Short-term variations of seston characteristics in a shallow tropical lagoon: Effect of wind-induced resuspension

In January 1993, a 24 hour survey based on an hourly sampling was conducted in a station (1 m depth) located in the Ebrié lagoon, Côte d'lvoire, in order to determine short-term changes in seston quality and quantity and relate them to physical conditions. Water samples were collected just below the surface and at 0.9m depth to assess the seston distribution in the water column. Located in an estuarine environment, the study site was submitted to periodic effect of tides with low currents. Under certain conditions, sediment resuspension by wind-induced waves was observed in the shallow parts of this ecosystem. From the analysis of hydrodynamic processes (tide, wind and sedimentation rates), four basic situations were distinguished in the course of the study: a period with no wind and high sedimentation rates, a period characterized by wind-induced resuspension, followed by a period of sedimentation and finallly, a period without wind during the second flood tide. The particle size spectra were similar throughout the study, the distributions being unimodal (mode of 4 μm diameter). The finer particles (<10 μm) were resuspended over the whole water column at the beginning of the resuspension period, whereas the larger particles were resuspended if the wind speed exceeded 3 m s^?1. Phytoplankton species were found to provide information on the physical factors controlling the pelagic system: small cells, such as those of cyanobacteria (Synechococcus sp.), were always present over the whole tidal cycles, whereas large motile cells were predominant in the water column during the resuspension period. The seston characteristics of this part of Ebrié lagoon are mainly controlled by tidal currents, but wind-induced resuspension can modify the structure of the pelagic food web.     

Size, composition and distribution of particles related to wind induced resuspension in a shallow tropical lagoon

The size, composition and distribution of particles in the watercolumn were surveyed in a shallow area (1 m depth) of a tropicallagoon (Cte d'Ivoire) during a sequence of wind-induced resuspension.Water samples were collected hourly near the surface duringone tidal cycle. Three characteristic periods were distinguished:a calm period with low wind speed (average 1.2 m s^–1 awindy period with wind speed >3 m^–1 s (range between4 and 6 m s^–1) inducing sediment resuspension and a relaxationperiod during the decrease of wind velocity. From the analysisof several parameters (particle size and volume, bacteria. pico-and nanophytoplankton, ciliates and detritus), sediment resuspensioncaused a regular injection of particles from the bed. The finestparticles (1.5–6 µm: chlorophytes such as Chiorellaspp., picocyanobacteria such as Synechococcus) were the firstto be affected by wind-induced turbulence, whereas large particles(6–12 µm: diatoms. cyanobacteria such as Lyngbiaspp.) were dispersed into the water column at the highest windspeed. The fate of the different seston components differedaccording to their size. Therefore, wind-induced resuspensioncould greatly influence the food web organization through thequantity, quality and size of edible particles available ata given time.     

Release of metals from polluted sediments in a shallow lake: quantifying resuspension

The contribution of erosion of bed sediment to the load of metals leaving Lake Ketelmeer, a shallow lake in the Netherlands fed by the IJssel branch of the River Rhine, is reported. Transport of suspended matter and associated trace metals was measured using both centrifuges and sediment traps at several locations in the lake. Mass balances of suspended matter and heavy metals were calculated using data from these field measurements.Metal/scandium-ratios were used to identify the source of the suspended matter in the lake. Since the bed sediment is more polluted than the suspended sediment entering the lake, higher metal/scandium-ratios were found for bottom sediment in the lake compared with those for suspended matter entering the lake from the River IJssel. Using the metal/scadium-ratio in suspended matter from the lake, it was calculated that bottom sediments made up 43% of the suspended matter leaving the lake. This implies an erosion flux of bottom sediment of 16 g m^–2 d^–1. For cadmium, mercury, chromium and zinc, this erosion process accounts for more than 50% of the pollutant load leaving Lake Ketelmeer and entering Lake IJsselmeer.     

A dynamic model for flow and wind driven sediment resuspension in a shallow basin

A model is presented, which describes the daily variations in suspended particulate inorganic matter (SPIM) in a large (61 km²), shallow (mean depth 3.4 m) wind-exposed lake basin at the western end of Lake Mälaren, Sweden. Field studies have shown that wind speed and river inflow are the two major factors leading to changes in SPIM. Wind speed and beam attenuation were measured at high (10 min) frequency, while river inflow were monitored at daily frequency to develop the model. From these field measurements, model-coefficients were determined for the SPIM transport, settling and resuspension. Large-scale variations in SPIM lasting many weeks could be explained by events of high river inflow with a correspondingly high particle load. The threshold for river transported SPIM to have effect on lake concentration was 150 m³ s^–1, while wind induced resuspended SPIM was related to the square root of wind speed.     

