Distribution and sea-to-air fluxes of nitrous oxide and methane from a seasonally hypoxic coastal zone in the southeastern Arabian Sea.

The seasonal variability, pathways, and sea-to-air fluxes of nitrous oxide (N2O) and methane (CH4) in the coastal environment, where coastal upwelling and mudbanks co-exist are presented based on the monthly time-series measurements from November 2021 to December 2022. Upwelling-driven hypoxic water's shoreward propagation and persistence were the major factors controlling the N2O concentrations, while the freshwater influx and sedimentary fluxes modulate CH4 concentrations. The N2O concentrations were high during the southwest monsoon (up to 35 nM; 19 ± 8 nM)), followed by spring inter-monsoon (up to 19 nM; 10 ± 5 nM), and lowest during the northeast monsoon (up to 13 nM; 8 ± 2 nM), whereas the CH4 levels were high during the spring inter-monsoon (8.4 to 65 nM), followed by southwest monsoon (6.8 to 53.1 nM) and relatively lower concentrations during the northeast monsoon (3.3 to 32.6 nM). The positive correlations of excess N2O with Apparent Oxygen Utilisation (AOU) and the sum of nitrate and nitrite (NOx) indicate that nitrification is the primary source of N2O in the mudbank regime. The negative correlation of CH4 concentrations with salinity indicates considerable input of CH4 through freshwater influx. CH4 exhibited a highly significant positive correlation with Chlorophyll-a throughout the study period. Furthermore, it displayed a statistically significant positive correlation with phosphate (PO43-) during the northeast monsoon while a strong negative correlation with PO43- during the spring inter-monsoon, pointing towards the role of aerobic CH4 production pathways in the mudbank regime. N2O and CH4 exhibited a contrasting seasonal pattern of sea-to-air fluxes, characterised by the highest N2O fluxes during the southwest monsoon (hypoxia) (13 ± 10 µM m-2 d-1), followed by spring inter-monsoon (12 ± 16 µM m-2 d-1), and the lowest during the northeast monsoon (0.6 ± 3 µM m-2 d-1). Conversely, the highest sea-to-air fluxes of CH4 were noticed during the spring inter-monsoon (74 ± 56 µM m-2 d-1), followed by the southwest monsoon (45 ± 35 µM m-2 d-1), and the lowest values during the northeast monsoon (19 ± 16 µM m-2d-1). Long-term time-series measurements will be invaluable in understanding the longer-term impacts of climate-driven variability on marine biogeochemical cycles in dynamic nearshore systems.

An integrated buoy-satellite based coastal water quality nowcasting system: India's pioneering efforts towards addressing UN ocean decade challenges.

The Indian coastal waters are stressed due to a multitude of factors, such as the discharge of industrial effluents, urbanization (municipal sewage), agricultural runoff, and river discharge. The coastal waters along the eastern and western seaboard of India exhibit contrasting characteristics in terms of seasonality, the magnitude of river influx, circulation pattern, and degree of anthropogenic activity. Therefore, understanding these processes and forecasting their occurrence is highly necessary to secure the health of coastal waters, habitats, marine resources, and the safety of tourists. This article introduces an integrated buoy-satellite based Water Quality Nowcasting System (WQNS) to address the unique challenges of water quality monitoring in Indian coastal waters and to boost the regional blue economy. The Indian National Centre for Ocean Information Services (INCOIS) has launched a first-of-its-kind WQNS, and positioned the buoys at two important locations along the east (Visakhapatnam) and west (Kochi) coast of India, covering a range of environmental conditions and tourist-intensive zones. These buoys are equipped with different physical-biogeochemical sensors, data telemetry systems, and integration with satellite-based observations for real-time data transmission to land. The sensors onboard these buoys continuously measure 22 water quality parameters, including surface current (speed and direction), salinity, temperature, pH, dissolved oxygen, phycocyanin, phycoerythrin, Coloured Dissolved Organic Matter, chlorophyll-a, turbidity, dissolved methane, hydrocarbon (crude and refined), scattering, pCO2 (water and air), and inorganic macronutrients (nitrite, nitrate, ammonium, phosphate, silicate). This real-time data is transmitted to a central processing facility at INCOIS, and after necessary quality control, the data is disseminated through the INCOIS website. Preliminary results from the WQNS show promising outcomes, including the short-term changes in the water column oxic and hypoxic regimes within a day in coastal waters off Kochi during the monsoon period, whereas effluxing of high levels of CO2 into the atmosphere associated with the mixing of water, driven by local depression in the coastal waters off Visakhapatnam. The system has demonstrated its ability to detect changes in the water column properties due to episodic events and mesoscale processes. Additionally, it offers valuable data for research, management, and policy development related to coastal water quality.

