Surface Chlorophyll Concentration Research Articles

Mechanism of skillful seasonal surface chlorophyll prediction over the southern Pacific using a global earth system model

This study investigates the physical mechanism involved in an Earth system model (ESM)-based global marine biogeochemical prediction system providing successful forecasts of surface chlorophyll concentrations over the southern Pacific. The significant correlation skill of the surface chlorophyll concentration over the south-central Pacific (SP region, 160°–110° W, 10°–5° S) appears up to a 15-month lead. In contrast to the previously known role of the vertical nutrient supply on the predictive chlorophyll concentration forecasts, the NO3 budget analysis indicates that this prediction skill over the SP region is mostly controlled by the meridional advection of nutrients. Further analysis indicates that the controlling mechanisms involved in chlorophyll variability over the SP region can be explained by atmospheric and oceanic dynamics during the ENSO events. During La Nina, equatorial NO3 anomalies are increased due to enhanced equatorial upwelling, and the climatological southward current then advects nutrient-rich waters from the equator to the SP region left( {{text{i.e.,,positive}}, - overline{v}frac{{partial {text{NO}}_{{3}}^{prime } }}{partial y}} right). In addition, anomalous easterly surface winds blow over the SP region as a circulation response to atmospheric diabatic heating anomalies during La Nina, which leads to southward current anomalies over the surface-layer ocean. This advects high climatological NO3 over the tropics to the subtropical south Pacific, which increases the NO3 anomalies left( {{text{i.e.,, positive}} - v^{prime}frac{{partial overline{{{text{NO}}_{3} }} }}{partial y}} right). This positive NO3 advection over the SP region is realistically simulated only at lead times shorter than 9 month, and the multi-season persistency of the nutrients contributes to the surface chlorophyll bloom at lead times longer than 1 year.

The daily resolved temperature dependence and structure of planktonic foraminifera blooms

Planktonic foraminifera (PF) life cycles are highly sensitive to marine conditions, which are evolving rapidly due to anthropogenic climate change. Even though PF shells in the sedimentary record serve as prominent proxies of past ocean conditions, very little is still known about their life cycles, particularly in oligotrophic environments. Here, we present a full annual record of PF fluxes (> 63 µm) from the oligotrophic Gulf of Aqaba, northern Red Sea, sampled at daily timescales during 2015–2016 using an automated time-series sediment trap. These results are coupled with daily surface chlorophyll-a concentrations, sea surface temperatures (SSTs), particulate organic carbon and bulk fluxes, together with monthly resolved vertical profiles of chlorophyll-a, temperatures and nutrient concentrations. The annual cycle of PF fluxes is controlled by SST changes that drive water column mixing and changes in food availability. PF species flux patterns and succession dynamics vary throughout the year, displaying large variability on previously undocumented daily-weekly timescales, and are not synchronized with lunar periodicity. On daily timescales, spring blooms show a complex structure and interplay between SSTs, chlorophyll-a surface concentrations and PF fluxes. These events deliver about a third of the total annual PF flux over a period of several weeks.

Circulation and Hydrography in the Western South Atlantic Shelf and Export to the Deep Adjacent Ocean: 30°S to 40°S

AbstractHigh sea surface chlorophyll concentration on the Argentine Continental Shelf frequently extends to the deep ocean in the vicinity of the Brazil/Malvinas Confluence (BMC). The offshore transport of shelf waters likely plays a key role in the biogeochemical balance of the western South Atlantic and promotes the offshore transport of planktonic species. We analyze data from an oceanographic survey carried out in the western South Atlantic shelf between 31°S and 38°S in October 2013. We describe the distribution and circulation of the water masses and focus on the exchanges with the open ocean. On‐shelf subsurface intrusions of oceanic waters and river discharge supply nutrients to the shelf. A low‐salinity tongue of Río de la Plata (RDP) waters extends northward to 32°S. Below these waters Subantarctic and Subtropical Shelf Waters (SASW and STSW) meet to form the Subtropical Shelf Front. The main SASW branch, oversaturated in oxygen and with high‐fluorescence mixes with a detachment of Brazil Current waters at 38°S and is exported offshore along the BMC. A second branch of SASW reaches 33°S mixing along its way with RDP and STSW and returns southward after splitting into an onshore and an offshore branch. The offshore branch is exported to the open ocean through the BMC. These export routes are in overall qualitative agreement with those indicated by a high‐resolution reanalysis. We estimate a net off‐shelf transport (geostrophic [96.2%] plus Ekman [3.8%]) of 3.44 Sv to the open ocean between 32.1°S and 37.7°S. The majority of the offshore flow occurs between 34.7°S and 37.7°S.

