Tropical Estuary Research Articles

Ecological habitat quality assessment of a tropical estuary using macrobenthic functional characteristics and biotic indices

Ecological habitat quality assessment of a tropical estuary using macrobenthic functional characteristics and biotic indices

Are tropical estuaries a source of or a sink for marine litter? Evidence from Sabaki Estuary, Kenya

Litter surveys were carried out in August and September 2020 to determine the contribution of Sabaki River and estuary in modifying the quantities of litter entering the oceans. The river discharged 0.035 items m−3s −1 translating to an estimated annual litter flux of between 6,622,560 and 614,952,000. The surveys in the estuary revealed that plastics contributed 90.8% of the total litter. Wet and dry zones had mean litter accumulation rates of 2.7 ± 1.1 and 4.4 ± 3.5 items m−1 day−1 respectively. 69.8% and 77.4% of branded litter were of Kenyan origin and food packaging material respectively. The litter turnover was slightly higher in the dry beach zone compared to the wet zone with a Whitakker Beta diversity of 0.36 and 0.33 respectively. Sabaki estuary acted as a sink for litter during flooding (through burial) and as a source (through exposure of buried litter due to wind and rain action).

Effects of Extreme Climatic Events on the Hydrological Parameters of the Estuarine Waters of the Amazon Coast

Oscillations in rainfall levels are one of the principal factors that determine seasonal shifts in physicochemical and hydrobiological variables, and exceptional climate events can have an enormous effect on the characteristics of small tropical estuaries. Given this, the present study investigated the responses of the physiochemical and hydrobiological variables of a small, shallow, and well-mixed Amazonian estuary with absence of any direct fluvial discharge to different climatic scenarios (typical conditions, and drier events). The study was based on 21 field campaigns conducted between April 2012 and April 2017. During rainy seasons of typical condition, when rainfall levels were higher and salinity lower than 7 (just after a weak La Niña), phytoplankton biomass was affected by increased turbidity (> 400 NTU) of the water, even when nitrogenous compounds were readily available (ammonium: 3.39 ± 1.71 µmol L−1, nitrite 0.90 ± 0.80 µmol L−1, and nitrate (3.91 ± 1.86 µmol L−1). In addition, the absence of any direct fluvial discharge appearance has contributed to the cumulation of phytoplankton biomass (chlorophyll-a > 20 mg m−3) into the estuary during the rainy season of the drier events and typical conditions, when turbidity was lower (< 200 NTU). Conversely, during the dry seasons of the drier events, when rainfall levels were lower, and extremes of salinity were recorded (> 38), phytoplankton biomass (chlorophyll-a < 10 mg m−3) was affected by the lower levels of nitrogenous compounds, even when phosphorous compounds were largely available. Overall, the present study has shown that extreme oscillations in rainfall levels cause abrupt shifts in the physicochemical parameters of the estuary, and affected the phytoplankton biomass, although high turbidity appeared to limit the buildup of this biomass. These effects are manifested immediately in the study area, given that the Taperaçu is a small estuary with a limited freshwater input, and it would seem reasonable to expect similar processes in other minor Amazonian estuaries when subjected to extreme fluctuations in rainfall levels.

Carbon dynamics and CO2 and CH4 exchange in the mangrove dominated Guayas river delta, Ecuador

