Sedimentation modified by wind induced resuspension in a shallow tropical lagoon (Cote d'Ivoire)

In shallow environments, under certain conditions of fetch, wind velocity, bathymetry and bottom characteristics, resuspension can be generated by wind induced waves. In the tropical Ebrié lagoon, austral trade winds are dominant almost all year long, and their velocity shows a marked diel pattern with maximum speed between noon and midnight. Only austral trade winds with a speed >3 m s⁻¹ allow particle resuspension which is effective for depths<1.5 m. In these areas, significantly higher values of chlorophyll biomass and mineral seston are noted during the windy sequences. Granulometric and mineralogical analyses showed that only the surficial sediment (0–3 cm) was involved in resuspension. This process induces several effects: 1) an increase of the suspended matter concentration in the water and thus a light attenuation due to a higher turbidity, 2) a redistribution in the whole water column of nutrients from the pore water and 3) a removal of the finer fractions from the superficial sediment. On the contrary, for depths>1.5 m, particle sinking is permanent in depressions which are spontaneously transformed into anoxic systems. At the lagoon scale, sedimentation is significantly modified by wind induced resuspension. According to the bathymetry and the distance from a river, three sedimentary facies are recognized. Their grain size distributions are parabolic in areas where resuspension occurs, logarithmic in areas where no resuspension is possible and hyperbolic in the hollows and the main channels. Finally, a large part of the allochthonous inputs (from drainage and rivers) and autochthonous pelagic production is trapped into the Ebrié lagoon and less than 10% of the particles entering the lagoon are exported toward the Atlantic Ocean.     

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.     

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.     

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.     

Influence of suspended particulate matter on nutrient biogeochemistry of a tropical shallow lagoon,Chilika, India

The behaviour of suspended particulate matter (SPM), salinity profile, dissolved nutrients, total (T.Chl-a) and size fractionated chlorophyll-a (F.Chl-a) were studied seasonally at Chilika Lagoon, east coast of India, during 2008–2009. The study showed large spatio-temporal variations among these parameters. The concentration of dissolved inorganic nitrogen and inorganic phosphate were found to be maximum during the monsoon, followed by post- and pre-monsoon, although the mean N:P ratios, which indicate the relative availability of N with respect to P, were 9.13 ± 3.09, 16.57 ± 11.53 and 5.47 ± 3.13, respectively. It was evident from the results that during pre-monsoon and postmonsoon, the lagoon exhibits nitrogen limitation. Mean T.Chl-a biomass in the lagoon showed distinct seasonality with maximum values during the pre-monsoon (23.12 ± 9.75 mg m^?3) followed by monsoon and post-monsoon. Irrespective of seasons, maximum T.Chl-a was found in the northern part of the lagoon. SPM concentrations during the monsoon were relatively higher in the freshwater dominated zones compared to seawater dominated areas, indicating its riverine sources. The correlation between SPM and various dissolved nutrients (p < 0.05) suggests its influence on the physico-chemical conditions at varying levels. It is summarized that seasonal variation of SPM and nutrients contributed by rivers, wind induced re-suspension events and in situ  regeneration processes play a crucial role in the lagoon biogeochemical cycle.     

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.     

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).     

一座热带浅水水库敞水区浮游甲壳动物群落结构

为了解热带中小型浅水型水库枯水期浮游动物群落组成与结构稳定性,于2006至2007的三个枯水期,对位于我国雷州半岛甘村水库敞水区进行了每周一次的高频采样,主要以个体大小作为变量分析了浮游甲壳类动物的群落结构的组成.三次枯水期采样中,敞水区中只检测到枝角类4种和桡足类4种.浮游甲壳类的体长范围多集中在400～700μm之间.枝角类的优势种为模糊秀体溞平均体长为533μm,桡足类成体优势种台湾温剑水蚤的平均体长为787μm,无节幼体和剑水蚤桡足幼体丰度与生物量也相对较高.水库浮游甲壳类种类数和丰度的峰值均出现在体长为300～400μm和400-500μm两个等级内.浮游甲壳类的种类和数量均是以小个体为主,除了营养水平还与水体中的捕食压力紧密相关,鱼类的捕食对于浮游甲壳类群落结构的形成作用显著.     

Resilience of a zooplankton community subjected to marine intrusion in a tropical coastal lagoon

One of the main disturbances on Imboassica Lagoon is the occasional artificial opening of its sand bar. Following two such events, two environmental gradients were observed. One was directly related to entry of marine water (salinity gradient); and the other to a decrease in dilution of the nutrient load, because of water level reduction (trophic status gradient). Two stations were sampled. At Station 1, located near the sand bar, salinity increase caused a decrease in the total zooplankton density and a shift in community composition due to a loss of relatively small individuals (i.e. rotifers) and the subsequent entrance of larger ones (i.e. copepods). High diversity was related to salinity increases. At Station 2, located near the mouth of a sewage canal, the total zooplankton abundance and dominance were related to the salinity increase, while the highest richness and the lowest dominance were obtained at a high trophic state. The zooplankton community showed high persistence and resilience, which together with other ecological features in the system, returned to the pre-disturbance state 2 months after the sand bar was closed. Canonical correspondence analysis was a useful tool to assess system resilience.     

