Mud Banks Research Articles

Multi-Decadal to Short-Term Beach and Shoreline Mobility in a Complex River-Mouth Environment Affected by Mud From the Amazon

On the 1500 km-long mud-dominated Guianas coast of South America, between the mouths of the two mega-rivers, the Amazon and the Orinoco, debouch numerous small rivers draining the humid tropical/equatorial Guiana Shield. The geomorphic development of the mouths of these rivers reflects interactions among water discharge, fluvial sediment load, and the alongshore migration of Amazon-derived mud banks alternating with inter-bank areas. The mouth of the Maroni River, astride the French Guiana-Suriname border, shows advanced estuarine infill and geomorphic development characterized by a western (downdrift) side comprising numerous recent cheniers and an eastern (updrift) side bound by an old (> 2000 years B.P.) chenier. A multi-decadal analysis of the beach bounding this chenier shows little net overall mobility notwithstanding significant decadal to sub-decadal variation. The overall stability reflects the diversion of sand supply from the Maroni River towards the downdrift coast and limited sand supply by the smaller Mana River further east, and the south bank of which was contiguous with this beach. The variability in beach multi-decadal mobility reflects the influence, on waves, of alongshore-migrating banks (strong wave dissipation, limited beach mobility) and inter-bank areas (limited wave dissipation, larger beach mobility), highlighted by a comparison, in the current bank phase, of offshore and inshore waves. Erosion of the beach between 2011 and 2017 coincides with the sealing of the mouth of the Mana by muddy progradation in 2011 and mouth relocation several kilometers eastward. The morphodynamics of the beach and shorter-term fluctuations in budget are related to: (1) interaction with estuarine sand dunes mobilized by strong tidal currents on the adjacent shallow shoreface, (2) the influence of the Maroni channel, and (3) rapid encroachment of the leading edge of the shore-attached mud bank on the eastern part of the beach. The beach morphodynamics and evolution highlight, thus, embedded levels of influence: the Maroni at the local scale, and the net westward sediment-transport system and bank and inter-bank alternations that affect the Guianas coast at a regional scale. The recent erosion poses a threat to the local communities by reducing beach space available for recreation and turtle-nesting.

Factors Controlling Storage, Sources, and Diagenetic State of Organic Carbon in a Prograding Subaerial Delta: Wax Lake Delta, Louisiana

AbstractWax Lake Delta, southern Louisiana, is a coastal delta that formed following the dredging of a river channel in 1941 and is a field model for investigating the geomorphology, ecology, carbon dynamics, and carbon storage capacity in young prograding deltas. However, it is unknown how the transition from subaqueous to subaerial sediments affects the sources and quality of the sequestered carbon. We investigated these variations within the sediments of Wax Lake Delta using amino acid, lignin, and stable carbon isotope compositions of the organic matter (OM). A principal component analysis of these proxies highlighted variability in organic carbon (OC) composition with changes in elevation. The transition from subaqueous to subaerial sediments at 0‐cm mean lower low water is an important component of the OM composition. In addition to the changes observed for OM source and quality, the OC loadings (OC/SA; mg C/m2) also increase as the delta aggrades and accumulates sediments with loadings typical of delta topsets and mobile mud banks (OC/SA < 0.4) to riverine sediments (0.5 < OC/SA < 1) and eventually to highly productive regions (OC/SA > 1). Linking this multiproxy approach with environmental variables such as elevation provides a path for incorporating OM dynamics into geomorphic models.

Microbial diversity in a coastal environment with co-existing upwelling and mud-banks along the south west coast of India.