Climate change drivers influencing Indian mackerel fishery in south-eastern Arabian Sea off Kerala, India

The Indian mackerel Rastrelligerkanagurta(Cuvier, 1816) is one of the most important marine fishery resources along the south-eastern Arabian Sea along the coast of Kerala, south India. The effect of selected environmental variables on the Relative effort (Effort) and weighted catch per unit effort (cpue)of the fish were investigated using simple correlation and Path analysis. Six major oceanographic variables, namely sea surface temperature (SST), sea surface chlorophyll-a concentration (SSC), sea surface salinity (SSS), Precipitation (Pr) Indian Ocean Dipole (IOD) and Southern Oscillation Index (SOI) (ENSO index) were selected for the present study. Among these SST had the highest direct negative effect (-0.282, p<0.01), followed by SSS (-0.152, p<0.1) and IOD (-0.006, p<0.01). The highest positive direct effect on the cpue was exhibited by Pr (0.514, p<0.001) followed by SSC and SOI (0.178, p<0.01). The environmental variables also exerted indirect effects on cpue through Effort. The residual variance indicated that there are spurious effects exerted by environmental variables not included in the study. According to the coefficient of determination (R2), the relative importance of the influence of causally dependent environmental variables on the cpue of Indian mackerel is Pr> SSC >SSS.

Linkage of the physical environments in the northern Antarctic Peninsula region to the Southern Annular Mode and the implications for the phytoplankton production

The long-term (almost 20 years) hydrographic and primary production data collected by the U.S. Antarctic Marine Living Resources (AMLR) program during the austral summer near Elephant Island and the South Shetland Islands were combined with satellite observations to assess interannual variability in environmental conditions and production of the northern Antarctic Peninsula (NAP). Correlation analyses show that interannual variability of the region is related to the dominant mode of the Southern Hemisphere extratropical climate variability, the Southern Annular Mode (SAM). Post 2000, significant correlations (r > 0.5, p < 0.1) are detected between SAM and environmental properties that potentially affect NAP phytoplankton production, particularly mixed layer depth (MLD), the extent of the nutrient-rich Circumpolar Deep Water (CDW), and photosynthetically available radiation (PAR). The relationship of these properties to SAM exhibits spatial variability. Near Elephant Island, interannual variations of the summertime MLD and PAR are significantly and positively correlated to the variation of the summer SAM index (r = 0.89 and p = 0.0003 for MLD; r = 0.64 and p = 0.04 for PAR). Significant correlations also exist between chlorophyll concentration and the summer SAM index (r = 0.7, p = 0.02), which are attributed to the SAM-related change in PAR and vertical mixing. Near the South Shetland Islands, the correlation between MLD and summer SAM index is weakened (r = 0.59, p = 0.05). Significant correlations are found between CDW extent and the spring SAM index as well as the annual SAM index (r > 0.7, p ≤ 0.01). Significant correlations also exist between chlorophyll concentration and the spring and annual SAM indices (r > 0.6, p ≤ 0.06). The statistical relationship between chlorophyll concentration and SAM is used with the predicted variation of SAM based on CMIP5 models to make projections of biological production change over the next 50 years in the NAP and adjacent areas. With an estimated SAM trend of 0.03 yr−1, in the next 50 years the surface chlorophyll concentration over the NAP and WAP outer shelf and slope will increase by as much as 0.5 mg m−3, and the integrated chlorophyll concentration over the upper 100-m water column will increase in the NAP area by 10 mg m−2 to 50 mg m−2.

Copepod species assemblage and carbon biomass during two anomalous warm periods of distinct origin during 2014–2015 in the southern Gulf of California

Copepod species assemblage and biomass were studied during a weekly time series done during an anomalously warm and low sea surface Chlorophyll-a concentration period 2014–2015, compared with 2002–2018, at Cabo Pulmo National Park, Gulf of California, Mexico. Forty-nine copepod species accounted for 64% of the total zooplankton abundance. Three small copepods (Oncaea venusta 12.6%, Paracalanus parvus 11.5%, Clausocalanus jobei 8.6%) out of 45 species numerically dominated during 2014. The large copepods Subeucalanus subtenuis and Subeucalanus subcrassus out of 38 species recorded during 2015 increased in abundance accounting for 69% of copepod Carbon biomass. A Canonical Correspondence Analysis demonstrated SST drove copepod species assemblages. Copepod abundance was significantly higher during the cold (Nov–May) that during the warm (Jun–Oct) season and lower during Jan–Aug 2014 regional heatwave than during the Equatorial El Niño (Nov 2014–Dec 2015). Acartia tonsa was the only significant species indicator during the 2014 regional heatwave and Clausocalanus furcatus, Copilia mirabilis, and Cosmocalanus darwinii were significant species indicators of El Niño 2015. Copepod species assemblage had 87% tropical species during the Jan–Aug 2014 due to the regional heatwave that started the tropicalization of copepod community increasing to 95% of tropical species during El Niño 2015.