Occurrence, sources and risk assessment of polycyclic aromatic hydrocarbons in the coral reef waters of the Lakshadweep Archipelago, Arabian Sea.

The compound effects of anthropogenic disturbances on global and local scales threaten coral reef ecosystems of the Arabian Sea. The impacts of organic pollutants on the coral reefs and associated organisms have received less attention and are consequently less understood. This study examines the background levels, sources, and ecological implications of polycyclic aromatic hydrocarbons (PAHs) in the coral reef ecosystems of Lakshadweep Archipelago. Water and particulate matter were collected from four coral Islands (Kavaratti, Agatti, Bangaram and Perumal Par) of Lakshadweep Archipelago during January and December 2022 and analysed for 15 PAHs priority pollutants. The 15 PAHs congeners generally ranged from 2.77 to 250.47 ng/L in the dissolved form and 0.44 to 6469.86 ng/g in the particulate form. A comparison of available data among the coral reef ecosystems worldwide revealed relatively lower PAHs concentrations in the Lakshadweep coral ecosystems. The isomeric ratios of individual PAH congeners and principal component analysis (PCA) indicate mixed sources of PAHs in the water column derived from pyrogenic, low-temperature combustion and petrogenic. The risk quotient (RQ) values in the dissolved form indicate moderate risk to the aquatic organisms, while they indicate moderate to severe risk in the particulate form.

Factors influencing nearshore hypoxia in the southeastern Arabian Sea: A sensor-based study.

Seasonal upwelling and the associated incursion of hypoxic waters into the coastal zone is a widely studied topic over different upwelling zones. However, its persistence or variations over short time scales are poorly addressed. The present study, therefore, brings out a first report on hourly variations in the temperature, salinity and dissolved oxygen recorded by an environmental data buoy equipped with sensors, deployed in the nearshore waters of Alappuzha (southeastern Arabian Sea) from April to August 2022. The characteristic feature of the Alappuzha coast is the development of mud banks during the southwest monsoon, providing a tranquil environment suitable for continuous sensor-based measurements when the sea remains turbulent elsewhere. The results showed that despite an advance in the upwelling intensity, there is a significant variation in the oxygen concentration in the study domain on a diurnal scale. In general, the nearshore region was under hypoxia during the first half of the day (00:00 to 12:00 h), which increased steadily to reach normoxic and supersaturated levels during the rest of the day (12:00 to 24:00 h). Statistical analysis showed that winds significantly correlate to the coastal environment's subsurface oxygen concentration. During the morning hours, the wind was weak, and the water column remained stratified over the subsurface hypoxic water layer. The situation changed in the afternoon (12:00 h onwards), as there was a steady increase in the local wind speed (>5 m/s), which was sustained during the rest of the day. A local wind speed >5 m/s can disturb the stratification and enhance the mixing process from 12:00 to 24:00 h. The total kinetic energy of 11.5 J/m3 is the threshold for this oxygen supersaturation. These findings emphasize the role of wind-induced mixing in alleviating coastal hypoxia, highlighting the need for further biogeochemical and ecological investigations into the impacts of alternating oxic-hypoxic conditions in nearshore waters.

Phosphorus cycling from a coastal upwelling zone in the Southeastern Arabian Sea.