Upwelling and mud banks are two prominent oceanographic features in the coastal waters along the south west coast of India during the southwest monsoon (MON) season. The present study investigates the microbial diversity in the coastal environments of Alappuzha, India, where upwelling and mud banks co-exist. Water samples were collected from three stations, M1, M2, and M3, on a weekly basis to estimate the physico-chemical parameters and microbial abundance (MA). Presence of cold waters (< 26°C) with high nitrate (6-8µM) and low dissolved oxygen (5µM) in the sub surface waters during monsoon (M) confirmed the presence of upwelling at all the three stations. Simultaneously, presence of unusually calm waters was seen at M2 alone during M indicating the formation of mud banks. The microbial diversity was determined from three stations, with distinct oceanographic conditions (M1: coastal reference station with only upwelling, M2: mud banks + upwelling, and M3: offshore reference station with only upwelling). The water samples were collected during two seasons, pre-monsoon (April) and M (July) and analysed using 16S rRNA-based Illumina high-throughput metagenomic sequencing. Proteobacteria was the most dominant phyla, followed by Bacteroidetes, Firmicutes, Cyanobacteria, Actinobacteria, and Verrucomicrobia in order, with variations in their relative abundance spatially and seasonally. Though the MA increased during M at all the stations, the relative abundance of most of the bacterial phyla except Proteobacteria decreased during M season. Interestingly, most of the sequences at M2 during mud banks were unclassified at the class level indicating the presence of unique microbial populations in this station. Prediction of metabolic activity revealed ammonia oxidation, nitrite reduction, sulphate reduction, xylan degradation, dehalogenation, chitin degradation, etc. as important functions. The metabolic activity throws light on the role of microbes in this environment thereby providing a system-scale perspective of microbial community interactions.

Inter-annual variations in the hydrochemistry of Alappuzha mud banks, southwest coast of India

The changes in the hydro-chemical properties of a near shore waters of Alappuzha, southwest coast of India, where the mud banks and coastal upwelling are co-existent, are presented based on two time-series measurements taken during 2014 and 2016. The meteorological data show that the wind speed was significantly high during 2014 as compared to that during 2016 (p < 0.01). As a result, the shoreward propagation and persistence of hypoxic upwelling waters were more prominent in the mud banks during 2014, even though the coastal upwelling was generally more intense during 2016. This indicates that irrespective of the intensity of upwelling, the incursion of hypoxic waters towards the shallow coastal environment is dependent on the local meteorological conditions such as winds. The coastal upwelling generally increased the concentrations of nutrients and chlorophyll a but marked increase in the nitrate levels during 2014 favoring a massive diatom bloom (Fragelaria oceanica) is noteworthy, as it is an indicator of the abundance of Indian oil sardines (Sardinella longiceps), the popular mud bank fishery.

Chenier Morphodynamics on the Amazon-Influenced Coast of Suriname, South America: Implications for Beach Ecosystem Services

The 350-km long coast of Suriname, South America, is part of a unique system in the world characterized by large-scale mud supply from the Amazon and exposed to Atlantic waves. Large banks migrate alongshore from the Amazon to the Orinoco delta, separated by 'inter-bank' zones. Banks dissipate waves, partially weld onshore, and are colonized by mangroves, whereas waves in inter-bank areas cause shoreline erosion, mitigated where rare cheniers develop. Cheniers assure coastal protection and recreational and ecosystem services, notably providing nesting sites for marine turtles. Cheniers are also under pressure from sand mining. In order to gain a better understanding of how these cheniers form and evolve, a study was conducted on Braamspunt beach, a major turtle-nesting chenier in Suriname constructed from sand supplied by the Maroni River. Satellite images between 1987 and 2018 show a reduction of the alongshore extent of the chenier, following sand supply cut-off from the Maroni by a mud bank migrating westward towards the mouth of the Suriname River, exacerbated by mud-blanketing of sand. Commercial sand mining has further affected the beach, but mined volumes are not known. Field surveys (high-resolution topography, drone photogrammetry, wave measurements) conducted in February, 2016 highlight two chenier types depending on sand availability, transport and wave reworking: a high-tide reflective/low-tide dissipative and longshore transport-dominated type 1 exhibiting berm scarping, and a low, overwash-dominated type 2. As the mud-bank's leading edge impinges on the inter-bank trailing edge, sand sequestering by mud and storage in type 2 washovers entail less available sand downdrift. Type 1 lost over 4200 m(3) of sand in just 3-4 days. An increasingly deficient budget induces progressive change to type 2, morphodynamically better adapted to diminution of available sand, and epitomizing chenier fragmentation and isolation within the muddy Guianas coastal plain. By affecting the morphology and sand budget of Braamspunt, these changes lower opportunities for turtle-nesting. Further reduction of the length of exposed chenier is apparently halted near the Suriname River mouth where mud is liquefied by the outflowing river and tidal jet, but the continued existence of this preserved beach could be endangered by sand mining.

