Tropical Estuary Research Articles

Semi-empirical models for remote estimating colored dissolved organic matter (CDOM) in a productive tropical estuary.

Inland waters are important components of the global carbon cycle as they regulate the flow of terrestrial carbon to the oceans. In this context, remote monitoring of Colored Dissolved Organic Matter (CDOM) allows for analyzing the carbon content in aquatic systems. In this study, we develop semi-empirical models for remote estimation of the CDOM absorption coefficient at 400nm (aCDOM) in a tropical estuarine-lagunar productive system using spectral reflectance data. Two-band ratio models usually work well for this task, but studies have added more bands to the models to reduce interfering signals, so in addition to the two-band ratio models, we tested three- and four-band ratios. We used a genetic algorithm (GA) to search for the best combination of bands, and found that adding more bands did not provide performance gains, showing that the proper choice of bands is more important. NIR-Green models outperformed Red-Blue models. A two-band NIR-Green model showed the best results (R2 = 0.82, RMSE = 0.22m-1, and MAPE = 5.85%) using field hyperspectral data. Furthermore, we evaluated the potential application for Sentinel-2 bands, especially using the B5/B3, Log(B5/B3) and Log(B6/B2) band ratios. However, it is still necessary to further explore the influence of atmospheric correction (AC) to estimate the aCDOM using satellite data.

Occurrence of extended-spectrum β-lactamases-producing Escherichia coli isolates over gradient pollution in an urban tropical estuary.

Bacterial resistance to antimicrobials is a global public health problem that surpasses the human context and can be increased by pollution. However, the lack of systematic monitoring of resistance in some aquatic matrices, such as tropical estuaries, makes it unknown whether its occurrence is associated with anthropogenic pollution in these environments. Therefore, we investigated the occurrence of extended-spectrum beta-lactamases (ESBLs) producing Escherichia coli as a resistance indicator for 12 consecutive months at three representative points of a pollution gradient in Guanabara Bay (GB), Brazil. Sixty-six E. coli strains were selected from 72 samples of GB waters in the presence of ceftriaxone (8 μg mL-1 ) and identified by MALDI-TOF MS. Of the 66, 55 (83.3%) strains were ESBL producers. They carried beta-lactamase/ESBL genes, with the predominance of blaCTX-M (54, 98.2%), especially the blaCTX-M-1,2 allele (49.1%). These strains were detected frequently (81.8%) from the point with the highest pollution levels. Furthermore, the marker for Class 1 integron, intI1 gene, was detected in 54.5% of ESBL producers. These data suggest an association between antimicrobial-resistant E. coli and sewage pollution in aquatic environments raising concerns about the possible risks of human exposure to these waters and fish consumption.

Role of inter-annual variability in the magnitude of river discharge on CO2 fluxes in the Godavari tidal river zone, India

Variations in the magnitude of river discharge may alter pH, pCO2 of surface water, and flux of CO2 to the atmosphere in the Tidal River Zone (TRZ). To test this hypothesis, a nearly decade-long time-series data of salinity, pH, and pCO2 data were collected and analyzed in the TRZ of Godavari River. The higher freshwater discharge occurred in the TRZ during the wet period (June to December) compared to the dry period (January to May). An increase in annual mean discharge was noticed between 2008 and 2019 due to surplus rainfall and the expansion of the hydropower plant. The surface water pH and pCO2 in the TRZ were controlled by the river discharge as evidenced by the linear positive and negative relationships with the rate of river discharge respectively. A strong relationship was observed between apparent oxygen utilization (AOU) and excess CO2 indicating a strong influence of heterotrophic activity on pCO2 levels in the TRZ. The respiration quotient (RQ) was above the aerobic respiration value of 1.35 indicating that terrestrial sources, such as sewage, and groundwater discharge, may have contributed to pH and pCO2 levels in the TRZ. The CO2 fluxes during the peak discharge period contribute to 48–88% of annual mean fluxes followed by moderate (12–52%) and dry periods (<4%). The annual mean CO2 emission amounted to 0.04 ± 0.05 TgC for the area of 36.9 × 106 m2 and increased at the rate of 0.005 TgC y−1 between 2008 and 2019 due to an increase in river discharge. The CO2 flux from the Godavari TRZ (0.04 TgC y−1) contributes to ∼0.2% of global tropical estuaries (18 TgC y−1). The CO2 flux contributed to ∼1.5% of the total carbon (both particulate and dissolved) exported from the Godavari TRZ to the Bay of Bengal and it is lower than the global mean contribution of 16%.