The effect of wave-reduction engineering on sediment resuspension in a large, shallow, eutrophic lake (Lake Taihu)

Frequent resuspension of sediments is recognized as an important process in large shallow lakes, impeding the recovery of eutrophic lakes. A large-scale project, including a wave barrier (3.3 km long) and a soft enclosure, was implemented to reduce wave energy and sediment resuspension in Lake Taihu, eastern China. The effects of the wave-reduction engineering on sediment resuspension and internal nutrient loading were investigated. Results showed that sediment resuspension rates as well as suspended solids (SS) in the areas protected by the wave barrier and the soft enclosure were significantly lower than in the unprotected areas. There was a positive relationship between total phosphorus (TP) and SS; thus internal loading of phosphorus was significantly reduced by the wave-reduction structure. High nutrient levels and phytoplankton biomass persisted during the experiment period, suggesting that additional measures, such as re-establishment of the macrophyte community, must be included to help restore the water quality in such a large, shallow and eutrophic lake.     

The effect of resuspension on algal production in a shallow lake

Waves cause erosion and resuspension of bottom sediments. In shallow lakes resuspension can take place over most of the lake area and the resuspended matter can stay in suspension for such a long time that the mean light intensity in the lake is reduced, causing reduced algal growth. The increase of suspended matter and light attenuation in the water of lake Tämnaren, Sweden, was found to be proportional to wind velocity to the third power. After each storm increased attenuation of light lasted for a week. The algal production was estimated to be reduced to only 15 % of what it would have been without increased turbidity due to resuspension.     

The effect of an emergent macrophyte (Typha angustifolia) on sediment resuspension in a shallow north temperate lake

1. The effects of emergent macrophytes on water turbidity and sediment resuspension in the shallow Kirkkojärvi basin of Lake Hiidenvesi were studied with sediment traps, and concomitant sediment and water samples. The study was conducted during May–August in three different zones of a stand of the emergent Typha angustifolia .
2. Within the stand (5 m from the edge), both the concentration of suspended solids and the rate of sediment resuspension were significantly lower than at the edge and outside the stand (5 m from the edge). The differences between the zones increased towards the end of summer together with the growing stem density. During the study period (82 days), 2210 g dw m⁻² of sediment was resuspended in the outer zone. At the edge and in the inner zone, the corresponding numbers were 1414 and 858 g dw m⁻², respectively.
3. With the resuspended sediment, 39.4 mg P  m⁻² day⁻¹ was brought to the water column outside the stand, 22.4 mg P  m⁻² day⁻¹ at the edge and 13.4 mg P  m⁻² day⁻¹ within the stand.
4. In early summer, the concentration of suspended solids had a highly significant positive effect on soluble reactive phosphorus (SRP) concentration in the water, whereas in late summer no effect was found. During the study period, phosphorus retention by emergent macrophyte stands corresponded to 3–5% of the present annual external phosphorus loading of the Kirkkojärvi basin.     

Macroinvertebrates associated with Chara in a tropical coastal lagoon (Imboassica lagoon,Rio de Janeiro,Brazil)

Imboassica lagoon is an urban coastal lagoon located in the municipality of Macaé (RJ), which has been exposed to a process of artificial eutrophication through the inflow of untreated sewage, as well as artificial openings of the sandbar that separates it from the ocean, provoking drastic modifications in this ecosystem. The sampling for the analysis of the community of macroinvertebrates associated with Charopyceae (Chara angolensis e C. fibrosa) were performed between October 1995 and October 1997, with a total of 9 samplings at two sampling stations: one located in the innermost area of the lagoon and another at the mouth of the main sewage channel, close to the sandbar. Throughout this period, four artificial sandbar-opening events were recorded, resulting in changes in the biomass of the macroalgae and in the densities of the macroinvertebrate populations. Through the analysis of the density of organisms per square meter, and density of organisms per 100 g dry weight of Chara, it was established that the biomass of the macroalgae has a direct influence on the density of the associated macrofauna. The greater development of the periphytic community in the area close to the inflow of sewage allows for the development of a community of macroinvertebrates with high densities, composed mainly of scraping organisms, such as the mollusc Heleobia australis (Hidrobiidae).     

