Northern Bay Research Articles

First report of the fish family Pomacentridae (Damsel fishes) from Sunderban biosphere reserve, India

Sundarban Biosphere Reserve is the largest single strand mangrove forest in the world and houses 350 species of fishes. The present work reports the presence of Indo-Pacific Sergeant fish (Family Pomacentridae) Abudefduf vaigiensis (Quoy & Gaimard, 1825) from the Sundarban Biosphere Reserve, West Bengal. This is the first report of this family and species from the northern Bay of Bengal region. The species was confirmed through a single specimen collected from Lothian Island in the southern side of Sundarban and the same was compared with a specimen collected from the Arabian Sea.

Observations of contrasted glacial-interglacial dissolution of foraminifera above the lysocline in the Bay of Bengal, northeastern Indian Ocean

Site U1446 (19°50′N, 85°44′E, at water depth 1 430 m) was drilled during Expedition 353 (Indian monsoon rainfall) of the International Ocean Discovery Program (IODP). It is located in the Mahanadi offshore basin, on the northern Bay of Bengal. Sedimentation rates and contents of biocarbonates are high at this relatively shallow site. Using a micropaleontological approach, we examined planktonic and benthic foraminifera in the upper around 40 m of this site, spanning the last around 190 ka. A striking feature of the foraminiferal record is the occurrence of strong but varying dissolution although the site is located well above the modern lysocline. Such strong dissolution has never been reported in this area. We estimated the flux of foraminifera and quantified the ratio of benthic foraminifera over total foraminifera (benthic/total foraminifera) along with the foraminifer fragmentation index in order to characterize past changes in this above-lysocline dissolution. This study reveals a clear glacial-interglacial contrast, with a stronger dissolution during marine isotope stages (MISs) 1 and 5 than during MISs 2–4 and 6. Such a difference in preservation is likely to have a strong impact on geochemical proxies measured on foraminifera. Our new observations call for an in-depth study of the causes of such above-lysocline dissolution in the region, and an evaluation of its impact on the foraminifera-based proxies used for paleoenvironmental reconstruction.

Formation and dissipation dynamics of the Asian tropopause aerosol layer

The Asian tropopause aerosol layer (ATAL) is characterized by enhanced aerosol concentrations in the Asian summer monsoon anticyclone in the upper troposphere and lower stratosphere at 13–18 km altitude. A growing body of evidence suggests that the aerosol enhancement is closely connected with deep convection during the monsoon. However, the origin of the aerosols is under debate, and the key factors that determine the ATAL variability remain poorly understood. We investigated the formation and dissipation mechanisms of the ATAL and the inter-annual variation from a dynamical viewpoint using satellite observations and meteorological reanalysis data from 2012 to 2018. We identified the northern Bay of Bengal and adjacent land area, where air pollution from the Indian subcontinent converges, as the major convection source area of aerosols to the ATAL. The spatial extent of the ATAL, represented by the mean attenuated scattering ratio from satellite measurements, appears to be related to a secondary circulation driven by the stratospheric quasi-biennial oscillation. The aerosols are not homogeneously distributed within the ATAL, and descending motion in the western part is found to play an important role in dissipation of the layer. These findings elucidate the ATAL dynamics and associated regional and global air pollution transports.

Impact of Remote Equatorial Winds and Local Mesoscale Eddies on the Existence of “River in the Sea” Along the East Coast of India Inferred From Satellite SMAP

AbstractThe Northern Bay of Bengal (NBOB) exhibits extreme salinity stratification caused by intense summer monsoon rainfall and associated river discharge. This accumulated fresh water plume in NBOB is believed to be transported along the East coast of India (ECI) by East India Coastal Current (EICC), known as River in the Sea (RIS). Though in situ sea Surface Salinity (SSS) measurements are sparse, the advancement of satellite monitoring system has provided a unique opportunity to unravel salient features of RIS. In the present study, in situ SSS at five selected regions around the Indian coast and open ocean are utilized to validate the Soil Moisture Active Passive (SMAP) (L3, v4.3) products. Further, this new SMAP data is used to examine year‐to‐year variability of RIS during 2015–2019 and the driving forces responsible for its existence. This SMAP SSS exhibited 1.52, 0.46, 0.38, 1.45 RMSD and 0.97, 0.82, 0.85, 0.86 correlation in NBOB1, NBOB2, Central BOB (CBOB) and Western BOB (WBOB), respectively with in situ measurements. This data also depicted the presence of “RIS” all along the ECI only during November in consistent with the magnitude of Kelvin wave induced EICC. The southward extension of RIS along the ECI is primarily based on the magnitude of remote equatorial wind induced forces. The mesoscale eddies in south‐western BOB (SWBOB) alters the shape of RIS and even inhibit its southward propagation completely.