Exceptional rates and mechanisms of muddy shoreline retreat following mangrove removal

AbstractProbably the largest regular shoreline fluctuations on Earth occur along the 1500 km‐long wave‐exposed Guianas coast of South America between the mouths of the Amazon and Orinoco Rivers, the world's longest muddy coast. The Guianas coast is influenced by a succession of mud banks migrating northwestward from the Amazon. Migrating mud banks dissipate waves, partially weld onshore, and lead to coastal progradation, aided by large‐scale colonization by mangroves, whereas mangrove‐colonized areas between banks (inter‐bank areas) are exposed to strong wave action and undergo erosion. On large tracts of this coast, urbanization and farming have led to fragmentation and removal of mangroves, resulting in aggravated shoreline retreat. To highlight this situation, we determined, in a setting where mangroves and backshore freshwater marshes have been converted into rice polders in French Guiana, shoreline change over 38 years (1976‐2014) from satellite images and aerial orthophotographs. We also conducted four field experiments between October 2013 and October 2014, comprising topographic and hydrodynamic measurements, to determine mechanisms of retreat. The polder showed persistent retreat, at peak rates of up to ‐200 m/yr, and no recovery over the 38‐year period of monitored change. Notwithstanding high erosion rates, mangrove shorelines show strong resilience, with recovery characterized by massive accretion. Retreat of the polder results in a steep wave‐reworked shoreface with a lowered capacity for bank welding onshore and mangrove establishment. Persistent polder erosion is accompanied by the formation of a sandy chenier that retreats landwards at rates largely exceeding those in inter‐bank situations. These results show that anthropogenic mangrove removal can durably modify the morphodynamics of muddy shorefaces. This limits the capacity for shoreline recovery and mangrove re‐establishment even when there is no sustained long‐term deficit in mud supply, as in the case of the Amazon‐influenced Guianas coast. © 2019 John Wiley &amp; Sons, Ltd.

Trophic relationships and basal resource utilisation in the Can Gio Mangrove Biosphere Reserve (Southern Vietnam)

Fatty acid biomarkers and dual stable isotopes (δ13C and δ15N) were used to identify the preferred food sources of consumers in a mangrove tidal creek and nearby unforested (mud bank) and forested areas located in the Can Gio Mangrove Biosphere Reserve (Southern Vietnam). We analysed 15 macro-invertebrates and 1 fish species representing primary consumers and their immediate predators in this area. Specific groups of fatty acids were used to trace the fate of various food sources (i.e., suspended particulate organic matter, mangrove litter and sedimentary organic matter). The δ13C and δ15N of consumers ranged from −26.9 to −18.8‰ and from 1.1 to 9.9‰, respectively. The trophic pathway based on mangrove litter, characteristic of mangrove ecosystems, is nutritionally sustaining various crab and snail species. In contrast, it appears that the most mobile species (fish and shrimps), living in the water column and possibly migrating with tides, are mostly feeding on suspended particulate organic matter, suggesting that this trophic pathway is of great importance for connectivity among tropical coastal ecosystems. Our study suggests that snails and crabs mainly act as mineralisers, processing high quantities of detrital material to meet their nutritional needs and thus releasing nutrients through the production of faeces, that are further mineralised by microorganisms, while locally grown phytoplankton reintegrates these compounds into its biomass and feeds migrating species. We highlight here a possible link between mangrove litter and coastal food webs.

The Advantages of Landsat 8-OLI-Derived Suspended Particulate Matter Maps for Monitoring the Subtidal Extension of Amazonian Coastal Mud Banks (French Guiana)

The coast of French Guiana is characterised by the northwestward migration of large mud banks alongshore and by high concentrations of suspended particulate matter (SPM) resulting from the strong influence of the Amazon River outflow. Surface OLI SPM concentration, linked to the footprint of the subtidal part of mud banks due to resuspension and migration processes, was used to develop a method to estimate the location of this footprint. A comparison of the results from this method with those obtained by locating the limit of the wave damping, which characterises muddy coasts, revealed good performance of the method based on recurring SPM values. The migration rates of the mud banks in French Guiana were calculated according to the delimitation of their subtidal parts, and showed slightly higher values (2.31 km/year) than suggested by earlier studies. In comparison with other methods, the migration rate estimated using the method proposed within the framework of this study takes into account the variability of the shape of the subtidal part for the first time. It was also shown that the mud banks existing on the coastal area of French Guiana present two different shapes. Our results clearly demonstrate the advantage of ocean colour data to describe mud banks according to their subtidal part, delimited using the assessment of SPM temporal variability.