Fish communities reveal the ecological quality of a small, temporarily closed tropical estuary in response to natural and anthropogenic interventions

Fish communities reveal the ecological quality of a small, temporarily closed tropical estuary in response to natural and anthropogenic interventions

Hydrodynamic Modelling: Estuary Dynamic Implication to Morphological Changes

Estuaries are transition zones between the sea and inland rivers, where oceanic tidal waves and fluvial flows control hydrodynamic processes. However, these natural changes and implications are not thoroughly understood, particularly in tropical estuaries. The interactions between tidal and riverine flows in the Kuala Pahang Estuary were examined using numerical modelling techniques applying TELEMAC2D. Our results model demonstrate that prolonged heavy rainfall significantly impacted water levels in the estuary system during the neap tide cycle. Moreover, the results show that the tidal range and current speed significantly influenced estuary morphological changes. The flood-ebb current can affect sediment transport near the river bank, leading to erosion. Meanwhile, the ebb-flood current significantly affected the bed sediment and suspension in the estuary mouth area while deposition happens during the slack water, either flood or ebb. Our results have a major impact on estuary morphology and sediment transport in the estuary water column, which will lead to a reduction in estuary water quality and the local economy, particularly fisheries.

Tracing terrestrial groundwater discharge and porewater exchange derived dissolved carbon export in a tropical estuary using multiple isotopes

Coastal terrestrial groundwater discharge and porewater exchange are two different submarine groundwater discharge (SGD) pathways. The contribution of both pathways in dissolved carbon export to the coastal water remains almost unknown. Here, we investigated terrestrial groundwater discharge, porewater exchange, and dissolved carbon export in a tropical estuary (Moyangjiang, China) using stable water (δ2H and δ18O) and radioactive (222Rn, 224Ra, and 228Th) isotopes. The average terrestrial groundwater discharge was estimated to be (3.2±0.2) × 106 m3 d−1 by a three end-member mixing model of δ18O and salinity. The porewater exchange was estimated to be 11.3±0.14 cm d−1 by the 224Ra/228Th disequilibrium method. Accordingly, dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) fluxes from terrestrial groundwater discharge were estimated to be (1.19±0.10) × 107 and (3.23±0.25) × 105 mol d−1, respectively. In comparison, when upscaling to the whole estuary, DIC and DOC export fluxes from porewater exchange were estimated to be (1.56±0.21) × 105 and (2.12±0.29) × 105 mol d−1, respectively. Terrestrial groundwater discharge derived DIC flux was two orders of magnitude greater than that from porewater exchange, and was three times greater than that from river discharge. Terrestrial groundwater discharge was recognized as a significant contributor to DIC export. Porewater exchange played a disproportionately important role in the delivery of DOC to the ocean, and its contribution to DOC needed to be emphasized. Overall, both pathways contributed a large amount of dissolved carbon export to the coastal water, and play an important role in delivering terrestrial carbon across land-ocean interface. Evaluating the contributions of different pathways of SGD to terrestrial solutes will help to improve the accuracy of SGD associated terrestrial solute fluxes.