Although estuaries are considered important pathways in the global carbon cycle, carbon dynamics in tropical estuaries is relatively understudied. Here, the tidal, seasonal and spatial variability of particulate organic carbon (POC), dissolved inorganic carbon (DIC), carbon dioxide (CO2) and methane (CH4), among other biogeochemical variables related to carbon cycling, were studied in the Guayas river delta (Ecuador) to document the sources, processing and fluxes of these carbon forms. All variables were studied during a semi-diurnal (13 h) tidal cycle and along river transects at low and high tides, all carried out during one dry and rainy season. POC and total suspended matter (TSM) strongly covaried and peaked at high tidal flow velocities during a tidal cycle and at high river discharge during the rainy season, suggesting that resuspension of bottom sediments and/or surface erosion in the river catchment were a dominant source of particulate matter in the water column. The δ13C of POC, (from ∼-22‰ to ∼-27‰) showed an increasing contribution of marine phytoplankton to the POC pool as moving downstream along the delta during the dry season. Upstream DIC concentrations (∼1200 μmol L−1) were high in the Guayas river delta as compared to other tropical estuarine systems, and the δ13C of DIC revealed a shift from a more phytoplankton dominated source in the dry season and downstream (∼-4‰) to a relatively more terrestrial source in the rainy season and upstream (∼-12.5‰). Both DIC and its δ13C showed slight but consistent deviations from conservative mixing that hint at inputs of 13C depleted DIC from mineralization along the delta. High values of the partial pressure of CO2 (pCO2) observed upstream and in the rainy season (∼5250 μatm), associated with O2 undersaturation (∼60%) and low δ13CDIC, suggest a strongly heterotrophic system, and resulted in high CO2 efflux to the atmosphere. CH4 concentrations were also higher during the rainy than dry season (93.5 ± 62.5 vs. 61.3 ± 39.5 nmol L−1), but unlike pCO2, showed tidal variations similar to TSM and POC, thus alluding to potential CH4 release from sediments during resuspension events at high tidal flow velocities. This explorative survey revealed complex drivers and biogeochemical processes acting upon various spatio-temporal scales which are necessary to consider for a complete understanding of the carbon biogeochemistry in estuarine systems. Similar surveys on estuarine carbon in data scarce regions are encouraged to constrain uncertainties in coastal zone carbon budgets.

Seasonal carbonate system vis-à-vis pH and Salinity in selected tropical estuaries: Implications on polychaete diversity and composition towards predicting ecological health

Salinity and pH play a fundamental role in structuring spatial patterns of physical properties, biota, and biogeochemical processes in the estuarine ecosystem. In this study, the influence of salinity-pH gradient and carbonate system on polychaete diversity in Ennore, Uppanar, Vellar, and Kaduvaiyar estuaries was investigated. Water and sediment samples were collected from September 2017 to August 2018. Univariate and multivariate statistical analyses were employed to define ecological status. Temperature, salinity, pH, and partial pressure of carbon-di-oxide varied between 21 and 30°C; 29 and 39 ppt; 7.4 and 8.3; and 89.216 and 1702.558 µatm, respectively. PCA and CCA results revealed that DO, chlorophyll, carbonate species, and sediment TOC have a higher influence on polychaete community structure. Forty-two species such as Ancistrosyllis parva, Cossura coasta, Eunice pennata, Euclymene annandalei, Lumbrineris albidentata, Capitella capitata, Prionospio cirrifera, P. pinnata, P. cirrobranchiata, and Notomastus sp. were found dominantly in all estuaries. Shannon index values ranged between 1.619 (UE-1) and 3.376 (VE-2). Based on these findings, high levels of carbonate species and low pH have a greater impact on polychaete diversity and richness values. The results of the AMBI Index revealed that stations UE-1, UE-2, UE-3 in Uppanar, EC-1, EC-2 in Ennore indicate “moderately disturbed”, while other stations are under the “slightly disturbed” category. This trend was quite evident in M-AMBI as well.

Spatial distributions and risk assessment of heavy metals and PAH in the southwestern Nigeria coastal water and estuaries, Gulf of Guinea

Spatial distributions and risk assessment of heavy metals (HM) and polycyclic aromatic hydrocarbon (PAH) in water and sediments of two southwestern Nigeria estuaries (Awoye A1-A6 and Abereke: AB1-AB7 and the adjoining coastal waters: I1–I12, C1–C7) were investigated. The physicochemical characteristics were measured in situ, while the HM and PAH were determined using atomic absorption spectrophotometer and gas chromatography coupled with flame ionization detection. The physicochemical characteristics of the water body were within the World Health Organization and United State Environmental Protection Agency stipulated limits for coastal water, except for pH (<6) and dissolved oxygen (<5 mg/l) attributed to a high level of human activities at the impacted stations (I1–I12). The HM in the water samples were within the permissible limits for coastal water bodies except for high Pb values (>1 mg/l) at the Awoye estuary linked to point source effluents of petroleum spills from downstream petroleum transportation. The calculated sediment contamination factor (CF) for HM indicated a considerable (3 < CF < 6) contamination of Cu (CF–Cu) at the sampling stations. Whereas the CF-Cr were within the moderate contamination range (1< CF < 3). The pollution load index (PLI) and ecological risk index (RI) suggest low-risk pollution of heavy metals to the aquatic ecosystems. The assessment of PAH revealed a dominant high molecular weight PAH composition in both surface water and sediments, mainly dominated by 4–6 rings PAH. The PAH diagnostic ratio BaA/(Ba + Chry) and IP/(IP + BghiP) points to the mixed (pyrogenic and petrogenic) origin of PAH across the two tropical estuaries and the coastal waters.