The present study examined the seasonal cycling of phosphorous (P) along the southwest coast of India (SWCI) based on two cruises during the southwest monsoon (SWM) and northeast monsoon (NEM) of 2018. During SWM, the entire SWCI experienced intense upwelling manifested by the incursion of cold, nutrient-rich, and hypoxic waters. During NEM, the region was transformed into a warm, well-oxygenated and nutrient-deplete environment. Dissolved inorganic phosphorus (DIP) was significantly high in the subsurface during SWM due to its release from sediments. The sediment P was high and showed an increasing trend towards the south, principally dependent on the sediment texture, organic carbon, and Fe concentrations. Bioavailable P, the sum of exchangeable (PEx) and reducible (PFe) fractions, was almost consistent (5-20%) over seasons, though PFe showed a marked reduction during SWM. Authigenic fraction (PAut) was the most dominant (46%), followed by detrital (PDet 41%) and residual (PRes 8%) fractions. Principal component analysis (PCA) of geochemical parameters for SWM was indicative of the high dissolution of Fe (oxy)hydroxides under hypoxia releasing P and its complexation with organic matter and Fe. PCA results for the NEM were different, as it indicated increased preservation of P-associated organic matter and Fe, alternately favouring the formation of PAut in sediments. The study's significance is the observation that the bottom water oxygen concentration can significantly influence sedimentary P cycling in tropical coastal upwelling zones.

Distribution, contamination status and bioavailability of trace metals in surface sediments along the southwest coast of India.

This study investigates the influence of upwelling induced seasonal hypoxia in the sediment-water interface on the distribution, bioavailability and geochemical partitioning of selected trace metals in surficial sediments along the southwest coast of India based on two successive cruises. The first cruise was during the southwest monsoon (SWM) season when coastal waters exhibited intermittent bottom hypoxia due to upwelling. The second cruise during the northeast monsoon (NEM) season was characterised by a uniformly warm and well-oxygenated water column in the study region. The results showed that grain size, organic carbon and Fe are the major factors influencing the distribution of trace metals in the surface sediments. Based on the geochemical indices (contamination factor, enrichment factor and geo-accumulation factor), the study region appears to be moderately contaminated by Ni, Cr, and Pb. Based on the ecological risk assessment criteria, the enrichment of Pb, Cr and Ni may cause adverse effects on the benthic organisms. The fractionation studies demonstrated that the major pathway of metal deposition in the sediment is lithogenic. The data also showed that labile and organic fractions are the second dominant forms, while other fractions (exchangeable and carbonate) are insignificant. The consistency in the reactive Fe concentrations during SWM and NEM could be due to the absence of Fe dissolution in sediments under mild reducing condition (intermittent hypoxia). In addition to the above, an enrichment of organic matter also leads to increased deposition of trace metals in sediments. Conversely, the secondary phase enrichment factor and risk assessment code calculated based on the metal fractionation data indicated low risk and contamination along the southwest coast of India except for Zn that showed moderate contamination in some transects. The study provides the need for regular geochemical assessment to control metal pollution in the coastal environment, which is a major resource of the fishery.

Aberrations in the microbiome of cyanobacteria from a tropical estuary polluted by heavy metals.

The effect of heavy metal pollution on the microbiome of cyanobacteria in Cochin estuary (CE) on the southwest coast of India is reported in the study. Statistically significant difference in heavy metal concentration was observed between water, suspended particulate matter (SPM) and sediment. The Zn, Cd, Cu, Ni and Cr were 2-6 times higher in the SPM compared with the sediment, while Pb was 10 to 25 times higher. Although nearly 60% of the species diversity of microbiome was common between cyanobacteria enriched from the upstream (S1S) and downstream (S11B), there was a difference in the major groups of heterotrophic bacterial associates. Proteobacteria was the dominant phylum (>80%) in S1S, while it was second only (27.5%) after Planctomycetes (37.4%) in S11B. The results of the current study indicate that the pollution can influence an ecosystem at the micro-niche level.

Phthalic acid esters - A grave ecological hazard in Cochin estuary, India.

