during this study. As the two species occupied the same space habitat most of the year, they are potentially in competition for food. A way to optimize the food utilization could be the time separation of their feeding activity, nocturnal in Acartia and diurnal in Brachionus. Another way could be selective feeding upon food particles depending on their size (Brachionus being able to use finer particles than Acartia) or their quality (Brachionus being more herbivorous than Acartia) as demonstrated in some grazing experiments carried out in parallel.     

Long-term phytoplankton changes in a shallow hypertrophic lake,Albufera of Valencia (Spain)

A long-term phytoplankton study was carried out in the Albufera of Valencia, a shallow hypertrophic lake (surface area 21 km², mean depth 1 m, total inorganic nitrogen load 155 g m^-2 y^-1, total inorganic phosphate load 15 g m^-2 y^-1) from 1980 to 1988. The lake functions as a reservoir for the surrounding rice cultivation. From 1940's to 1988, its phytoplankton assemblage has been altered from a mesotrophic to a hypertrophic character, as consequence of the increasing pollution. For 1980–88, annual variations in the phytoplankton were less pronounced than seasonal changes. The hypertrophic and morphometric features of the lake favoured the stability of the phytoplankton assemblage and chlorophyll a levels during the study period. Seasonal and horizontal distribution of the total phytoplankton abundance and biomass were highly influenced by the hydrological cycle of the lagoon. Compared with other shallow nutrient rich lakes, the Albufera of Valencia is similar to the shallow hypertrophic lakes of the Netherlands.     

Distribuzione del fitoplancton in un'area salmastra della Sicilia nord-orientale

Abstract

Phytoplankton distribution in a brackish area of North-Eastern Sicily -The abundance and taxonomic composition of the phytoplankton have been studied in relation to temperature, salinity and dissolved oxygen in the brackish pools of Oliveri-Tindari, a very restricted area on the Tyrrhenian coast of Sicily (Italy). The phytoplankton distribution in these pools has been subject to relatively few investigations. Although this area is not influenced by sewage and industrial effluents, some pools showed episodical high densities of phytoplankton, due to the influence of organic inputs (streams and water-bearings), and anoxic conditions.

The microalgal communities were generally characterized by chlorophyceans, although sometimes other taxa prevailed.     

Pheopigment dynamics,zooplankton grazing rates and the autumnal ammonium peak in a Mediterranean lagoon

The dilution technique was used to estimate chlorophyll and pheopigment, net and gross production as well as zooplankton grazing over a 12-month period in a coastal lagoon in Southern France. Chlorophyll a (Cha) based gross growth rates of phytoplankton ranged from undetectable in February to 2.6 day⁻¹ in June, corresponding to 3.8 divisions per day. Cha-based grazing rates ranged from undetectable in February to 1.1 d⁻¹ in June. The seasonal growth pattern of picoplankton was similar to that of the whole community, with a peak in July, corresponding to four divisions per day. Grazing processes represented from 20 to 150% of the phytoplankton daily growth, and the grazing pressure was stronger on small phytoplankton cells than on larger cells. Gross growth rates of phytoplankton were related to zooplankton grazing rates, and both were related to water temperature. Mesozooplankton which escaped sampling or oysters had to be also invoked as additional sinks for the primary production. In the fall, pheopigment concentrations greater than chlorophyll concentrations coincided with high ammonium levels in the water column. Pheopigment a production rates were highly correlated to chlorophyll -based microzooplankton grazing rates. The pheopigment a to chlorophyll a ratio was correlated with ammonium concentrations and could be used an index of the balance between ammonium supply (degradation) and demand (uptake by phytoplankton). In addition, pheopigment degradation rates in absence of grazing could be related to irradiance, indicating photo-degradation of these compounds.     

Impact of a low salinity year on the metabolism of a hypersaline coastal lagoon (Brazil)

Araruama Lagoon is a large, continuously hypersaline coastal lagoon (mean salinity 52 g l^-1). However, during certain years the evaporation-precipitation balance is modified and results in lower salinity conditions, e.g. 1989–1990 when the mean salinity measured 41 g l^-1. These atypical conditions are responsible for a different behavior of the system, normally characterized by the presence of extensive microbial algal mats, low pelagic primary production compared to benthic primary production, and low turbidity. However, during this study (1989–1990), the presence of microbial algal mats was restricted to the shallow eastern embayment of the lagoon representing c.a. 8% of the total area, the mean concentration of suspended matter was relatively high (21.2 mg l^-1) when compared with a typical year, and primary production measured was 447 mg C m^-2 d^-1, higher than benthic primary production during hypersaline years. These conditions therefore influence the trophic state of the system, during low salinity conditions, the lagoon was meso-eutrophic.