&lt;i&gt;Fragilariopsis&lt;/i&gt; Sp. Bloom Causes Yellowish-Brown Waters Off Alappuzha, South-Central Kerala Coast, India, during the Mud Bank-Upwelling Phase

Mud banks (Chakara) of Kerala are calm coastal waters that form in several isolated stretches along the coast usually during the southwest monsoon (SWM) period (June–September). They are characterized by the damping of incident waves, generating localized calm sea environment conducive for fishing activities, while the high monsoon waves create hostile environment in the rest of the region. Here, we present the scientific basis of the yellowish-brown discoloration of water column that occurs off Alappuzha, Kerala annually during the peak and late SWM associated with coastal upwelling-mud bank event. The discoloured waters that occur off Alappuzha associated with these events are locally known as ‘pola vellam’, which is nothing but diatom blooms. In 2014, pola vellam was actually caused by the bloom of Fragilariopsis (= Fragilaria ) sp.; hereafter Fragilariopsis , which was widespread in the study region, even beyond the mud bank domain. This bloom feature is attributable to the nutrient enrichment associated with intense coastal upwelling that was dominant over a larger spatial extent in the study domain, including the mud bank. FlowCAM-based plankton data strongly support the above view, as the abundance, biovolume and biomass of Fragilariopsis had similar temporal trend both in the mud bank and non-mud bank regions. The general ecology and importance of Fragilariopsis bloom in the study domain, from the point of view of commercial fisheries, is also elaborated in this communication.

Control of "blue carbon" storage by mangrove ageing: Evidence from a 66-year chronosequence in French Guiana.

The role of mangroves in the blue carbon stock is critical and requires special focus. Mangroves are carbon-rich forests that are not in steady-state equilibrium at the decadal time scale. Over the last decades, the structure and zonation of mangroves have been largely disturbed by coastal changes and land use conversions. The amount of time since the last disturbance is a key parameter determining forest structure, but it has so far been overlooked in mangrove carbon stock projections. In particular, the carbon sequestration rates among mangrove successional ages after (re)establishment are poorly quantified and not used in large-scale estimations of the blue carbon stock. Here, it is hypothesized that ecosystem age structure significantly modulates mangrove carbon stocks. We analysed a 66-year chronosequence of the aboveground and belowground biomass and soil carbon stock of mangroves in French Guiana, and we found that in the year after forest establishment on newly formed mud banks, the aboveground, belowground and soil carbon stocks averaged 23.56±7.71, 13.04±3.37 and 84.26±64.14 (to a depth of 1m) Mg C/ha, respectively. The mean annual increment (MAI) in the aboveground and belowground reservoirs was 23.56×Age-0.52 and 13.20×Age-0.64 Mg Cha-1 year-1 , respectively, and the MAI in the soil carbon reservoir was 3.00±1.80Mg Cha-1 year-1 . Our results show that the plant carbon sink capacity declines with ecosystem age, while the soil carbon sequestration rate remains constant over many years. We suggest that global projections of the above- and belowground reservoirs of the carbon stock need to account for mangrove age structures, which result from historical changes in coastal morphology. Our work anticipates joint international efforts to globally quantify the multidecadal mangrove carbon balance based on the combined use of age-based parametric equations and time series of mangrove age maps at regional scales.

Mud Banks along the southwest coast of India are not too muddy for plankton

Considering Alappuzha Mud Bank in the southern Kerala coast as a typical case of biologically productive Mud Banks that form along the southwest coast of India during the Southwest Monsoon (June - September), the present study addresses several pertinent missing links between the physical environment in Mud Banks and their influence on plankton stock. This study showed that very strong coastal upwelling prevails in the entire study domain during the Southwest Monsoon, which manifests itself in the form of significantly cool, hypoxic and nitrate-rich waters surfacing near the coast. The upwelled water persisting throughout the Southwest Monsoon period was found to have fuelled the exceptionally high phytoplankton stock in the entire study area, including the Mud Bank region. Having accepted that Mud Banks are special because of the calm sea surface conditions and relatively high turbidity level in the water column around them, the present study showed that except at points close to the sea bottom, turbidity level in the Alappuzha Mud Bank was below the critical level to inhibit the plankton stock. The suspended sediments that form in the Mud Bank occasionally could be attributed to the disturbance of the bottom fluid muddy layer and their vertical spurts.