Ecobiology and distribution of Streptomyces species from distinctive metal polluted sediment of tropical estuary, Northwest coast of India

The present study aims to isolate diverse actinomycetes communities from the estuarine sediments of Patalganga located at Northwest coast of India. Total 40 actinomycetes were isolated from 24 sediment samples by dilution plating on six different isolation media. Among them, morphologically 18 distinct selected isolates of actinomycetes were verified by 16S rRNA gene sequencing and identified as Streptomyces spp. The relationship between diversity of total actinomycetes population (TAP) and its antagonistic behaviour with the physico-chemical characteristics of sediment samples were investigated. Multiple regression analysis revealed that the influencing physico-chemical factors comprises of sediment temperature, sediment pH, organic carbon and heavy metals. The results of statistical analysis showed that TAP is positively correlated (p < 0.01) with sediment organic carbon whereas, negatively correlated with Cr (p < 0.05) and Mn (p < 0.01). Based on Principle component Analysis (PCA) and cluster analysis results, the obtained six stations can be divided into three groups. Among them mobile fraction of metals, TAP could be the main factor reflecting the lower and middle estuary. The recovery of large number of actinomycete isolates suggests that Patalganga Estuary could be potential source of bioactive compounds with biosynthetic capabilities.

Water Chestnut (Trapa natans L.): Functional characteristics, nutritional properties and applications in food industry: A review

An annual floating-leaved aquatic plant, the water chestnut (Trapa natans L.), is found in temperate and tropical freshwater wetlands, rivers, lakes, ponds, and estuaries. Hydrophytes that produce starch called water chestnut has the potential to serve as a reliable food supply, particularly in flood-prone wasteland areas. It is loaded with minerals and essential nutrients. Water chestnuts are not actually nuts, despite their name. They are aquatic tuber vegetables that may be found in shallow lakes, paddy fields, marshes, and ponds. Water chestnuts are indigenous to numerous islands in the Indian and Pacific oceans, as well as Southeast Asia, Southern China, Taiwan, Australia, and Africa. When the corm, or bulb, acquires a dark brown hue, they are picked. They are a typical ingredient in Asian recipes including stir-fries, chop suey, curries, and salads because of their crisp, white meat, which may be eaten raw or cooked. The huge, nourishing seed of the water chestnut, a native of Eurasia and Africa, has been widely collected since the Neolithic, and it is now grown for food throughout Asia. Water chestnuts have several advantages over other foods, including being particularly nutrient-dense and low in calories. A wonderful source of fibre, water chestnuts may aid in promoting bowel movements, lowering blood cholesterol, neuro-protective, controlling blood sugar levels, and maintaining the health of your gut. In addition, carbohydrates account for the majority of the calories in water chestnuts. Although they are abundant in fibre, potassium, manganese, copper, vitamin B6, and riboflavin, uncooked water chestnuts are 74% water, which means they are often low in calories. Due to its usage in the treatment of gastrointestinal illnesses, genitourinary system disorders, liver, kidney, and spleen disorders, Trapa natans is one of the most significant medicinal plants in Indian Ayurveda.

Assessment of eutrophication from phosphorus remobilization after resuspension of coastal sediments from an urban tropical estuary

Dredging activities cause sediment resuspension, which can change the bioavailability of nutrients such as phosphorus (P) in aquatic ecosystems due to remobilization. This study evaluated the remobilization of P in the solid and dissolved phase before and after sediment resuspension in the Meriti and Iguaçu River estuaries and the Rio de Janeiro and Niterói harbor in Guanabara Bay (Rio de Janeiro, Brazil). Three water and sediment samples were collected at each point. Dissolved phosphorus (DP), total phosphorus (TP), organic phosphorus (OP), and inorganic phosphorus (IP) were analyzed before and after resuspension. Resuspension directly impacted the fine-grained samples, causing the release of P into the water column after resuspension, increasing eutrophication of the estuary and risk to biota. The phosphorus enrichment index (PEI) was calculated in the four areas, and in all areas, the index was above 1, which means high ecological risk. The area with sandy granulometry and a lower percentage of organic matter showed an increase in the index after resuspension. The resuspension may impact the increase of eutrophication in some areas, due to the remobilization of the sediment and the adsorbed contaminants.