Coastal morphological changes: Assessing long-term ecological transformations across the northern Bay of Bengal

Major rivers from the Himalayas carry a high volume of sedimentation, and deposit it across the Bay of Bengal in Bangladesh. This has caused significant changes in the morphology of the bay, including the development of islands across the bay area. However, few studies have been carried out on the morphological changes, especially the development of new islands across the northern Bay of Bengal. This study, therefore, aimed to assess the coastal morphological changes and ecological succession of the newly formed islands of the bay area. We used state of the art cloud computing technologies, using the Google Earth Engine (GEE) platform. Publicly available annual composites of Landsat 8, Landsat ETM+, and TM data from 1989 to 2018 were used for analysis. The findings showed significant changes in the morphology of the coastal area over a period of 30 years. There was a 1.15% increase in land area between 1989 and 2018. New islands were formed across the bay, and a few old islands disappeared between 1989 and 2018. The majority of the offshore islands developed in the estuary of the Meghna River. Among the quickly grown islands, Bhashan Char, Char Nizam, Jahajerchar, and Urir Char are prominent. Initially, the islands appeared as barren areas without any vegetation, but different types of vegetation have been observed growing on the newly formed islands recently. The findings of this study are important for the conservation and development planning of newly formed islands.

Modeling spatio-temporal variability of suspended matter and its relation with hydrodynamic parameters in the northern Bay of Bengal

Variation of the non-algal suspended particulate matter (SM) was assessed, both on a spatial and temporal scale using satellite datasets. Located on one of the biggest deltas around the world, enormous rivers, and sediment discharge make this region’s suspended matters more prone to variations. Noticeable latitudinal transformations were found in SM as it ceased to exist after 21.5° N, with discrete seasonal changes that included summer SM concentration as high as 90–95 g/m3, while in winter it was 70–75 g/m3 on average near the river mouth. Such contrasting values are because of the variation in input from the nearby major rivers including Ganges–Brahmaputra, which in terms brings suspended load, particulate matters, nutrients, and other material fluxes along with it. However, other regions had a seasonally averaged concentration of 36 g/m3. Temporal analysis of SM revealed that it consisted of an increasing pattern between 2004 and 2010, but it gradually altered to decreasing trends from 2012 to 2018. Secchi depth, chlorophyll-a, nitrate, phosphate, silicate, sea level anomaly, current speed, temperature, and salinity are the hydrodynamic parameters those are considered in this article to find which one is well correlated with SM. Nitrate and silicate proved a significant positive correlation with SM, whereas chlorophyll-a, salinity, and Secchi depths were negatively correlated over the whole period. Eventually, further wavelet analysis gave rise to an interesting sign of the SM on chlorophyll-a for specific regimes of the time.

Seasonal and annual variability in chlorophyll-a in the shelf region of the Northern Bay of Bengal using MODIS-Aqua data