Upwelling induced changes in the abundance and community structure of archaea and bacteria in a recurring mud bank along the southwest coast of India

The present study reports the influence of upwelling on the abundance and community structure of archaea and bacteria in a mud bank formed along the coastal waters of Alappuzha, the southwest coast of India. The signatures of upwelling were observed with the onset of southwest monsoon, characterized by a decrease in sea surface temperature and dissolved oxygen in bottom waters and increase in salinity. Nitrite, nitrate, and phosphate showed a significant increase during upwelling compared to pre-upwelling conditions. There were no significant differences in the abundance of archaea across seasons (pre- and during upwelling conditions) and regions (Station S1, inside and Station S2, outside the mud bank), while the bacterial abundance showed a six-fold increase for S1 and four-fold increase for S2 during upwelling. The PCR–DGGE results show the existence of a transient population of archaea and bacteria in the mud bank region during upwelling. The step up multiple regression models could explain 66.5% and 56.3% of the temporal variations in archaeal and 96.5% and 53.63% in bacterial distribution inside and outside of mud bank respectively, at pre-upwelling conditions. About 67% of the temporal variations in the distribution of archaea and 41.4% in bacteria inside and 49.9% in bacteria outside of mud bank during upwelling conditions could be explained by the same model. The temporal variation in the distribution of archaea outside mud bank during upwelling conditions could not be explained by the present regression model. Our studies indicate that the upwelling induces changes in the abundance and community structure of bacteria in the mudbank, which are modulated by the interactive effects of hydrographic variables. Further studies to unravel the interactive effect of hydrographic variables on the biogeochemical activities of microorganisms would be appreciated.

Correction to: Formation Mechanism of Mud Bank Along the Southwest Coast of India

In the original article, V. Samiksha’s family name was incorrect. It is correct as reflect in this erratum. The original article has also been updated.

Formation Mechanism of Mud Bank Along the Southwest Coast of India

Mud bank formation during the southwest monsoon along the southwest coast of India remains an enigma to the researchers and coastal community in spite of several earlier studies. The present study attempts to unravel the mystery through a high-frequency, season-long time-series observation at Alappuzha, located at the southern part of the west coast of India, a region of frequent occurrence of mud bank. Using 7-month-long weekly time-series observation, we identified strong winds and high waves associated with onset of the southwest monsoon and subsequent three episodic atmospheric low-pressure events (LPEs).With the help of in situ time-series data, we show that the strong winds and high waves associated with southwest monsoon pre-conditions the near shore bottom sediment to bring it into suspension. The high amplitude waves associated with the southwest monsoon, while propagating from the deep water to shallow water region, interact with the bottom initiating bottom-sediment movement and its suspension due to wave refraction and shoaling. The sporadic occurrence of the atmospheric LPEs enhances the process of suspension of bottom sediment in the near shore region leading to the formation of fluid mud. Simulations with a cohesive sediment transport model yielded realistic estimates of sediment transport, in the presence of an onshore current, a pre-requisite for transporting the fluid mud toward the coast. The prevailing onshore upwelling current during the southwest monsoon provides the favorable pre-requisite conditions for transporting the fluid mud through depression channel network towards the coast. Once sufficient quantity and thickness of fluid mud is accumulated in the near shore region, it acts as a wave damper for subsequent high monsoon waves, as indicated by the time-series wave data, leading to the formation of tranquil mud bank region. Depression channel networks extending from the shelf to the coast off Alappuzha, Kochi, Ponnani, Beypore, and Ullal were found in the bathymetric charts, thus explaining why mud banks occur only at few locations in spite of the prevalence of similar monsoon conditions.