Distribution and ecological risks of Phthalic acid esters (PAEs) are poorly studied in estuarine environments in India. An attempt is made to chart the sources and assess the ecological risk of six PAE congeners (∑6PAEs), present in dissolved and particulate forms in a tropical ecosystem (Cochin Estuary, India). Terrestrial input, as attested by a clear seasonality with substantial enrichment during monsoon (2-28 µg/L and 31-1203 µg/g; dissolved and particulate PAEs respectively) and post-monsoon (1-7 µg/L and 7-321 µg/g; dissolved and particulate PAEs respectively), was identified as the primary source. DnBP (di-n-butyl phthalate) and DEHP (diethylhexyl phthalate) were found to be the dominant species except for dissolved PAEs at pre-monsoon season. Statistical analysis identified two major clusters, in the ∑6PAEs, composed of medium to high molecular weight PAEs (derived from plastic products) and low molecular weight PAEs (derived from cosmetic products). Calculated Risk Quotient (RQ) indicated values indicated moderate to high ecological risk for DnBP and DEHP congeners is a grim pointer to their detrimental effects on human health through consumption of contaminated organisms. Although substantial enrichment of suspended matter gets flushed out of the estuary during monsoon, there is a net PAE accumulation in the estuary during post-monsoon following an increased sedimentary restitution. Data of PAEs generated herein raises a challenge for immediate enactment of statutory legislation to curb and regulate hazardous contamination of estuaries by phthalic acid esters.

Trace metal distribution in the sediment cores of mangrove ecosystems along northern Kerala coast, south-west coast of India.

Core sediment samples were collected from five mangrove ecosystems along northern Kerala coast (Kunjimangalam: S1, Pazhayangadi: S2, Pappinissery: S3, Thalassery: S4, and Kadalundi: S5) to assess the status of heavy metal pollution. S1 recorded comparatively lower metal concentration at surface (except Pb) due to low organic content and sandy texture, while the reverse was true for S3. Higher metal contents were recorded at S5 (0-5 cm), which was attributed to its unique biogeochemical behavior. Enrichment factor and geoaccumulation index indicated moderate enrichment for Cd, and the pollution load index revealed progressive deterioration of sediment quality at S5 (0-5 cm). There was no harmful effect of trace metals on biological community (except Ni) according to Sediment Quality Guidelines. Major processes controlling trace metal accumulation in these systems are diagenetic processes, precipitation of heavy metals as sulfides, and the presence of Fe, Mn-oxy hydroxides, which act as adsorption sites for other metals.

Distribution and contamination status of phthalic acid esters in the sediments of a tropical monsoonal estuary, Cochin - India.

Phthalic acid esters (PAEs) are a group of endocrine-disrupting chemicals listed as priority pollutants by United States Environmental Protection Agency (USEPA, 2009). This study provides baseline information on seasonal distribution and contamination status of six phthalic acid esters (∑6PAEs) in sediments of a tropical estuary (Cochin-India). In general, the sediments accumulated more PAEs during the post monsoon (mean 2325 ngg-1; between 1402 and 3121 ngg-1) and monsoon (mean 1372 ngg-1; between 331 and 4015 ngg-1) periods indicating land run off as the major transport pathway. Moderate run off and comparatively high residence time lead to effective sorption and settling of PAEs in the surface sediments during post monsoon season. Despite a high discharge of PAEs in to the water column, their deposition on to the sediments occurs at a lower rate during monsoon than that post monsoon season. PAEs were (mean 810 ngg-1; between 44 and 1722 ngg-1) lowest in pre monsoon season. The pre monsoon season is characterized by a minimal runoff consequent to the trapping of these organic pollutants in the river catchment area. The mid and high molecular PAEs (DEHP-Di ethylhexyl phthalate, BBP-Benzyl butyl phthalate and DnBP-Di-n-butyl phthalate) were the dominant congeners relative to the low molecular weight congeners (DMP-Dimethyl phthalate and DEP-Diethyl phthalate). DEHP and BBP levels exceeded permissible risk levels indicating a serious ecological hazard to the estuarine ecosystem.

Role of heavy metals in structuring the microbial community associated with particulate matter in a tropical estuary.