Abstract Chlorophyll-a (Chl-a) concentration is an important issue in ocean ecosystem management and research. This study investigates seasonal and annual variability in Chl-a and its relationship with sea surface temperature (SST) and river discharge in the shelf region of the Northern Bay of Bengal (BoB), as well as validates satellite data against in-situ data. Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua satellite data on Chl-a concentration and SST from 2002–2018 were used in this study. River discharge data were obtained from the Bangladesh Water Development Board (BWDB). The annual Chl-a concentration ranged from 2.08 to 2.94 mg m−3, with an average of 2.43 ± 0.24 mg m−3. The Chl-a concentration was found higher (2.21 ± 0.56 mg m−3) during the northeast monsoon (October–February) and lower (1.81 ± 1.14 mg m−3) during the pre-monsoon season (March–May). The study revealed a declining trend in Chl-a concentration from 2002 to 2018, and the rate of change was −0.0183 mg m−3 year−1. Chl-a concentration showed a weak inverse relationship with SST, both annually and seasonally, especially in the pre-monsoon season. River discharge masked the effect of SST on Chl-a variability during the southwest and northeast monsoon. A reasonable correlation (r = 0.78) was found between the MODIS-Aqua data and in-situ Chl-a observations.

Influence of warm core eddy on the vertical distribution of autotrophic pico- and nanoplankton in the Bay of Bengal

ABSTRACT The distribution and ecology of autotrophic pico- and nanoplankton were studied in the warm core eddy-affected northern Bay of Bengal (BoB) during the peak south-west monsoon of 2012. Small phototrophs showed diverse response patterns to the contrasting environmental setting in the northern BoB during the south-west monsoon. The convergence of surface waters in the eddy-influenced region was evidenced in terms of a deeper mixed layer and lower concentrations of . Synechococcus and picoeukaryotes were found to be more abundant in coastal waters as compared with the warm core eddy-influenced and offshore waters. Prochlorococcus was more abundant in the eddy-influenced and the offshore regions possibly as a consequence of their ability to survive in low-nutrient/oligotrophic waters. The abundance of Synechococcus increased noticeably in the coastal and subsurface waters due to their affinity towards higher concentrations of nutrients. The present data therefore support the view of a community shift induced by warm core water eddies and show their importance in promoting primary production by Prochlorococcus in the mixed layers of the BoB.

Hydrodynamics and sediment dynamics in Barataria Bay, Louisiana, USA

Barataria Bay is a receiving basin for a large Mid-Barataria Sediment Diversion scheme in Louisiana, USA. In this region, data on sediment transport and hydrodynamics are scarce but essential for the design and planning of future sediment diversion and marsh creation. Four months of bottom boundary layer observations were conducted to study winter and spring hydrodynamics and sediment dynamics in this coastal bay. Hourly waves, tides, currents, and bottom suspended sediment concentrations were measured using multiple optical and acoustic sensors attached to two tripod platforms. High temporal resolution data indicated that the salinity in the northern bay was mainly controlled by northerly winds, and tidal currents kept the salinity high in the southern bay during the winter cold front season. In spring, frequent, pervasive southerly winds and the westward shelf transport of less saline water emerging from the Mississippi River Delta lowered the salinity in the southern bay. Spectral analysis showed that wave-current combined shear stress played the most critical role in triggering sediment resuspension. The Style-Glenn 1-D bottom boundary layer model was applied in sediment flux calculations, and showed that net sediment transport mainly occurred during cold front passages. During two 3-day cold-front events, southward sediment fluxes accounted for 56% of the total sediment flux during the 35-day winter period, revealing their nonlinear, event-driven, and episodic nature. The direction of the sediment transport generally rotated, and its magnitude changed considerably, when southeasterly winds shifted to intensified northwesterly winds. Long-duration southerly pre-frontal winds facilitated wetland sedimentation by transporting sediment to the northern bay during high water level conditions impacted by flooding spring tides or southerly winds. Conversely, northerly winds during cold fronts dominated bidirectional tidal currents and led to southward net sediment transport and possible escape of sediment from the bay. The timing of diversion openings, the orientation of receiving basins, and dominant wind directions in relation to fetch, as well as the dynamic water levels should be considered in the planning and management of future diversion operations in coastal areas.