Seasonality in autotrophic mesoplankton in a coastal upwelling-mud bank environment along the southwest coast of India and its ecological implications

Mesoplankton refers to both autotrophic and heterotrophic plankton with a body size between 200 – 20,000µm. Here, we applied a FlowCAM to identify the autotrophs present in the mesoplankton size class in a coastal environment along the southwest coast of India (off Alappuzha, Kerala), which is characterized by intense coastal upwelling and mud bank formation during the Southwest Monsoon. 18 time series sampling (weekly/biweekly) sessions were carried out spanning over the Pre-Southwest Monsoon (April) to the Late-Southwest Monsoon (September) period in 2014. The study showed that during the Pre-Southwest Monsoon when nitrate level was relatively low in the study area, the mesoplankton community was entirely contributed by zooplankton, mostly consisting of copepods. During this time, the only autotrophic mesoplankton found in the water column, that too inconsistently, was Trichodesmium erythraeum. However, the entire scenario changed with the onset of the Southwest Monsoon due to hydrographical transformation and nutrient enrichment caused by the coastal upwelling. Especially during the Peak (July) and Late-Southwest Monsoon (August), the mesoplankton composition changed with a significant dominance of larger diatoms such as Fragilaria and Coscinodiscus. The autotrophic mesoplankton abundance was noticeably low during the Pre-Southwest Monsoon (av. 3145 ± 2396 individual m−3 and av. 2045 ± 1907 individual m−3 in M1 and M2, respectively), as compared to the Southwest Monsoon (av. 30436 ± 5983 individual m−3 and av. 32346 ± 11664 individual m−3 in M1 and M2, respectively). Similar was the seasonal trend in the autotrophic mesoplankton biomass, which increased from a low Pre-Southwest Monsoon value (av. 8.45 ± 7.1mgCm−3and av. 4 ± 3.7mgCm−3 in M1 and M2, respectively) to a significantly high value during the Southwest Monsoon (av. 117.04 ± 40.2mgC m−3 and av. 136.9 ± 68.1mgCm−3 in M1 and M2, respectively). The FlowCAM analysis results also showed that the dominant copepods present in the study region during the Southwest Monsoon (Temora turbinata, Oithona similis, Pseudodiaptomus serricaudatus and Centropages tenuiremis) were smaller in size than those present during the Pre-Southwest Monsoon period (Acartia danae, A. erythraea, and Centropages orsini). The ecological implication of the present observation is that the smaller-sized copepods grazing on larger phytoplankton during the Southwest Monsoon (upwelling) period cause sloppy feeding and lead to inefficient grazing of the larger diatom stock, eventually facilitating frequent phytoplankton blooms along the southwest coast of India.

Wave energy dissipation due to mudbanks formed off southwest coast of India

Mudbanks (MBs) are a unique natural phenomenon, and form along the southwest coast of India during the southwest monsoon. They are characterized by a calm-water region, bordered by a rough sea. In order to quantify the wave energy dissipation, wave data were collected at two water depths (15 m and 7 m) before and during the period of formation of MBs off Alappuzha, Kerala. The observations indicate that MBs exist even in deeper water beyond 15 m water depth, contrary to earlier findings that they only form in depths of 0–5 m. The analysis showed 65–70% wave height attenuation. As spectral density evolves with shoaling, energy dissipation was examined using the concept of wave energy flux. The combination of high frequency dissipation and nonlinear energy transfer from higher-frequency to low-frequency waves resulted in a reduction of energy across a wide frequency range. The WAVEWATCH III® (WW3) model with wave-mud interaction physics was used to capture the signature of wave energy dissipation due to MBs. The accuracy of prediction of significant wave heights (Hs) of the WW3 model was verified using Hs of measured waves and ERA- Interim (ECMWF Reanalysis Interim data). The model accurately reproduced both the wave heights in the MB region and their general characteristics. The measurements and model results complement each other in explaining changes associated with an apparent shift of the MBs.