Study of trace metal contamination and ecological risk assessment in the sediments of a tropical river estuary, Southwestern India

The present study aims to assess the extent of trace metal pollution in the sediments of Sita-Swarna estuary, west coast of India, and investigate their possible ecological risk on the aquatic environment. The sediment cores were analyzed for sand, silt, clay, organic carbon, and trace metals (Al, Fe, Mn, As, Cd, Co, Zn, Pb, Ni, Cr, and Cu) at 2-cm intervals. The study revealed that sediments have deposited in relatively violent to very violent hydrodynamic energy conditions. Factor analysis indicated that the metal distribution is mainly controlled by Fe–Mn oxyhydroxides and organic carbon. Further, the geochemical approach, pollution indices, and statistical evaluation revealed moderate pollution in the catchment. From an ecotoxicological perspective, the estimated risk index (RI) value was found to less than 150, indicating low risk for aquatic life. Thus, this baseline study would help to adopt strategies in pollution control and protect the fragile marine environment.

Phytoplankton community structure is governed by salinity gradient and nutrient composition in the tropical estuarine system

Primary production in the estuarine ecosystems is principally represented by phytoplankton. The presence or function of phytoplankton in estuarine production is mainly regulated by factors such as salinity, light, turbidity, temperature, nutrients, grazers and the dynamics of the water basin. In order to understand the impact of salinity and hydrological characteristics on phytoplankton community structure in tropical estuaries, surface water samples for phytoplankton and biogeochemical parameters were collected from 26 estuaries along the Indian coast during non monsoon/no discharge period. Large spatial gradient in salinity was observed within the estuary (upper, middle and lower regions of estuary) from 2 to > 15 due to sea water intrusion through tidal currents. A higher concentration of suspended particulate matter (SPM) is observed in the lower estuarine regions, such as Tapti, and Narmada due to higher tidal amplitude. Higher phytoplankton biomass/abundances in upper estuarine region is associated with higher concentration of nutrients, and lower salinity whereas higher SPM, grazers and salinity is noticed associating with lower phytoplankton abundance in lower estuarine region. Phytoplankton community structure is found to vary significantly with reference to salinity as green algae and diatoms were dominated in the low saline upper estuary, diatoms and blue-green algae were abundant in the moderate saline region of middle estuary and diatoms alone were prominent in the high saline lower estuarine region. Moreover, diatoms in the upper estuarine region is exhibiting high heterogeneity with Melosira spp., Thalassiosira spp., Navicula spp. and Nitzschia spp. while the middle estuary and lower estuary were homogenous, mainly dominated by Thalassiosira spp. Higher species diversity (lower species dominance) was observed in upper estuary due to the diverse taxa of green algae and blue-green algae along with diatoms, whereas lower diversity was noticed in the lower estuarine region. Present study reveals that availability of light and grazing pressure is predominately governing phytoplankton abundance, while variations in nutrient concentrations and salinity is more influencing phytoplankton community structure.