Environmental influence on the picophytoplankton community structure in the central and northern Bay of Bengal

The Bay of Bengal (BoB) is a low productive, oligotrophic ecosystem. The picophytoplankton (Pico; <3μm) play a significant role in the biogeochemistry of such ecosystems. The information on the Pico is exiguous from this Bay. Here, the seasonal and spatial variations in abundance and carbon biomass of the Pico groups were investigated (July’08 to April’09) in the central and northern BoB in relation to the environmental conditions. Surface samples were collected from twenty-two stations across the Chennai–Port Blair–Kolkata sector. The Synechococcus group was dominant during most of the year, both in abundance and biomass, with few exceptions when Prochlorococcus dominated the Pico community. The Synechococcus abundance and carbon biomass were higher in the nutrient-rich coastal regions and correlated negatively with salinity. The picoeukaryotes exhibited a distribution pattern similar to Synechococcus, although relatively lower in numbers and biomass. The Prochlorococcus abundance and biomass were higher in the open ocean regions of the central BoB (CBoB). The relatively lower abundance of Prochlorococcus in the northern BoB implies an influence of the freshwater influx, as depicted from the positive correlation with salinity. During the winter monsoon, a cyclonic storm coincided with a deep mixed layer characterized by high concentrations of nutrients and chlorophyll-a in the CBoB. The Pico abundance and biomass were relatively higher during this period, with the Prochlorococcus dominating the former and Synechococcus, the latter. The real-time observations of the surface distribution of Pico abundance and carbon biomass in the BoB revealed seasonal variations that were modulated by the episodic mesoscale features.

Precipitation Characteristics and Moisture Source Regions on Mt. Everest in the Khumbu, Nepal

Summary Precipitation is critical to the water towers of the Hindu Kush-Himalaya-Karakoram region, exerting an important control on glacier mass balance and the water resources for 1.65 billion people. Given that hydroclimatic extremes and water stress have emerged as key hazards in the context of climate change, Nepal's Khumbu region overlaps key vulnerabilities. Here, we investigate the region's precipitation characteristics and moisture sources through analysis of data from a new high-altitude network of automatic weather stations, which allows for a more complete understanding of the climatological precipitation data that are critical information for local communities in the Khumbu region, visitors, and downstream populations. Our findings demonstrate that the northern Bay of Bengal is potentially an important moisture source during the monsoon period (June to August) and that westerly trajectories over land predominate for precipitation events during the post-monsoon, winter, and pre-monsoon seasons.

Variability of hydrometeorological situation during fish kill events in the summer seasons of 2010–2018 in the Sea of Azov

Hydrometeorological situations during the fish kill events that took place in the Belosaray, Obitochny and Kazantip Bays in the summer seasons of 2010–2018 have been analyzed. Average monthly background water and air temperatures and wind velocity are considered, as well as their average daily values before, during and after a fish kill event. It is shown that background characteristics of water temperature and wind velocity influenced the development of a fish kill event more directly and definitely than air temperature, which influence was only indirect. In the majority of cases, hydrometeorological situation, leading to fish kill event, developed according to the typical pattern with increasing air and water temperature and decreasing wind activity. In the northern bays, the Belosaray and Obitochny Bays, fish kill events were recorded during the first half of summer season, and in the southern bay, the Kazantip Bay, they occurred in the second half of it. Since 2012, due to significant decrease in vertical thermohaline stability and increase in water homogeneity, no significant fish kill events were recorded under various temperature and wind conditions. Satellite imaging of sea surface temperature confirmed typical course of fish kill event development: the temperature increased in the pre-event period and then, after the event, stayed high in the southern bays and decreased in the northern ones.

Spatio-temporal variation of transparent exopolymer particles (TEP) and their sinking flux in a temperate bay: Jiaozhou Bay, China