Particulate matter (PM), which are chemically and biochemically complicated particles, accommodate a plethora of microorganisms. In the present study, we report the influence of heavy metal pollution on the abundance and community structure of archaea and bacteria associated with PM samples collected from polluted and non-polluted regions of Cochin Estuary (CE), Southwest coast of India. We observed an accumulation of heavy metals in PM collected from CE, and their concentrations were in the order Fe > Zn > Mn > Cr > Pb > Cu > Cd > Co > Ni. Zinc was a major pollutant in the water (4.36-130.50 µgL-1) and in the particulate matter (765.5-8451.28 µgg-1). Heavy metals, Cd, Co, and Pb were recorded in the particulate matter, although they were below detectable limits in the water column. Statistical analysis showed a positive influence of particulate organic carbon, nitrogen, PM-Pb, PM-Zn and PM-Fe on the abundance of PM-archaea and PM-bacteria. The abundance of archaea and bacteria were ten times less in PM compared with planktonic ones. The abundance of PM-archaea ranged between 4.27 and 9.50 × 107and 2.73 to 3.85 × 107 cellsL-1 respectively for the wet and dry season, while that of PM-bacteria was between 1.14 and 6.72 × 108 cellsL-1 for both seasons. Community structure of PM-bacteria varied between polluted and non-polluted stations, while their abundance does not show a drastic difference. This could be attributed to the selective enrichment of bacteria by heavy metals in PM. Such enrichment may only promote the growth of metal resistant archaea and bacteria, which may not participate in the processing of PM. In such cases, the PM may remain without remineralization in the system arresting the food web dynamics and biogeochemical cycles.

Noctiluca and copepods grazing on the phytoplankton community in a nutrient-enriched coastal environment along the southwest coast of India.

The relative grazing impact of Noctiluca scintillans (hereafter referred only Noctiluca) and copepods (Acrocalanus gracilis, Paracalanus parvus, Acartia danae and Oithona similis) on the phytoplankton community in an upwelling-mudbank environment along the southwest coast India is presented here. This study was carried out during the Pre-Southwest Monsoon (April-May) to the Late Southwest Monsoon (August) period in 2014. During the sampling period, large hydrographical transformation was evident in the study area (off Alappuzha, Southwest coast of India); warmer Pre-Southwest Monsoon water column condition got transformed into cooler and nitrate-rich hypoxic waters during the Southwest Monsoon (June-August) due to intense coastal upwelling. Copepods were present in the study area throughout the sampling period with a noticeable increase in their abundance during the Southwest Monsoon. On the other hand, the first appearance of Noctiluca in the sampling location was during the Early Southwest Monsoon (mid-June) and thereafter their abundance increased towards the Peak Southwest Monsoon. The grazing experiments carried out as per the food removal method showed noticeable differences in the feeding preferences of Noctiluca and copepods, especially on the different size fractions of phytoplankton. Noctiluca showed the highest positive electivity for the phytoplankton micro-fraction (av. 0.49 ± 0.04), followed by nano-fraction (av. 0.17 ± 0.04) and a negative electivity for the pico-fraction (av. -0.66 ± 0.06). In total ingestion of Noctiluca, micro-fraction contribution (83.7%) was significantly higher compared to the nano- (15.7%) and pico-fractions (0.58%). On the other hand, copepods showed the highest positive electivity for the phytoplankton nano-fraction (av. 0.38 ± 0.04) followed by micro- (av. -0.17 ± 0.05) and pico-fractions (av. -0.35 ± 0.05). Similarly, in total ingestion of copepods, nano-fraction (69.7%) was the highest followed by micro- (28.9%) and pico-fractions (1.37%). The grazing pressure of Noctiluca on the total phytoplankton was found to be 27.7% of the standing stock and 45.6% of the production, whereas in the case of copepods, it was 9.95% of the standing stock and 16.6% of the production. The study showed that the grazing pressure of Noctiluca on the total phytoplankton as well as larger phytoplankton fraction was 2.8- and 8-folds higher than that of the copepods. This suggests the leading role of Noctiluca as an effective grazer of larger phytoplankton along the southwest west coast of India, especially during the Peak/Late Southwest Monsoon.

Dynamics of polycyclic aromatic hydrocarbons (PAHs) in surface sediments of Cochin estuary, India.