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.     

Trophic functioning and nutrient flux in a highly productive tropical lagoon

Chiku Lagoon is a highly productive tropical lagoon with high fishery yields. Trophic networks and stoichiometrically linked water-salt-nutrient budgets were constructed to relate the functioning of the food web to nonconservative behavior of nutrients in the lagoon. Network analysis showed that the lagoon is more dependent on phytoplankton than detritus and periphyton to generate food sources for consumers. Nevertheless, detritivory is more important than herbivory in the food web. Transfer efficiency is high at low trophic levels, but declines at higher levels due to the high fishery pressure. Thus, only a small fraction of organic matter (15%) is recycled, and this all through detrital pathways, most of which involve only two compartments. Summation of individual rate measurements for primary production and respiration yielded an estimate of +249 g C m^-2 year^-1, suggesting an autotrophic ecosystem. An alternative biogeochemical approach demonstrated that the lagoon is a large sink for total dissolved nitrogen and phosphorus, and the net system metabolism was calculated to be +144 g C m^-2 year^-1, thus providing a biogeochemical explanation for the high productivity of Chiku Lagoon. Our results suggest that the high fishery yield in Chiku Lagoon can be attributed to high planktonic productivity induced by the high rate of nutrient loading, and the straight-through pathways of the food web.     

Temporal and spatial fluctuations of phytoplankton in a tropical coastal lagoon, southeast Brazil.

Spatial and temporal variability of the phytoplankton community in the tropical coastal Imboassica lagoon, an environment naturally isolated from the ocean by a narrow sandbar, was analysed every two weeks for 19 months by sampling three sites. During this study, the lagoon received direct input of marine water three times, resulting in remarkable salinity, nutrient concentrations and phytoplankton biomass variations in both temporal and spatial aspects. The phytoplankton biomass presented relatively low values ranging, on average, from 0.54 mg x L(-1) in the station closest to the sea (station 1) to 1.34 mg x L(-1) in the station close to a macrophyte bank (station 3). Diatoms and cryptomonads dominated in stations 1 and 2 (located relatively close to station 1, yet receiving the runoff of domestic sewage), and euglenoids, cryptomonads and dinoflagellates at station 3. Stations 1 and 2 usually presented the same dominant species but station 2 presented a higher phytoplankton biomass. On the other hand, station 3 showed more similar results concerning phytoplankton biomass with station 2, however the dominant species were usually different. The high fluctuations of salinity and the reduced nutrient availability are pointed out as the main factors structuring the dynamics of the phytoplankton community at the Imboassica lagoon.     

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.     

Spatial, seasonal and diel distribution patterns of two species of mojarras (Pisces: Gerreidae) in a Mexican tropical coastal lagoon

Spatial, seasonal and diel distribution patterns were analysed over a 2‐year period for Diapterus auratus and Eucinostomus melanopterus in Pueblo Viejo lagoon, a tropical coastal lagoon in Veracruz, Mexico. A total of 216 samples were collected with 329 individuals of D. auratus and 170 of E. melanopterus. Diapterus auratus individuals were captured throughout a wider range of environmental conditions, and consequently its spatial niche breadth was significantly greater (P < 0.001) than that of E. melanopterus. In general, more individuals were captured at localities with submerged vegetation (Ruppia maritima), with 70% for D. auratus and 87.2% for E. melanopterus, although only the latter species showed significant spatial differences. A spatial segregation influenced by distance from the lagoon inlet and salinity was observed between the species. It is considered that this segregation may allow resource partitioning and minimize interspecific competition. A relatively low niche overlap (4.9%) was observed for the two species. Seasonal capture peaks of D. auratus were related to rainfall and the productivity pattern in the lagoon system. Seasonal fluctuations in monthly mean number of D. auratus were correlated with preceding variations in local rainfall (r = 0.86, P < 0.02). Moreover, seasonal capture peaks of D. auratus coincided with ecosystem primary production peaks. Although E. melanopterus exhibited no significant seasonal differences, its maximum numbers also coincided with an ecosystem primary production peak and recruitment patterns. In 24‐h cycle analysis, both species were captured mainly around mid‐day, with minor peaks at dawn and/or at dusk. Generally, few individuals were collected during hours of darkness. It was proposed that diel abundance variation of gerreids might be related to both light penetration in the water column and feeding activity. Food availability within Pueblo Viejo lagoon appears to be an important factor influencing fish abundance.     