This study investigates transparent exopolymer particles (TEP) distribution, seasonal variability and its sinking flux in a shallow coastal sea, Jiaozhou Bay, China. The TEP concentrations in the bay ranged from 75 to 553 μg xanthum gum equivalents per liter (Xeq. L−1), which was within the range of values reported in other coastal seas. Spatially, high levels of TEP were always observed in the northern bay during all four seasons. In the southern and outer bay, TEP levels were relatively low. Significant positive correlations were observed between TEP and chlorophyll a during spring, summer and fall, suggesting that phytoplankton was the primary source of TEP during these three seasons. Seasonally, TEP concentrations were highest in summer (mean = 275 ± 142 μg Xeq. L−1), followed by winter (mean = 209 ± 53 μg Xeq. L−1), and lowest in spring (mean = 142 ± 43 μg Xeq. L−1) and fall (mean = 180 ± 49 μg Xeq. L−1). High phytoplankton biomass and temperature contributed to the high TEP levels in summer, while the relatively high amount of TEP in winter was most probably derived from re-suspension of sedimentary particles. For the whole year, TEP carbon (TEP-C) contributed to a mean of ~26% of POC in the bay, representing the second most important contributor to POC pool after phytoplankton (mean: 38%). Sinking flux of TEP was measured with sediment trap, and the results revealed that sinking TEP contributed to a mean of 20% of total POC flux for the four seasons in the bay. This study highlights the fact that TEP-C could represent a significant fraction of the POC pool in the coastal sea, and their sedimentation could play an important role in the carbon sedimentation in these areas.

Effect of landfall location and coastal topography on surge response in the Northern Bay of Bengal

Storm surges caused by frequent cyclone events have posed severe threats to the inhabitants of the northern Bay of Bengal. In this study, a high-resolution, two-dimensional, hydrodynamic storm surge model was developed using the Semi-implicit Cross-scale Hydroscience Integrated System Model (SCHISM) to investigate storm surges in the Bay of Bengal. After validation, the storm surge of the study area was simulated for 11 idealized cyclone cases, the tracks of which were approximately perpendicular to the head of Bay of Bengal when landing at high-tide and low-tide time, respectively. The simulation results show that when landing at the high-tide time, the cyclone produced a peak surge height of about 85% of that when landing at the low-tide time, on average. It is also shown that when cyclone landed during the low-tide time, there was a delay of about 90-130 min for the arrival time of peak surge at the right side of the landfall position, compared with that landed during the high-tide time. Finally, we found that, for the area of the head of the bay, the surge response in the middle-eastern part was weak than that in other parts. This study demonstrates the importance of including the tide-surge interaction in the simulation of storm surge and provides the difference of the surge responses between different shore sections of the study area, which may be useful for the prediction and mitigation of the storm surge disaster.

Phytoplankton community dynamics in a coastal bay under upwelling influence (Central Chile)

Marine coastal areas have high social-economic relevance and ensuring the good quality of these areas has gained importance in the past decades due to the increase of anthropogenic pressures. Understanding the dynamics of these areas is crucial to support management decisions. In this context, phytoplankton communities are generally used as key indicators of environmental quality as they respond in short time-scales and are at the base of food webs. This work intends to improve our understanding on how phytoplankton communities respond to environmental conditions, particularly in upwelling areas. Seawater samples were collected from Algarrobo Bay, Central Chile, for both phytoplankton pigments and microscopy analysis. Data were analyzed in combination with physico-chemical parameters and two distinct hydrographic regions were identified for the study area: i) a region occupying most of the northern bay section, characterized by lower temperatures and dissolved oxygen (DO), higher concentration of nutrients and higher N:P, where a mixed community with diatoms, chrysophytes, cryptophytes, haptophytes and other small flagellates were observed; and ii) a central-southern section, with higher temperatures and DO, lower nutrients and lower N:P, where diatoms, in particular the centric diatom Chaetoceros Ehrenberg, 1844 sp. (~92.5% on average), dominated the phytoplankton assemblage. Other common but much less abundant diatom genus included Skeletonema Greville, 1865, Eucampia Ehrenberg, 1839, Dactyliosolen Castracane, 1886, Nitzschia Hassall, 1845, Cylindrotheca Rabenhorst, 1859 and Asterionellopsis Round, 1990. Apart from diatoms, the most abundant taxa belonged to flagellates, mainly chrysophytes (228.4 × 103 cells L−1 on average) and cryptophytes (213 × 103 cells L−1 on average). Overall, results suggest that the southern part of the Algarrobo Bay, with higher residence times and nutrient discharges, may act as an upwelling shadow area, where phytoplankton species can accumulate.