Dominance of coastal upwelling over Mud Bank in shaping the mesozooplankton along the southwest coast of India during the Southwest Monsoon

Mud Banks are littoral zones along the Kerala (southwest) coast of India with increased suspended sediments during the Southwest Monsoon (SWM), which facilitate remarkable damping of incident waves creating a localized calm sea environment conducive for fishing activities. In this paper, we used the response of mesozooplankton community as a whole and copepod species in particular as a tool to characterize the relative importance of Coastal Upwelling and Mud Bank in shaping the biological manifestations along the southwest coast of India. The geographical area studied was off Alappuzha (Kerala coast) and altogether 18 field samplings (weekly/fortnightly) were carried out in three locations in the region (M1, M2 and M3) from the Pre-SWM to the Late-SWM of 2014. M1 was a reference location at the same depth contour (6m) and 10km north of location M2 that represented the region where the Mud Bank forms during the SWM. Location M3 was situated beyond the offshore boundary of the Mud Bank zone (12m depth), perpendicular to location M2. The physical characteristics of the Mud Bank, such as relatively high suspended sediments and calm sea conditions, existed at M2 by mid-June and before that all the three locations had comparable physical characteristics in the water column. Coastal Upwelling was prevalent over all the three locations by early-June, characterized by cool and less oxygenated waters, more so in the subsurface waters. The mesozooplankton community in the study domain was composed of 15 heterogeneous groups; their biomass and abundance were high during the Peak- (av. 6.02±5.19mlm−3 and av. 7285±3248Nom−3) and Late-SWM (av. 4.91±4.35mlm−3 and av. 6802±2727Nom−3) compared to the Pre-SWM (av. 0.63±0.55mlm−3 and av. 3045±1584Nom−3) and Early-SWM (av. 0.58±0.46mlm−3 and av. 2047±1675Nom−3). Mesozooplankton abundance was dominated by copepods (>70%) and the number of larger zooplankton carnivores (medusae, siphonophores and chaetognaths) was found to be low (<5%). Altogether, 43 species of copepods were recorded from the present study. Based on the abundance and species distribution of copepods in all three locations, cluster and SIMPROF analyses delineated four environmental conditions (clusters) typical of Pre-SWM (cluster 1), Early-SWM (cluster 2), Peak-SWM (cluster 3) and Late-SWM (cluster 4). SIMPER analysis demarcated the discriminating species of copepods in the environmental clusters delineated by cluster/SIMPROF. The discriminating copepods were composed of Acartia danae, A. erythraea, Centropages orsini (cluster1); Temora turbinata (the upwelling indicator) (cluster 2); Oithona similis (cluster 3); Temora turbinata, Pseudodiaptomus serricaudatus, Centropages tenuremis (cluster 4). The dominance and distribution pattern of copepods observed in all three study locations were very similar and the detailed Multivariate Redundancy analysis confirmed the dominant influence of Coastal Upwelling and not the Mud Bank in structuring the mesozooplankton/copepod community in the study domain. The present study provides the first in-depth information on the mesozooplankton (especially copepods) from a Coastal Upwelling - Mud Bank system along the southwest coast of India, evidencing the dominant role of Coastal Upwelling in shaping the zooplankton community during the SWM.

Influence of upwelling induced near shore hypoxia on the Alappuzha mud banks, South West Coast of India

The results of the first time-series measurements spanning 18-weeks (22 April to 20 September 2014) from a coastal environment (Alappuzha, southwest India), where two process of upwelling and mud banks are concurrent during summer monsoon are presented. The upwelling signals were evident from the 2nd week of May onwards and the intensity varied widely from 1st week of June to 3rd week of September. The formation of mud banks was observed during the peak upwelling period (14th June) and once formed, they were persistent till the end of monsoon season (September 2014). The time-series evolution of dissolved oxygen levels showed the incursion of hypoxic waters well in to the mud banks during peak upwelling period. The considerable reduction in the dissolved methane levels (300 to <50nM) during hypoxia is an anomaly, which may be due to some microbial processes. The incursion of hypoxic waters in to the Alappuzha mud banks is the first observation and is significant because, it can now explain why mud banks are not always good fishing grounds. The present study shows that the fish availability is mainly dependent on the incursion and prevalence of hypoxic waters in the mud banks.