Application of multiple indices to the evaluation of trophic and ecological status in a tropical macrotidal estuary (Equatorial Margin, Brazil)

ABSTRACT The Paciência river estuary (PRE) is an Amazonian estuary with relevant socio-economic importance; however, it is undergoing a continuous anthropogenic pressure such as the input of untreated domestic effluents. Hence, the phytoplankton community and trophic indices were analysed during three seasons in 2017 to assess the water quality and understand the degree of human influence in a macrotidal estuary. The trophic state of the PRE was determined by applying the trophic state index (TSI), trophic index, and Karydis trophic index, and ecological status by diversity metrics (diversity, evenness, richness, and dominance). Multivariate analysis indicated that TSI and dominance index were the most sensitive indices to changes in environmental conditions. The precipitation regime characterised the estuary in two different trophic scenarios. PRE was hypereutrophic in the dry season and mesotrophic in the rainy/transitional seasons based on the TSI. A high dominance index was recorded in the dry season with blooms of Thalassiosira rotula, reflecting an ecological disturbance. A higher phytoplankton diversity was recorded in the rainy/transitional seasons, indicating a tendency toward a lower stressed ecosystem. Therefore, understanding the trophic state variability in tropical estuaries and its response to changes in phytoplankton ecology is essential to support the management of coastal ecosystems.

Assessment of the trophic status in a tropical estuarine system

Estuaries are extremely valuable ecosystems and have been affected by several anthropogenic activities from urban growth and industrial development, such as the release of untreated efuents which decrease water quality. This study aimed to evaluate the seasonal and temporal trophic status variation in the Formoso River Estuarine System (FRES), a tropical estuary. Three campaigns were carried out in each season (rainy and dry), during which surface water was sampled for abiotic parameter analyses. The TRIX index was used for trophic status assessment. For the temporal evaluation, a review of literature addressing the matter in the study area was conducted. Between 2017 and 2018, the TRIX index varied from a mean of 6 ± 0.2 upstream to 5 ± 0.4 downstream during the rainy season, indicating eutrophic conditions throughout most of the FRES extension, especially near the Formoso River, and from 6 ± 1 to 3 ± 0.8 during the dry season. Temporal analyses showed that in 2002 the overall mean TRIX index was 5.72 ± 0.4, decreasing in 2005 and 2014, and followed by an upwards trend. We conclude that the FRES is impacted by the release of untreated efuents such as sewage and fertilizers from the city of Rio Formoso and from various agricultural activities, which are more intense during the rainy season. Nevertheless, over time there were more critical scenarios pertaining to the trophic status. The present diagnosis reinforces the importance of public sanitation policies in estuarine regions and spatial planning instruments for pollution control in fishery areas.

The estimation of total suspended matter from satellite imagery of tropical coastal water Berau Estuary, Indonesia

Water in coastal and estuary areas needs to be investigated as human activities is allegedly decrease their quality. This has led to optical remote sensing for mapping optical water quality using empirical and semi-analytical approaches. Therefore, this study aims to estimate the Total Suspended Matter (TSM) concentration from Medium Resolution Imaging Spectrometer (MERIS) data using a spectral unmixing approach combined with a radiative transfer model. This approach was implemented in the equatorial tropical coastal water, the Berau estuary, Indonesia, by generating a look-up table (LUT) of top-of-atmosphere radiance spectra through the coupled forward models, and the endmembers were selected from the LUT. The spectral unmixing algorithm was employed to the MERIS data for decomposing the image into haze and sediment components. The data were then transformed into images of a constant haze level corresponding to 50 km visibility, and the atmospheric correction was applied. Furthermore, the TMS concentration was retrieved using the inverse semi-analytical Kubelka-Munk model. The result gave overestimated TSM concentration values on clear waters. However, in turbid waters, a lower RMSE was obtained, and the coefficient of determination was higher than in clear waters.

Blue Carbon Storage in a Tropical Coastal Estuary: Insights for Conservation Priorities

Blue Carbon Storage in a Tropical Coastal Estuary: Insights for Conservation Priorities

Spatio-temporal variation in the density and diversity of decapods captured with artisanal traps in an Amazon estuary