Polycyclic aromatic hydrocarbons (PAHs) showed significant seasonal dynamics in surface sediments of a tropical ecosystem (Cochin estuary, south west coast of India). Concentrations ranged from 304 to 5874ngg-1 in pre-monsoon, 493 to 14,149ngg-1 in monsoon, and 194 to 10,691ngg-1 in post-monsoon. The estuary was moderately contaminated with low molecular weight PAH fractions, which increased rapidly during the monsoon season indicating land/river runoff as the major transport pathway. The dominance of 2-3 ring fractions in the PAH indicated petrogenic and low temperature combustion processes as major sources, while the very low levels of 5-6 ring components indicated low contribution from pyrolytic sources. Low molecular weight fractions were higher in concentration than the Effective Range-Median (ERM) levels, whereas high molecular weight PAHs were lower than the Effective Range-Low values (ERL). Calculated carcinogenic toxicity equivalents (TEQ) values ranged from 1 to 971ngg-1 in the surface sediments.

Phosphorous fractionation in mangrove sediments of Kerala, south west coast of India: the relative importance of inorganic and organic phosphorous fractions.

The study of phosphorous dynamics in mangrove ecosystems of the northern Kerala coast aims to delineate its relationships with other biogeochemical parameters. Our intension is to check the validity of the hypothesis that these mangrove ecosystems act as an efficient trap of organic phosphorous by acting as P sink. The dissolved inorganic phosphate displayed higher concentration in monsoon that could be correlated with higher P leaching from mangrove litter as well as terrigenous input during wet season. Fe(OOH)≈P was much higher in monsoon (235.23 to 557.70 µg g(-1)) and lower in pre-monsoon (36.50 to 154.97 µg g(-1)), and displayed significant contribution towards the inorganic sedimentary P fractions. In monsoon, adsorption of P on iron hydroxides is enhanced by fresh water conditions, but pre-monsoon is characterised by the reductive dissolution of iron oxy hydroxides and the subsequent efflux of P to water column. CaCO3≈Pinorg may be present as an inert fraction in the sediment matrix, and did not display any interrelationship with other geochemical parameters. The abundant total organic P (25 to 73 %) fractions, largely derived from P bound with humic/fulvic acid, played a major role in immobilising P and regulating its dynamics in the nearby estuarine and coastal environment.

Evaluation of heavy metal enrichment in Cochin estuary and its adjacent coast: multivariate statistical approach.

Temporal and spatial variations of heavy metals in the Cochin estuary and its adjacent coastline during three seasons were studied to investigate the impact of anthropogenic heavy metal pollution. Total organic carbon, sand, silt, clay and 10 metals (Cd, Pb, Cr, Ni, Co, Cu, Zn, Mn, Mg and Fe) in the surface sediments were analysed. Multivariate statistical analyses like canonical correspondence analysis, principal component analysis and cluster analysis were used for source identification, integration of geochemical data and clustering of stations based on similarities. Enrichment factor, contamination factor and geoaccumulation index were used to assess the contamination level. From the study, it can be understood that estuary and coast are highly polluted especially with Cd, Zn, Pb and Ni. Anthropogenic influence of heavy metals was evidenced from both the principal component analysis and cluster analysis. Finer fractions (mud) of the sediment and the associated Fe oxy hydroxides might be playing major role in the transport of heavy metals in the system. Very high enrichment factor value observed suggested high anthropogenic pressure in the study area. All the stations in the northern part of the estuary showed very high enrichment factors indicating heavy load of Zn and Cd in this area which might have reached from the industrial area lying to the north side of the Cochin estuary. Pollution indices suggested that both the estuary and its adjacent coast were showing low contamination with respect to Cr, Mg, Mn and Fe; all other metals were causing low to extremely high levels of pollution in the study area.

Distribution and sources of aliphatic hydrocarbons and fatty acids in surface sediments of a tropical estuary south west coast of India (Cochin estuary).