Discriminating the biophysical impacts of coastal upwelling and mud banks along the southwest coast of India

Abstract Coastal upwelling and mud banks are two oceanographic processes concurrently operating along certain stretches of the southwest (Kerala) coast of India during the Southwest Monsoon period (June–September), facilitating significant enhancement in plankton biomass. Mud banks have scientific and societal attention from time immemorial, predominantly due to the large fisheries associated with them. In this paper, for the first time, the specific biophysical roles of these oceanographic processes have been discriminated, based on a focused 18 weekly/fortnightly time-series study (April to September 2014) in a mud bank-upwelling area (off Alappuzha, southwest coast of India). In conjunction with standard hydrographical and satellite remote sensing data, we utilised a FlowCAM to track the biophysical linkage in terms of plankton composition abundance and size structure at three locations (M1, M2 and M3) in the study area. During the Pre-Southwest Monsoon (April–May), the entire study area was warmer with low nitrate concentration in the surface waters, which caused lower biomass of autotrophs compared to the Southwest Monsoon (June–September). By the onset of the Southwest Monsoon (June), drastic hydrographical transformations took place in the study domain due to the Coastal upwelling, reflected as the surfacing of significantly cool, high nutrient and hypoxic waters. Concurrently, mud bank formed at location M2 due to the presence of relatively high-suspended sediments in the region, creating a localised calm environment conducive for fishing activities. In response to the hydrographical transformations in the entire study area during the Southwest Monsoon, the autotrophic plankton biomass and size structure experienced significant change. The micro-autotrophs biomass that was low during the Pre-Southwest Monsoon (av. 0.33 ± 0.2 mgC L− 1 at surface and av. 0.07 ± 0.04 mgC L− 1 at subsurface) noticeably increased during the Southwest Monsoon (av. 1.6 ± 0.4 mgC L− 1 at surface and av. 1.3 ± 0.2 mgC L− 1 at subsurface). The seasonal mean bio-volume of micro-autotrophs followed the same pattern in all three locations with the dominance of smaller individuals during the Pre-Southwest Monsoon (av. 55 ± 4.4 × 103 μm3 individual− 1 at surface and av. 67.1 ± 38.4 × 103 μm3 individual− 1 at subsurface). Relatively large phytoplankton dominated during the Southwest Monsoon (av. 77.3 ± 5.5 × 103 μm3 individual− 1 at surface and av. 90.3 ± 4.9 × 103 μm3 individual− 1 at subsurface). Similar spatial change in the plankton composition, biomass and size structure in all three locations suggested the dominant role of coastal upwelling, and not the mud bank, in shaping the dominant autotrophic plankton in the study domain. The nutrient enrichment in the region associated with the coastal upwelling facilitated the predominance of a classical short food chain capable of supporting large fish stocks. In addition to relatively calm mud bank, the hypoxic, cool upwelled waters that existed in the subsurface induce the fish stock to concentrate in the surface waters, making the relatively calm mud bank and adjoining areas a rich fishing ground for the native fishers.

Hydro-sedimentary processes of a shallow tropical estuary under Amazon influence. The Mahury Estuary, French Guiana

Along the Guianas coast, coastal dynamic is characterized by the migration of mud banks originating from the Amazon. This singular feature affects the dynamic and the morphology of local estuaries and can induce rapid bathymetric evolution in lower estuaries. Since 2012, the navigation channel of the Mahury Estuary (French Guiana) is enduring a severe siltation whose origin comes from a mud bank crossing the estuary mouth. This study aims to determine how the migration of a mud bank through an estuary mouth could influence the transport and fluxes in the estuary. Field measurements were performed over a year with the monitoring of the salt intrusion length, mooring surveys during spring-neap cycles and shipboard profiling surveys during semi-diurnal cycles. Salt intrusion lengths underline a significant seasonal variation characterized by the transition from a steady-state length during high river discharge and a wide range of lengths with the tidal range during low to moderate river discharge. During the rainy season, measurements indicate a fluvial-dominated condition with low suspended-sediment concentrations most of the semi-diurnal cycle. Residual sediment fluxes are usually seaward excepted when river discharge is below seasonal average. During the dry season, maximum suspended-sediment concentrations are higher in the middle part of the estuary. Residual sediment fluxes are landward along the estuary and stronger during neap tides in the estuary mouth and few kilometers upstream. In this area, a persistent density stratification traps sediments in the bottom layer and generates a gravitational circulation during neap tides, which enhances landward transports up to 2.56 t m−1 over a semi-diurnal cycle. In the middle estuary, landward fluxes are most significant during the dry season and also during the rainy season when the river discharge is below the seasonal average. Although this study includes temporal and spatial limitations, it underlines significant mud inflows in the middle part of the estuary during low to moderate river discharges. Comparison with old data suggests higher sediment loads in the estuary during the migration of a mud bank but must be confirmed by further studies during the interbank period.