Ocean heat content and its role in tropical cyclogenesis for the Bay of Bengal basin

Anthropogenic factors continue to warm the oceans by storing sufficient memory of past accumulated effects resulting in higher Sea surface temperature (SST) and Ocean heat content (OHC) invigorating tropical cyclogenesis and tropical cyclone intensity. Prior studies indicate that an increase in accumulated tropical cyclone heat potential, a measure of OHC is directly related to high intense storms of longer durations. The present study is an attempt to understand the role of OHC in tropical cyclogenesis for the Bay of Bengal (BoB). Unlike the other global ocean basins, the BoB is a semi-marginal sea in the North Indian Ocean (NIO) having differential water mass characteristics attributed due to tremendous influx from riverine discharges, seasonal reversing monsoonal wind system and complex ocean circulation features. This study provides a detailed assessment on SST and OHC distributions during the pre- and post-monsoon seasons covering a period of 21 years (1991–2016) over four identified sub-domains, the potential locations for tropical cyclogenesis. The daily ERA-Interim, ORAS4, ORAS5 and 3-h RAMA buoy datasets were used in the present study. The spatial distribution and variations of SST and OHC anomalies during pre- and post-monsoon seasons in the BoB were also examined. Differential nature of inter-annual SST variability was noticed in the study region. Highest variability in SST is evidenced during both pre- and post-monsoon seasons for the northern Bay region showing a warming bias of ∼ 1.7 °C in the past 2 decades. Contrasting variability is also noticed for the eastern Bay during pre-monsoon that is almost three times higher as compared to the post-monsoon season. Maximum OHC variability is found for the southern and central-eastern Bay regions. It is seen that OHC variability has a strong tele-connection with Oceanic Nino Index associated with El Nino/La Nina events in the Pacific basin with their amplitudes found to increase from northern to southern regions of the Bay.

Distributional patterns of humpback whales (Megaptera novaeangliae) along the Newfoundland East Coast reflect their main prey, capelin (Mallotus villosus)

AbstractOn the Newfoundland foraging ground, humpback whales (Megaptera novaeangliae) primarily consume capelin (Mallotus villosus), which experienced a population collapse in the early 1990s, associated with altered timing of spawning and spawning migration. We examined whether humpback whale movement and distribution match these prey changes. Combining tour company whale sighting reports and photographs, citizen science reports of capelin spawning and scientific monitoring, whales were found to move northward along the east coast and whale aggregation presence within bays was associated with spawning capelin presence, being later in northerly bays. Whale aggregations arrived 8–20 days later than spawning capelin in northern bays, however, suggesting inconsistent tendencies to track high abundance spawning capelin aggregations during migration. Repeated scientific surveys during July–August 2009, 2010, 2012, 2014–2017, within a biological hotspot associated with capelin spawning sites in Notre Dame Bay, revealed that whale presence was influenced by the date of capelin spawning rather than capelin abundance metrics (i.e., biomass, number of shoals, shoal density, shoal area). A photo‐identification catalog compiled during July–August, 2003–2017, revealed a 22% return rate of whales to the hotspot. Overall, findings suggest that capelin spawning sites are important foraging areas for humpback whales in coastal Newfoundland under these altered prey conditions.

Prediction of monthly Hilsa (Tenualosa ilisha) catch in the Northern Bay of Bengal using Bayesian structural time series model

Monthly catch of Hilsa in the Northern Bay of Bengal was modeled and predicted using the Bayesian structural time series (BSTS) with chlorophyll and rainfall as explanatory covariates. Chlorophyll and rainfall were used in the model with a lag time of eleven months and one month, respectively. These lag time values were determined from the exploratory data analysis. In this study, four separate models were developed to predict the Hilsa catch and compare the results. In the first model, the catch was predicted based on trend and seasonality of the previous years’ catch data. The second and third models predicted monthly catch with chlorophyll (having a lag time of 11 months) and rainfall (having a lag time of 1 month), respectively, as the only covariate in the model. The fourth model had both chlorophyll and rainfall as covariates with a lag time of 11 months and 1 month, respectively. The training mean absolute percentage error (MAPE) of the fourth model (measuring model fit) was observed as 0.2827, while the test MAPE (measuring model accuracy) was 0.2786. The coefficient values for both chlorophyll and rainfall were very close (0.999 and 1.064) which indicated the almost equal effect of both the parameters on the Hilsa catch. This study indicated that more abundance of the food material (phytoplankton) during the post-monsoon season would promote the growth and maturity of more Hilsa that would successively migrate towards river or estuary during the subsequent monsoon season and the catch would likely increase with the intensification of the southwest monsoon.