Macrobenthic communities in a tropical lagoon (Tahiti, French Polynesia, central Pacific)

Soft bottom communities were sampled quantitatively in Tahiti lagoon (French Polynesia) at 18 stations in five zones around the island over 1 year. In addition, various environmental parameters (silt/clay fractions, organic content, chlorophyll and phaeopigment content) were sampled at the same stations over 2 years. The temporal and spatial variabilities of the macrobenthic communities are described and related to these environmental parameters. Each zone ran from the fringing reef to the inner flat of the barrier reef. The macrofauna exhibited a high richness (392 taxa) with an average mean biomass of 1.8 g AFDW m⁻² (grams ash-free dry weight per square metre). These communities exhibited temporal but not seasonal fluctuations. The biomass of the macrofauna increased from the fringing to the barrier reefs, and the density of individuals was significantly higher on the fringing reefs. Ordination techniques highlighted four groups of stations characterised by distinctive species composition, density and biomass. The first group included stations located on the inner flat of the barrier reef and in the shallow lagoon area and was characterised by highly diverse communities dominated by polychaetes. The second group primarily included stations from the industrial and hotel zones. This group had the lowest diversity and was also dominated by polychaetes, especially the capitellid Dasybranchus sp. 1. The third group was dominated by gastropods and bivalves. The final group of stations was represented only by station 51, in zone 5, which was characterised by mobile soft sediments and wave action and was dominated by the decapod Hippa cf. pacifica. Presumably, these mobile sediments facilitate the development of this benthic community, which does not occur elsewhere in the lagoon. The diversity and biomass of these benthic communities are low compared with most other areas in the South Pacific. Accepted: 10 May 2000     

Trophic ecology and food consumption of fishes in a hypersaline tropical lagoon

This study evaluated the trophic ecology (diet composition, trophic strategy, similarities and overlap between species, feeding period and food consumption) of six benthivorous fish species in Araruama Lagoon, the largest hypersaline tropical lagoon on the east coast of South America, with an area of 210 km² and an average salinity of 52. The burrfish Chilomycterus spinosus fed on Anomalocardia flexuosa shell deposits, ingesting associated fauna. The caitipa mojarra Diapterus rhombeus differed from all other species, having not only the highest proportions of algae and Nematoda, but also feeding on polychaete tentacles. The two mojarras Eucinostomus spp. showed similar trophic strategies, feeding mostly on Polychaeta. The corocoro grunt Orthopristis ruber also fed mainly on Polychaeta, but differed from Eucinostomus spp. in secondary items. The whitemouth croacker Micropogonias furnieri fed mainly on small Crustacea at night, showing a high number of secondary prey items with low frequencies and high prey‐specific abundance. The daily food consumption (g food g^?1 fish mass) for Eucinostomus argenteus was 0·012 and was 0·031 and 0·027 for M. furnieri in two different sampling events. The diet similarities between Araruama Lagoon and other brackish and marine environments indicate that hypersalinity is not a predominant factor shaping the trophic ecology of fishes in this lagoon. The stability of hypersaline conditions, without a pronounced gradient, may explain the presence of several euryhaline fishes and invertebrates well adapted to this condition, resulting in a complex food web.     

Variability in benthic diazotrophy and cyanobacterial diversity in a tropical intertidal lagoon

Benthic nitrogen fixation has been estimated to contribute 15 Tg N year(-1) to the marine nitrogen budget. With benthic marine nitrogen fixation being largely overlooked in more recent surveys, a refocus on benthic diazotrophy was considered important. Variations in nitrogenase activity (acetylene reduction-gas chromatography) in a tropical lagoon in the western Indian Ocean (Zanzibar, Tanzania) were monitored over a 3-year period (2003-2005) and related to cyanobacterial and diazotrophic microbial diversity using a polyphasic approach. Different nitrogenase activity patterns were discerned, with the predominant pattern being high daytime activities combined with low nighttime activities. Analyses of the morphological and 16S rRNA gene diversity among cyanobacteria revealed filamentous nonheterocystous (Oscillatoriales) and unicellular (Chroococcales) representatives to be predominant. Analyses of the nifH gene diversity showed that the major phylotypes belonged to noncyanobacterial prokaryotes. However, as shown by cyanobacterial selective nifH-denaturing gradient gel electrophoresis analysis, cyanobacterial nifH gene sequences were present at all sites. Several nifH and 16S rRNA gene phylotypes were related to uncultured cyanobacteria or bacteria of geographically distant habitats, stressing the widespread occurrence of still poorly characterized microorganisms in tropical benthic marine communities.