Effects of different macrophyte growth forms on sediment and P resuspension in a shallow lake

The effects of floating-leaved, submerged and emergent macrophytes on sediment resuspension and internal phosphorus loading were studied in the shallow Kirkkojärvi basin by placing sedimentation traps among different plant beds and adjacent open water and by sediment and water samples. All the three life forms considerably reduced sediment resuspension compared with non-vegetated areas. Both among submerged (Ceratophyllum demersum, Potamogeton obtusifolius, Ranunculus circinatus) and emergent (Typha angustifolia) plants, resuspension rate was on average 43% of that in the adjacent open water, while within floating-leaved plants (Nuphar lutea) the corresponding value was 87%. The effects of submerged and emergent vegetation increased in the course of the growing season together with increasing plant density. Among floating-leaved vegetation, such seasonal trend in resuspension effects was not observed. Compared with the non-vegetated area, floating-leaved, submerged and emergent plants reduced internal phosphorus loading on average by 21, 12 and 26 mg m⁻² d⁻¹, respectively. The effects of floating-leaved plants on resuspension-mediated internal phosphosrus loading were thus comparable to the effects of the other two life forms.     

Nutrient release and resuspension generated by ruffe (Gymnocephalus cernuus) and chironomids

1. Benthivorous fish may play an important role in internal nutrient loading. Ruffe are highly specialised, feeding exclusively on bottom animals; thus all nutrients released via their feeding are derived from the bottom and are new to the water column. The fish can also release nutrients from the sediment through resuspension while searching for food. 2. The aim of this study was to estimate experimentally in the laboratory the effect on water quality of resuspension and nutrient release by ruffe and bottom animals (chironomids). 3. Ruffe released nutrients during 8 h experiments as follows: total P 1.4, dissolved PO₄ 0.6, total N 24.0 and NH₄‐N 15.9 μg g^?1 WW h^?1. A decreasing trend in mass‐specific release was observed over time, probably because of starvation. The mass‐specific release of total N and NH₄‐N decreased as the mean weight of fish increased. The mean ratio of excreted N : P was 32. 4. In 26 h experiments with sediment and both ruffe and chironomids, ruffe increased nutrient concentrations and turbidity values significantly but chironomids had an effect only on turbidity. Neither ruffe nor chironomids affected the ratio of inorganic N : P concentrations. An interaction between ruffe and chironomids was found for turbidity. 5. According to these results, benthivorous fish may increase nutrient concentrations in the water column and need to be taken into account when estimating internal loading.     

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.     

The effect of different submerged macrophyte species and biomass on sediment resuspension in a shallow freshwater lake

Our study aim was to elucidate the effects of different species of submerged macrophytes and biomass levels on sediment resuspension. For this purpose experiments were conducted in four different enclosures (Potamogeton maackianus enclosure-PE, Vallisneria spinulosa enclosure-VE, manipulated enclosure-ME and aquaculture enclosure-AE). A sediment trap method was employed and the experiments were conducted from summer to winter in a shallow freshwater lake located in central China. A total of 813, 1277, 613 and 693 g DW m⁻² of sediment was resuspended in VE, AE, ME and PE, respectively. Our results showed that P. maackianus was more effective than V. spinulosa in restraining sediment resuspension. Macrophytes reached their maximum effectiveness of reducing resuspension at a certain species-specific biomass threshold above which biomass effects on resuspension were negligible. The threshold biomass was estimated as 300 g m⁻² for P. maackianus. Accordingly, within a lake management and aquaculture aspect, we conclude that as long as biomass does not fall below this threshold its consumption will not influence sediment resuspension. In the mid-lower reaches of the Yangtze River macrophyte coverage protects the lake sediment against adverse effects of monsoon wind; if the vegetation is eroded aquaculture sediment resuspension increases significantly.     