Prevalence of pollution indicator and pathogenic bacterial load in a tropical estuary, Bay of Bengal: a multivariate statistical approach

The present study aimed to estimate the current level of pollution, human pathogenic bacteria and their seasonal variability of the Dhamra estuary. The hydrological parameters like water temperature (WT), pH, salinity, dissolved oxygen (DO), nitrite (NO2-N), nitrate (NO3-N), ammonia (NH4-N) and phosphate (PO4-P) were studied to understand its influence on the microbial community. The total viable count of bacteria in water (TVC-W), sediment (TVC-S), Escherichia coli in water (EC-W) and sediment (EC-S) and Streptococus faecalis in water (SFLO-W) and sediment (SFLO-S) were analyzed in order to distinguish the pollution indicator bacteria in Dhamra estuary. The collected data was inferred through person correlation matrix (PCM), principal component analysis (PCA) and Euclidean distance cluster. The results reflected that the bacteriological community had a significant correlation with hydrological parameters during pre-monsoon than post-monsoon and summer. Cluster analysis showed a positive link between the environmental variables. The result of PCA showed 6, 5 and 5 factors during pre-monsoon, post-monsoon and summer accounts for 86.83%, 87.81% and 85.62% respectively of the total variance. The estuary shows that both water and sediment were having a significantly high number of EC and SFLO in water and sediment and exceeded the permissible limits of the World Health Organization (WHO). The result shows that the estuary is under threat due to a load of pollution and pathogenic bacteria.

Surface flow modulation by river discharges in a tropical micro-tidal estuary

The typical bidirectional pattern of subtidal flow observed at the mouths of estuaries is produced by inflows during high-tide (flood) and outflows during low-tide (ebb) periods. This flow pattern can be modulated in the presence of a strong river discharge, as observed for the Alvarado Estuary (western Gulf of Mexico) during the rainy season. Herein, six transect repetitions were obtained using a towed Acoustic Doppler Current Profiler (ADCP) at an average speed of approximately 2 m/s during a diurnal tidal cycle from September 20 to 21, 2009. The water column during this period was strongly stratified at the mouth of the estuary due to the amount of discharge from the Papaloapan River. Furthermore, the temperatures within the estuary were directly related to the distance from the Papaloapan River. Based on salinity data obtained at four locations along the estuary, the Alvarado Estuary could be considered for the first time as a transitional system from oligohaline to mesohaline (salinities between 5 and 18). The currents and backscatter intensities of the ADCP revealed concentrated areas of suspended particles in the deepest area of the estuary during flood tide periods. During ebb tide periods, estuarine water primarily exited through its mouth (∼90%), followed by the artificial channel (∼10%) located in the northernmost area of the estuary. The residence time during this season was 3.99 days. Finally, the typical estuarine flow pattern observed during flood and ebb tide periods was only depicted near the bottom of the mouth of the estuary. However, the near-surface flow was always toward the ocean (outflow), regardless of the tidal stage.

Multiple-biomarkers show the importance of blue carbon to commercially important fishery species

Coastal blue carbon ecosystems (BCE) support nearshore food webs and provide habitat for many commercially important fish and crustacean species. However, the complex links between catchment vegetation and the carbon food-base of estuarine systems are difficult to disern. We employed a multi-biomarker approach (stable isotope ratios - δ13C and δ15N, fatty acid trophic markers – FATMs and metabolomics – central carbon metabolism metabolites) to test links between estuarine vegetation and the food sources available to commercially important crabs and fish occurring within the river systems of the near-pristine eastern coastline of the Gulf of Carpentaria, Australia. Stable isotope analysis confirmed the dietary importance of fringing macrophytes to consumer diet, but showed that this is modulated by their dominance along the riverbank. FATMs indicative of specific food sources further confirmed the differences among upper intertidal macrophytes (driven by concentrations of 16: 1ω7, 18:1ω9, 18:2ω6, 18:3ω3 & 22.0) and seagrass (driven by 18:2ω6, 18:3ω3). These dietary patterns were also reflected in the concentration of central carbon metabolism metabolites. Overall, our study demonstrates the congruence of different biomarker approaches to resolve biochemical links between blue carbon ecosystems and important nekton species, and provides fresh insights into the pristine tropical estuaries of northern Australia.