Abstract: Aim The variability in density and species diversity of decapod crustaceans was investigated on four islands with different degrees of anthropogenic disturbance around the city of Belém, State of Pará. Methods Samples were obtained from 15 creeks using artisanal traps, every three months between October 2013 and May 2014 on Combu, Onças, Cotijuba and Mosqueiro islands. Results Salinity and temperature little varied, which is common for a tropical Amazon estuary strongly influenced by freshwater inflow. A total of 8,367 decapods were captured, with one record of an exotic species Penaeus monodon. In all seasons, decapod density and richness tended to increase from Combu to Mosqueiro, with increasing proximity to the sea and higher salinity and pH. Except for Combu, species richness and Margalef diversity tended to be slightly greater in the wet season at all islands, especially Onças. Eveness and Shannon diversity did not vary greatly between seasons but were lowest at Onças in the dry season and highest at Combu, decreasing to Mosqueiro, in the wet season. In general, ecological indices are similar in the dry and transition dry to wet seasons, and in the wet season, dominance occurs at Mosqueiro Island. Macrobrachium acanthurus, C. bocourti and P. gracillis were associated with the wet season, whereas M. surinamicum prefers the dry season. M. amazonicum and Macrobrachium sp. have no well-defined seasonal pattern of occurrence at all the islands. Conclusions Despite anthropogenic disturbances and proximity to large human populations, especially on Mosqueiro Island, the density and diversity of decapod crustaceans appear to be reasonably unaffected for the moment.

Feeding ecology, trophic interaction and resource partitioning among four omnivorous finfish species of a tropical Estuary

A crucial aspect of sustainable resource management is understanding the trophic interactions amongst fish in the estuarine ecosystem. The goal of this study was to look into the food preferences, feeding strategies, trophic partitioning, as well as dietary overlap among four omnivorous species that live in the Cochin Estuary: Mugil cephalus (n = 73), Planiliza parsia (n = 35), Planiliza planiceps (n = 65) and Chanos chanos (n = 55) through the analyses of gut-content. Index of relative importance demonstrated that prey items in Mugil cephalus and Planiliza planiceps guts were dominated by Bacillariophyceae whereas in Planiliza parsia and Chanos chanos guts were dominated by Myxophyceae. The highest diet value (7.5) was recorded in Chanos chanos, while the lowest was recorded in Mugil cephalus (5.69). The highest niche breadth value (0.77) was recorded in Planiliza planiceps while the lowest value was recorded in Planiliza parsia (0.52). Pianka's overlap, evaluated with the help of null models structured by Ecosim 7.0, showed remarkable niche overlap between Mugil cephalus and Planiliza planiceps (0–0.92, P &lt; 0.001), between Planiliza planiceps and Planiliza parsia ( 0–0.77, P &lt; 0.05) and between Mugil cephalus and Planiliza parsia ( 0–0.7, P &lt; 0.05). The fish's trophic niche width, along with prey-specific abundance confirmed that they are generalist feeders. The present results differed from the general hypothesis related to the omnivorous species and concluded that these four omnivorous species live in the same niche zones of the Cochin Estuary with no interspecific conflict.

Impact of a Megacity on a Tropical Estuary Assessed Using a Combination of Chemical Analysis and In-Vitro Bioassays

Impact of a Megacity on a Tropical Estuary Assessed Using a Combination of Chemical Analysis and In-Vitro Bioassays

Fish biodiversity of a tropical estuary under severe anthropic pressure (Doce River, Brazil)

Abstract The Doce River has undergone severe changes over the last centuries (e.g., flow regulation, pollution, habitat and species loss). Here, we present the first comprehensive fish biodiversity assessment of the Doce River estuary and a summary of the main impacts and their drivers for the whole river since the early 18th century. Carangiformes, Siluriformes and Eupercaria incertae sedis were the most representative orders for the 115 species recorded. Most species are native (87.8%), euryhaline/peripheral (80%) and zoobenthivorous (33.9%). Threatened (Paragenidens grandoculis, Genidens barbus, and Lutjanus cyanopterus) and near threatened (Cynoscion acoupa, Dormitator maculatus, Lutjanus jocu, Lutjanus synagris, and Mugil liza) species are peripheral. Thirteen species are exotic at the country (Butis koilomatodon, Coptodon rendalli, and Oreochromis niloticus) or the basin level (e.g., Pygocentrus nattereri and Salminus brasiliensis). The catfish Cathorops cf. arenatus is reported for the first time on the eastern coast of Brazil and Paragenidens grandoculis, considered extinct in the Doce River, was discovered in the estuary.