Densities of Large Marine Gastropods in Seagrass, Oyster Reef, and Sandy Habitats in Tampa Bay and along the Gulf Coast of Florida

Marine gastropods in Florida are harvested by the marine-life trade, tourists (for shells), and traditional commercial fisheries, but stocks are not formally assessed for most species. The main goal of this project was to estimate biological variables that may be used to evaluate the Species of Greatest Conservation Need status of the banded tulip Cinctura lilium and six other large marine gastropod species that are commonly encountered in the targeted habitats. The principal target, C. lilium, was listed in the 2012 State Wildlife Action Plan. The densities and size structures of C. lilium and six co-occurring species of large marine gastropods in Tampa Bay were examined to evaluate population dynamics, seasonal variation, and habitat preferences of these species. Long-term trends in the densities of large, predatory marine gastropods in 10 regions from 2009 to 2017 were included. In Tampa Bay, strong habitat preferences were observed for all species studied: C. lilium and Melongena corona were most likely to be observed on oyster reefs, and Fasciolaria tulipa was found only in seagrass, whereas Strombus alatus, Triplofusus gigantea, Sinistrofulgur sinistrum, and Fulguropsis spirata were more common on soft sediments. Seasonal variations in densities were weak; seasonal differences in size structure were more pronounced. The smallest individuals of C. lilium appeared in spring and summer; F. spirata, S. alatus, and S. sinistrum reached a detectable size in fall. The smallest detectable M. corona appeared in winter. Regionally, there were more gastropods in southwestern bay grass beds (Tampa Bay, Sarasota Bay, and Pine Island Sound) than in coastal seagrass beds (Big Bend, Citrus, Hernando, Pasco, and coastal Pinellas) or northern bays (St. Joseph Bay and St. Andrew Bay), and each region had characteristic species. As a group, tulip snails exhibited modest local declines in densities at two sites, whereas the whelks and T. gigantea varied annually but did not exhibit consistent trends during 2009 to 2017. The present study establishes a baseline of abundance for seven species of common marine gastropods and recommends expanded monitoring in multiple habitats to improve future analyses of abundance trends.

Hourly maximum individual wave height in the Indian shelf seas—its spatial and temporal variations in the recent 40 years

The spatial and temporal variability of the high individual wave heights, which pose a hazard in India’s shelf seas, has not been investigated. In this study, the spatial and temporal variation of hourly maximum individual wave height (Hmax) in the Indian shelf seas is examined based on 40 years ERA5 Re-Analysis data from European Centre for Medium-Range Weather Forecasts. Waverider buoy data from five locations are used to assess ERA5 Hmax. The variability of Hmax in different temporal scales; daily, monthly and interannual and their linkages with major ocean-atmosphere coupled interaction in the Pacific and the Indian Ocean are presented. Wave climatology throughout India’s western shelf seas shows average Hmax exceeding 2 m during most of the year. Maximum Hmax in the western shelf seas is during May–July, whereas in the southern tip and the eastern shelf seas are during the tropical cyclones and are either in October–December or May. Maximum Hmax is 17.94 m, and it occurred at the northern Bay of Bengal on 25 May 2009 during the passage of tropical cyclone Aila. Spatial variations in Hmax between locations spaced at 1.5° latitude are less than 10% in the northwestern and southeastern shelf seas, but large variations (~ 28%) are in the southwestern and northeastern shelf seas. Annual mean Hmax shows contrasting trends when data for 1979 to 2018 and 1992 to 2018 is considered. Indian Ocean Dipole influence on the Hmax suggests negative correlations in the western shelf seas and positive correlations in the eastern shelf seas. The correlation with climate indices is similar for both significant wave height and Hmax.