Age and growth study of two fin fish species from a tropical estuary

Data on two fin fish species were collected from the Cochin estuary (part of the Vembanad–Kol wetland system) from January 2018 to December 2019. The present study presents the first findings on fisheries demographics in these estuaries, based on length–frequency​ data, to calculate vital demographic dynamic factors and estimate the stock of these commercially important fish species. A variety of evaluations was conducted using the FiSAT II program. Pseudetroplus maculatus had an asymptotic length (L) of 97.6 mm as well as a growth constant (K) of 1.2 year-1. For all the species studied, the age at zero-length (t0) had been calculated and varied from −0.095 to −0.2. Gerres setifer had longevity (tmax) of 5.66 years and a growth performance index (ϕ) of 4.03, whereas P. maculatus had 2.5 and 4.06. P. maculatus had overall mortality (Z), natural mortality (M), and fishing mortality (F) of 3.04, 1.45, and 1.58 year1, respectively, while G. setifer had 1.14, 0.77, and 0.37 year1. The present exploitation level is greater than the optimum exploitation rate, according to the relative yield per recruit (Y/R) study, indicating over-exploitation of three examined finfish species and moderate exploitation for two species. Management measures such as mesh size limitations, restricted fishing efforts, and the proclamation of restricted seasons are required to save these species from extinction.

The importance of blue carbon as a food source to oysters (Saccostrea cucullata) inside a tropical pristine Australian estuary

This study investigated the importance of blue carbon (carbon captured from the atmosphere by algae, mangrove and seagrass leaves) as a component of particulate organic matter (POM) along the pristine King George River (Kimberley region, Australia), and how this modulates the diet of common suspension feeding bivalves (Mussels: Brachidontes sp. and Oysters: Saccostrea cucullata). These species utilise a range of different carbon sources (e.g., mangrove and seagrass detritus and plankton), which dictate the composition of locally available POM), which could then be used to evaluate shifts in consumer diet away from blue carbon within the estuary, to an increasing reliance on marine phytoplankton in offshore areas. The differences in δ13C of different organs (i.e., mantle, gill and adductor muscle) of oysters further affirm a greater discrepancy between ‘slow-turnover’ muscle and ‘fast-turnover’ gill and mantle tissue at offshore sites, when compared to inner estuarine sites.

Role of salinity on zooplankton assemblages in the tropical Indian estuaries during post monsoon

The estuary is the transition zone between the riverine and marine environments, in which the zooplankton act as a trophic connector in the energy transfers from primary producers to secondary consumers. Zooplankton biovolume and species assemblages with reference to physical, chemical and biological properties in the Indian estuaries are rarely studied. To examine the zooplankton variability in abundance and diversity, we therefore, investigated seventeen Indian estuaries during the post monsoon of the year 2012. Based on salinity conditions, estuaries were classified into oligohaline, mesohaline and polyhaline. A marked spatial gradient in salinity was observed between the upstream and downstream estuaries. Relatively, salinity was high in downstream areas, resulting in high zooplankton biovolume and diversity perceived in downstream areas. In contrast, nutrient concentrations were higher in the upstream than the downstream estuaries, resulting in high phytoplankton biomass (in terms of chlorophyll-a) perceived in the upstream estuaries. Zooplankton abundance was numerically dominated by Copepoda, constituting approximately 76 % of the total zooplankton count. Zooplankton population was highly similar between upstream and downstream in the oligohaline estuaries. In contrast, heterogeneous assemblages were noticed between upstream and downstream in the mesohaline and polyhaline estuaries. Under oligohaline conditions, surface waters dominated by Acartia clausi, A. dane, A. plumosa, Cyclopina longicornis, Oithona rigida and Tigriopus sp. In contrast, under mesohaline and polyhaline conditions, Acartia tonsa, Acartia southwelli, Acartia spinicauda, Paracalanus spp. Centropages typicus, Temora turbinate, Oithona spinirostris and O. brevicornis become the key dominant species. Eucalanus spp., and Corycaeus spp. were indicator species in the downstream estuaries. Our findings suggest that zooplankton diversity and numerical abundance were chiefly governed by salinity rather than phytoplankton biomass (chlorophyll-a) in the Indian estuaries during the post monsoon.