Responses of a macrobenthic community to seasonal freshwater flow in a wet-dry tropical estuary

Wet-dry tropical estuaries are extreme environments driven by wet season rainfall and runoff events. The biota associated with these systems are highly adapted to large fluctuations in salinity. There are few examples globally where water extraction for human needs has not yet had a major impact on freshwater flow volumes and seasonal variability in these systems. Therefore, this study examined the importance of seasonally variable flow on intertidal macrobenthic (>0.5 mm mesh size) abundance and species composition in a pristine, wet-dry tropical estuary and nearshore environment. The focus was on intertidal mud- and sandflats where macrobenthos are known to be an important food source for endangered migratory shorebirds and fisheries species in the region. Macrobenthos on the intertidal flats were sampled on four occasions spanning the late-dry, late-flood, and early-dry periods across four years. Across all sites, total macrobenthic abundance was significantly lowest during the late-flood period, when the salinity was low, and highest in the late-dry season, under hypersaline conditions. In the muddy estuarine intertidal flats, the dominant macrobenthic group was polychaetes. In contrast, in the sandy nearshore intertidal flats, bivalves dominated in the late-dry and early-dry. However, during the late-flood, when salinities were at their lowest, the nearshore intertidal flats experienced almost complete loss of bivalves. Although flow had short-term negative effects on some species, it is clear that macrobenthos recruitment back into the intertidal flats had begun once the tide re-established in the early-dry. The differences in the flood response between polychaetes and bivalves likely reflects differences in their life history strategies, with the burrowing and swimming behaviour of polychaetes likely to be providing the ability to seek refuges during low salinity periods. Macrobenthic abundance was also correlated with benthic chlorophyll a concentrations, likely reflecting that both primary and secondary production is impacted by flow. This study suggests that macrobenthos abundance, and species dominance is driven by the seasonal changes in flow in this pristine system, demonstrating the important role of freshwater inputs.

Nutrient dynamics in a tropical estuary: a microcosm approach

Nutrient dynamics in a tropical estuary: a microcosm approach

Spatial and temporal variation of salinity stratification in a tropical estuary

Longitudinal and vertical distribution of salinity field in the Vellar River Estuary (VRE) was investigated across the lower, mid and upper estuarine zone, monthly from January to December 2011. The hydrological survey of VRE revealed shallow bathymetry with a complex topography and a mean tidal amplitude of around 0.9 to 1 m. The depth-averaged tidal velocity had a magnitude that was observed with a maximum value of 0.56 m/s at the estuarine mouth and was indirectly proportional to the increasing distance from the mouth. The wind speed fluctuated from 1 to 4.1 km h-1 with a mean of 2.5±0.82 km h-1. The temperature ranged from 25.32 °C to 32.93 °C with a mean of 28.89±2.26 °C while salinity varied between 0 to 34 psu with a mean of 13±12.77 psu and negatively correlated with rainfall (r = -0.69; p = 0.04). A higher stratification number, i.e. ns ~ 1 at the lower estuarine zone indicated the formation of the salt wedge in the monsoon while during the rest of the year the estuarine water column was observed to be homogeneous. Wave height, tide and currents were estimated through 2D hydrodynamic models and they were significantly correlated with in-situ data. Recorded seasonal variation in salinity stratification of VRE from the present work reflected on shaping the primary governing factors on estuarine circulation at spatial and temporal scales.

Niche overlap of benthic macrofauna in a tropical estuary: diurnal variation

The complexity of estuaries allows for the establishment of diverse communities composed of species with different survival strategies. The vertical migration of animals in the sediment is linked to competition, escape from predators and adaptations to diurnal physio-chemical changes related to variations in water levels. The present study aimed to evaluate niche overlap and amplitude, as well as the composition and structure, of communities of polychaetes and molluscs between sediment aliquots during the day and at night. Data sampling was performed in the Tubarão River estuary. The highest individual occurrence was registered during the diurnal period. Communities of polychaetes varied significantly between sediment aliquots during the day and at night, while molluscs did not show diurnal variation. Niche overlap results for polychaetes showed higher values between aliquots during the night, while molluscs showed greater overlap during the day. This indicates that polychaetes and molluscs have different mechanisms of coexistence. This may be related to different attributes of species allowing for the division of resources among individuals. Examination of niche overlap provides insights into coexistence of mechanisms within benthic macroinvertebrate communities.