Seasonal and areal features of the lagoonal environment in Lake Nakanoumi,a shallow coastal lagoon in Japan

Lake Nakanoumi is a shallow coastal lagoon connected with the Japan Sea by a narrow channel. Over the past decade, land reclamation resulted in a 33% reduction of the lagoon's surface area. The remaining water basin of Lake Nakanoumi is scheduled to be artificially freshened to supply irrigation water for the newly reclaimed lands. This paper deals with the seasonal and areal features of the lagoonal environment prior to the beginning of the artificial desalinization.     

Stoichiometry of aerobic mineralization (O/C) of aquatic macrophytes leachate from a tropical lagoon (São Paulo –Brazil)

The temporal variation of stoichiometry between consumed oxygen and oxidized carbon was investigated for the aerobic mineralization of leachates from aquatic macrophytes. Seven species of aquatic plants, viz. Cabomba piauhyensis, Cyperus giganteus, Egeria najas, Eichhornia azurea, Salvinia auriculata, Scirpus cubensisand Utricularia breviscapa, were collected from Òleo lagoon located in the floodplain of Mogi-Guacu river (São Paulo State, Brazil). After being collected, the plants were washed, oven-dried and triturated. In order to obtain the leachate, the fragments were submitted to an aqueous extraction (cold). Mineralization chambers were incubated at 20 °C containing leachates dissolved in water samples from Òleo lagoon to a final concentration of ca. 200 mg l^–1on carbon basis. The chambers were maintained under aerobic conditions; the concentrations of the organic carbon (particulate and dissolved) and the dissolved oxygen were measured during approximately 80 days. Elemental analysis of the detritus and the concentrations of the remaining material (DOC and POC) were used to determine the amounts of mineralized organic carbon. The data were analyzed with first-order kinetics models, from which the daily rates of consumption (carbon and oxygen) and the stoichiometry (O/C) were determined. In the early phase of mineralization the O/C rates increased before reaching a maximum, after which they tended to decrease. For the mineralization of leachates from C. giganteus, S. auriculata and U. breviscapa, the decrease was relatively slow. For all substrata the initial values were smaller than 1, and ranged from 0.42 (S. cubensis) to 0.81 (C. piauhyensis). The maximum values were within the range from 0.58 (U. breviscapa) to 23.1 (E. najas) and at their highest 26th (C. piauhyensis) and 106th (C. giganteus) days. These variations are believed to be associated with the chemical composition of the leachates, with their transformations and alterations of metabolic pathways involved in the mineralization.     

Effects of a seasonal salinity change on periphyton biomass in a shallow tropical lagoon

In the Ebrié Lagoon (Cǒte d'Ivoire), growth of periphyton on bamboo stuck in the sediment is at the basis of the acadja, a low-cost system developed in order to enhance aquaculture of omnivorous fish like the tilapia Sarotherodon melanotheron. A survey based on hydrological, bacterial and algal data (including phytoplankton and periphyton) was conducted from November 1992 to June 1993 in an experimental structure. Climatic and hydrological seasonality was marked during the study. Succeeding to a rainy season, the oligohaline situation starting in November was characterized by low bacterial and chlorophyll biomass attached to the bamboo (respectively 1 and 18 mg m⁻²). The mesohaline situation observed from January featured a sharp increase in periphyton biomass, with a maximum in April (16 and 177 mg m⁻² for bacterial and algal biomass, respectively). The flood of the Agnéby river, induced in June by the local rains, originated again oligohaline conditions. The biomass decrease observed in May and June resulted from a shift towards low salinity (from 9.8 to 1.8 psu), a decrease in light availability (combined effects of a decreasing solar radiation during the rainy season and an increase of water turbidity due to the flood) and a lower phytoplankton biomass (therefore limiting the secondary epiphytism potentialities). In this shallow tropical environment characterized by high nutrient concentrations (due to local hydrology and organic nature of the substrate), the combination of seasonal variations of climatic (light availability), hydrological (salinity) and biological (abundance or lack of epiphytic algae) seems to control the periphyton biomass growing on bamboo. Therefore, marked seasonality in the production of resource available for the target fish would limit the interest of the acadja as an aquaculture system in brackish ecosystems.     