Magnetic susceptibilites of suspended sediment and microplastic abundance in a tropical volcanic estuary

Coastal and estuarine ecosystems are seriously threatened by pollutants, according to the literature. In the estuary, the zone that traps suspended particulate matter, namely, estuarine turbidity maxima (ETM), needs to be identified to prevent ecological threats, namely migration and species death. Magnetic measurements of sediments in volcanic rivers have also attracted attention because the sediment contains a high abundance of magnetic minerals. The method in this paper provides a low-cost, rapid, and nondestructive approach to environmental monitorings and assessments. However, the validity of magnetic measurements has not been investigated for their use in identifying the ETM zone in estuaries in volcanic regions. Therefore, this research aims to identify ETM zones in the estuaries of volcanic regions using magnetic properties to correlate the characteristics to microplastic abundance within suspended and surface sediments. The measurements consist of magnetic susceptibility at low frequency, magnetic susceptibility at high frequency, frequency-dependent magnetic susceptibility analyses, and acquisition of a hysteresis curve. Magnetic minerals were also identified using X-ray diffraction and scanning electron microscopy-energy dispersive spectroscopy. The results showed that the values of magnetic susceptibility at low frequency significantly correlate with total dissolved solids and microplastics abundance in sediments. The frequency-dependent magnetic susceptibility values were significantly correlated with total dissolved solids, microplastics abundance, turbidity, and microorganism cells. This phenomenon indicates that the magnetic properties of suspended sediments can be used to identify an ETM zone in estuaries in tropical volcanic regions.

Impact of a megacity on the water quality of a tropical estuary assessed by a combination of chemical analysis and in-vitro bioassays

Tropical estuaries are threatened by rapid urbanization, which leads to the spread of thousands of micropollutants and poses an environmental risk to such sensitive aqueous ecosystems. In the present study, a combination of chemical and bioanalytical water characterization was applied to investigate the impact of Ho Chi Minh megacity (HCMC, 9.2 million inhabitants in 2021) on the Saigon River and its estuary and provide a comprehensive water quality assessment. Water samples were collected along a 140-km stretch integrating the river-estuary continuum from upstream HCMC down to the estuary mouth in the East Sea. Additional water samples were collected at the mouth of the four main canals of the city center. Chemical analysis was performed targeting up to 217 micropollutants (pharmaceuticals, plasticizers, PFASs, flame retardants, hormones, pesticides). Bioanalysis was performed using six in-vitro bioassays for hormone receptor-mediated effects, xenobiotic metabolism pathways and oxidative stress response, respectively, all accompanied by cytotoxicity measurement. A total of 120 micropollutants were detected and displayed high variability along the river continuum with total concentration ranging from 0.25 to 78 μg L−1. Among them, 59 micropollutants were ubiquitous (detection frequency ≥ 80 %). An attenuation was observed in concentration and effect profiles towards the estuary. The urban canals were identified as major sources of micropollutants and bioactivity to the river, and one canal (Bến Nghé) exceeded the effect-based trigger values derived for estrogenicity and xenobiotic metabolism. Iceberg modelling apportioned the contribution of the quantified and the unknown chemicals to the measured effects. Diuron, metolachlor, chlorpyrifos, daidzein, genistein, climbazole, mebendazole and telmisartan were identified as main risk drivers of the oxidative stress response and xenobiotic metabolism pathway activation. Our study reinforced the need for improved wastewater management and deeper evaluations of the occurrence and fate of micropollutants in urbanized tropical estuarine environments.