Surface sediments samples from the Cochin estuary were measured for elemental, stable isotopic and molecular biomarkers (aliphatic hydrocarbons and fatty acids) to study the sources and distribution of sedimentary organic matter. Concentrations of total organic carbon (TOC), total nitrogen (TN) and stable isotopic ratios of carbon (δ(13)C) ranged from 0.62 to 2.74 %, 0.09 to 0.25 % and -27.5 to 21.7 ‰, respectively. Sedimentary n-alkanes ranged from 6.03 to 43.23 µg g(-1) with an average of 16.79 µg g(-1), while total fatty acids varied from 22.55 to 440.69 µg g(-1). The TOC/TN ratios and δ(13)C suggest a mixture of marine- and terrestrial-derived organic matter in the surface sediments with increasing contributions from marine-derived organic matter towards the seaward side. Long-chain n-alkanes derived from higher plants predominated the inner part of the estuary, while short-chain n-alkanes derived from planktonic sources predominated the bar mouth region. The even carbon preference of the C12-C22 n-alkanes may refer to the direct biogenic contribution from bacteria, fungi and yeast species and to the potential direct petroleum inputs. The presence of odd mid-chain n-alkanes in the sediments indicates the organic matter inputs from submerged and floating macrophytes (water hyacinth). Various molecular indices such as carbon preference index, terrestrial to aquatic ratio, average chain length and the ratios of mid-chain n-alkanes support the aforementioned inferences. The high contribution of odd and branched chain fatty acids along with very low contribution of polyunsaturated fatty acids, suggest the effective utilisation of algae-derived organic matter by bacteria and the effective recycling of labile organic matter in whole settling and deposition processes. The distributional variability of n-alkanes and fatty acids reveals the preferential utilisation of marine-derived organic matter and the selective preservation of terrestrial-derived organic matter in surface sediments of the Cochin estuary.

Phosphorous speciation in surface sediments of the Cochin estuary.

Sequential chemical extraction using chelating agents were used to study the P dynamics and its bioavailability along the surface sediments of the Cochin estuary (southwest coast of India). Sediments were analyzed for major P species (iron bound P, calcium bound P, acid soluble organic P, alkali soluble organic P and residual organic P), Fe, Ca, total carbon, organic carbon, total nitrogen and total sulfur contents. An abrupt increase in the concentration of dissolved inorganic P with increasing salinity was observed in the study region. Iron-bound P exhibited a distinct seasonal pattern with maximum values in the monsoon season when fresh water condition was prevailed in the estuary. As salinity increased, the percentage of iron-bound P decreased, while that of calcium-bound P and total sedimentary sulfur increased. C/P and N/P ratios were low which indicate that large amounts of organic matter enriched with P tend to accumulate in surface sediments. The high organic P contribution in the sedimentary P pool may indicate high organic matter load with incomplete mineralization, as well as comparatively greater percentage of humic substance and resistant organic compounds. Principal component analysis is employed to find the possible processes influencing the speciation of P in the study region and indicate the following processes: (1) the spatial and seasonal variations of calcium bound P and acid soluble organic P was mainly controlled by sediment texture and organic carbon content, (2) sediment redox conditions control the distribution of iron bound P and (3) the terrigenous input of organic P is a significant processes controlling total P content in surface sediments. The bioavailable P was very high in the surface sediments which on an average accounts for 59 % in the pre-monsoon, 65 % in the monsoon and 53 % in the post-monsoon seasons. The surface sediments act as a potential internal source of P in the Cochin estuary.

Distribution and sources of sedimentary organic matter in a tropical estuary, south west coast of India (Cochin estuary): a baseline study.

Surface sediments samples were collected from 9 stations of the Cochin estuary during the monsoon, post-monsoon and pre-monsoon seasons and were analyzed for grain size, total organic carbon (OC), total nitrogen (TN) and stable isotopic ratios of carbon (δ¹³C) and nitrogen (δ(¹5N) to identify major sources of organic matter in surface sediments. Sediment grain size is found to be the key factor influencing the organic matter accumulation in surface sediments. The δ¹³C values ranges from -27.5‰ to -21.7‰ in surface sediments with a gradual increase from inner part of the estuary to the seaward side that suggest an increasing contribution of marine autogenous organic matter towards the seaward side. The δ¹5N value varies between 3.1‰ and 6.7‰ and it exhibits complex spatial and seasonal distributions in the study area. It is found that the dynamic cycling of nitrogen through various biogeochemical and organic matter degradation processes modifies the OC/TN ratios and δ¹5N to a considerable degree. The fraction of terrestrial organic matter in the total organic matter pool ranges from 13% to 74% in the surface sediments as estimated by δ¹³C based two end member mixing model.