Modulation of dissolved oxygen levels in a hypertidal estuary by sediment resuspension

Hyperconcentrated benthic layers, which form during neap tides, recruit much of the fine sediment population of the turbidity maximum of a hypertidal estuary. Measurements of tidal amplitude and suspended solids concentration reveal that resuspension of the hyperconcentrated layers occurs between three and eight tides after neap tides rather than during spring tides (12 to 15 tides after neaps). During these resuspension events, dissolved oxygen levels are reduced but recover by spring tides. Peak solids concentrations and critically depressed dissolved oxygen levels are out of phase with tidal current amplitude. Thus observations close to neap and spring tides do not necessaraly capture the extremes of the envelope of water quality conditions.     

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.     

Spatial structure of fish assemblages in a tropical estuarine lagoon: combining multivariate and geostatistical techniques

The spatial structure and seasonal changes of estuarine fish assemblages in the Ciénaga Grande de Santa Marta (CGSM) were analysed based on four seasonal comprehensive surveys conducted in 1993-1994 and 1997. Geostatistical and multivariate techniques were used to: (a) determine seasonal changes in spatial distribution of the species richness, and (b) identify assemblages of estuarine fish and their relation to abiotic factors. Potential biotic interactions affecting the assemblage structure were also explored. A total of 11075 individuals representing 39 species were collected, with Eugerres plumieri, Diapterus rhombeus, Micropogonias furnieri, Mugil incilis, Cathorops spixii, Elops saurus and Anchovia clupeoides as dominant species between seasons. Spatial distribution of fish richness differed between rainy and dry seasons in each year, whereas species mapping showed spatial patchiness in 1993-1994 and gradients during 1997. Strong evidence of species saturation was found in all seasons, suggesting biotic interactions limiting species richness at a threshold density of ca. 50 ind./5000 m². Marine, marine-estuarine and freshwater species were classified in each season according to their capability to cope with salinity fluctuations. Associations defined by functional feeding guilds were also identified. Empirical and statistic evidence showed that fish assemblages differed between seasons within each year, and each assemblage was always dominated by a small number of species, notably E. plumieri in both years 1993-1994 and 1997. Between-season differences in fish assemblage structure in the CGSM seem to be driven by abiotic factors; however, evidence of species saturation could suggest the existence of density-dependent factors operating together.     

Effects of wind on a shallow lake ecosystem: Resuspension of particles in the Loosdrecht Lakes

Horizontal variation of seston concentration in the shallow, eutrophic Loosdrecht Lakes (A=9.8 km²; =1.9 m) was studied in relation to windspeed and effective fetch. Simple wave theory was applied in order to predict resuspension using wind data from a nearby meteorological station. Most results were consistent with the theory, but a clear limit for the occurrence of resuspension could not be established. Generally, changes in epipelon—the particles at the sediment-water interface — were not directly related to computed frequency of resuspension at the sampling station. The frequency was estimated for 37 grid points over the entire lake. Resuspension was computed to affect high percentages of the lake area in winter. In summer the frequency was much lower, but in June and July 1984 there were days with nearly 50% of the lake area subject to resuspension. The resulting input of particulate organic carbon into the water column during these days was estimated to equal 12–25 times the daytime phytoplankton carbon fixation. Most of the resuspended matter appeared to be redeposited rapidly. The computed frequency of resuspension for the 37 locations of the lake varied between 7 and 48 days in 1984.     

Patterning limnological characteristics of the Chilika lagoon (India) using a self-organizing map

In this study, a self-organizing map (SOM) was utilized to classify habitats in the Chilika lagoon located in India, the largest lagoon ecosystem in Asia (maximum length, 64.3 km; mean width, 20.1 km). The lagoon was marginally eutrophic (nitrate, 0.25 ± 0.22 mg L⁻¹; orthophosphate, 0.26 ± 0.22 mg L⁻¹; n = 1,980, respectively) for six years (1999–2004), and it used to be warm, shallow, turbid and predominantly brackish. The SOM model successfully identified the changing patterns of limnology in the lagoon using the monthly limnological dataset from 30 study sites (July 1999–December 2004). Comparative re-sectoring evaluation of current monitoring sites was accomplished based on the outcome of the modeling. The new site clustering that emerged from the model was similar to conventional ones, and several sites were reorganized. Water physicochemistry was affected by freshwater inflow during monsoon and the new lagoon mouth constructed in September 2000, which resulted in variations in site characteristics in terms of limnology. The results of this study may provide information on the limnological patterns in Chilika lagoon, and they leave room for further study into functional changes in the lagoon ecology with respect to changes in climatic factor, freshwater flow and lagoon morphology.