Seen from the Stakeholder’s revelation — valuation of ecosystem services in a small tropical Indian estuary

The small estuaries represent the least recognized transitional systems and receive limited research priority and management focus among the coastal ecosystems worldwide. Though they provide numerous ecosystem services to the local inhabitants, these services are not adequately recorded compared to large estuaries and coastal systems. In this study, we evaluated the primary ecosystem services (ES) of Terekhol (TRE), a small tropical estuary along the west coast of India. The key ES identified were provisioning—fish and shellfish, crops and mangroves, raw materials (sand and salt), water for navigation; regulating—carbon​ sequestration, disturbance regulation and water quality control, erosion control; supporting—breeding grounds for fish and biodiversity and habitat; recreational & tourism, and cultural & information services. We used a hybrid approach to evaluate these ES using primary and secondary sources of information and scientific literature. The provisioning, regulating, supporting, recreational & tourism, and cultural & information services yielded annual economic values of US$4894018.7, US$2523095.4, US$14248456.6, US$58714.3 and US$35520 respectively. The perception of key stakeholders (fishers/farmers/tourism operators/researchers and students/ biodiversity management committee members) were assessed and documented. Most stakeholders (75%–90%) believed that TRE is essential for ferry service, raw materials, biodiversity and species, fish/shellfish, recreation, science, and education. Only 10%–15% indicated that the estuary is important for the ES, such as water quality control, sediment erosion control, and carbon sequestration. This knowledge could play a vital role in policy formulation and the decision-making process to manage and conserve the ecosystem of TRE. Moreover, the study could also trigger scientific research on the ecosystem evaluation of the least recognized transitional systems of the world.

Spatially distributed water quality responses to freshwater discharge in a tropical estuary, Hilo Bay, Hawai'i.

Hilo Bay, Hawai'i, is an estuary of great importance to its neighboring coastal community, but it is threatened by impaired water quality indicated by excessive turbidity and chlorophyll a associated with river discharges of sediments and nutrients. The Wailuku River in the western half of the bay is the primary source of freshwater discharge, hypothesized here to form a surface water-dominant half of the bay with different water quality traits than the groundwater-dominant, eastern half of the bay where the spring-fed Wailoa River discharges. The water quality of both halves of the bay over different flow conditions of the Wailuku River is examined in this study using spatially distributed water quality sampling which collects hundreds of samples in either half of the bay at a distance of about every 40m. The dense sample shows significant differences between the two halves of the bay, with greater salinity dilution and turbidity in the surface water-dominant area. Both salinity and turbidity have a predictable relation to discharge, with salinity decreasing and turbidity increasing in higher flow conditions. Chlorophyll a, however, has a more complex relation to discharge, as chlorophyll a concentrations are greatest in high-flow conditions, but this may be because the water quality samples were collected in different seasons. Furthermore, significantly greater chlorophyll a concentrations in the groundwater-dominant half of the bay in low-flow conditions show that discharge may be spuriously correlated to chlorophyll a, and further studies of the effects of surface water discharge on chlorophyll a concentrations are warranted.

First insights into an exceptionally deep blue hole in the Western Caribbean: The Taam ja’ Blue Hole

Coastal karst formations may remain unexplored by the global marine scientific research community. Knowledge provided by fishermen coupled with scientific research on karstic formations prompted us to explore the geomorphometric and physicochemical features of a submerged blue hole in the large tropical estuary of Chetumal Bay in the southeastern coast of the Yucatán Peninsula, Mexico. Differently from the well-known literature on cenotes in the Yucatán Peninsula, no references are made to blue holes in the region. Sampling and surveying of the blue hole were conducted in September 2021 by scuba, echosounders, CTD profilers, and the collection of water samples. This study revealed a previously unknown maximum water depth for the Taam ja’ Blue Hole, as it is now named, of 274.4 meters below sea level (mbsl), making it the second deepest known blue hole in the world. The Taam ja’ Blue Hole has a nearly circular shape at its surface, encompassing an area of ~13,690 m2. The sides of the blue hole are steep, with slopes &amp;gt;80° that form a large conic structure covered by biofilms, sediments, limestone, and gypsum ledges. The blue hole mouth locates at ~4.5-5.0 mbsl, at which water properties change significantly with temperature and salinity gradients of 4.9°C/m and 9.8 PSU/m, respectively. Hydrographic profiles show a stratified water column inside the blue hole consisting of a hypoxic layer (5-20 mbsl), a chemocline (50-80 mbsl), and an anoxic layer (&amp;gt;110 mbsl). Evaluation of anions and cations indicated that the water inside corresponds mainly to that of diluted seawater.