Multisatellite-Based Feeding Habitat Suitability Modeling of Albacore Tuna in the Southern Atlantic Ocean

Decision strategies in fisheries management are often directed by the geographic distribution and habitat preferences of target species. This study used remote sensing data to identify the optimal feeding habitat of albacore tuna in the Southern Atlantic Ocean (SAO) using an empirical habitat suitability model applying longline fisheries data during 2009–2015. An arithmetic mean model with sea surface temperature (SST) and sea surface chlorophyll-a concentration (SSC) was determined to be suitable for defining the albacore habitat in the SAO. The optimal ranges of SST and SSC for the habitat were approximately 16.5 °C–19.5 °C and 0.11–0.33 mg/m3, respectively. The study revealed a considerable positive trend between the suitable habitat area and standardized catch per unit effort (r = 0.97; p &lt; 0.05); due to the west-to-east and northward development of the suitable habitat, albacore schools moved to the northeast of the SAO, thus increasing catch probability in April to August in that region. Overall, the frontal structure of SST and SSC plays an essential role in the formation of potential albacore habitats in the SAO. Our findings could contribute to the establishment of regional ecosystem-based fisheries management in the SAO.

Surface Chlorophyll Enhancement in Mesoscale Eddies by Submesoscale Spiral Bands

AbstractOceanic submesoscale processes (with scales ~1–10 km and approximately days) have been progressively recognized as an important upwelling mechanism to close the upper ocean nutrient budget and sustain the primary production in euphotic layer. Spiral band is one of the most typical submesoscale structures of mesoscale eddies, characterized with enhancement of surface chlorophyll concentration. By combining satellite ocean‐color, satellite‐altimetry, and surface drifter data, we find that the oceanic spiral chlorophyll bands emerge globally and share a series of structural and kinematic features with atmospheric spiral rain bands of tropical cyclones, which indicate that they are footprints of the vertical motions induced by vortex Rossby waves embedded in eddies. As vortex Rossby waves are closely related to eddy evolution under background deformation, further observational analysis indicates that an intense eddy energy variation and a strong background deformation field constitute the favorable conditions for the emergence of the spiral chlorophyll bands.

Abundance of Pelagia noctiluca early life stages in the western Mediterranean Sea scales with surface chlorophyll

Pelagia noctiluca is the most successful and well-studied jellyfish in the Mediterranean Sea. This species tolerates a wide range of water temperatures and succeeds in low to medium food regimes, but factors driving its distribution and population dynamics remain poorly understood. Here we applied a multiscale analytical approach using survey data and a physical-biochemical coupled model to assess how environmental factors affect the 3-dimensional distribution and seasonal abundance of P. noctiluca early life stages. The surveys took place after the spring bloom, when warm water favors fecundity and growth, but food shortage limits the reproductive investment and early survival. We found that most early life stages of P. noctiluca remained above the shallow thermocline and upper mixed layer where temperature is warm. Their spatial distribution was positively correlated with surface chlorophyll concentration, and over 90% of the variation in interannual abundance was explained by basin-scale productivity in June. Warmer water during winter and spring seasons coupled with protracted spring blooms increase the population of P. noctiluca, and this explains the trend of increasing outbreaks observed in the western Mediterranean Sea over the past decades.

Unprecedented Surface Chlorophyll Blooms in the Southeastern Arabian Sea During an Extreme Negative Indian Ocean Dipole

AbstractObservations of ocean color in the southeastern Arabian Sea (SEAS) during the late boreal summer and fall of 2016 reveal the highest surface chlorophyll concentrations (&gt;1.0 mg m−3) ever reported in the region in the past two decades of the satellite era. Anomalous chlorophyll blooms were triggered by two cold‐core eddies, along the periphery of the coastal upwelling zone. Observational evidence suggest that the eddy formation and propagation can be attributed to the local wind forcing and the remote effects originating around Sri Lanka and the southern tip of India (STI) during the extreme negative Indian Ocean Dipole (IOD) of 2016. The eddy‐induced anomalous shoaling of thermocline indicates strong upwelling of subsurface nutrients and chlorophyll and enhanced light availability for phytoplankton growth favoring the observed biological response. The study signifies the unprecedented impacts of extreme climatic events on oceanic chlorophyll and the need for better understanding of the associated biophysical interactions.

Reconstructing Global Chlorophyll-a Variations Using a Non-linear Statistical Approach

Monitoring the spatio-temporal variations of surface chlorophyll-a concentration (Chl, a proxy of phytoplankton biomass) greatly benefited from the availability of continuous and global ocean color satellite measurements from 1997 onward. These two decades of satellite observations are however still too short to provide a comprehensive description of Chl variations at decadal to multi-decadal timescales. This paper investigates the ability of a machine learning approach (a non-linear statistical approach based on Support Vector Regression, hereafter SVR) to reconstruct global spatio-temporal Chl variations from selected surface oceanic and atmospheric physical parameters. With a limited training period (13 years), we first demonstrate that Chl variability from a 32-years global physical-biogeochemical simulation can generally be skillfully reproduced with a SVR using the model surface variables as input parameters. We then apply the SVR to reconstruct satellite Chl observations using the physical predictors from the above numerical model and show that the Chl reconstructed by this SVR more accurately reproduces some aspects of observed Chl variability and trends compared to the model simulation. This SVR is able to reproduce the main modes of interannual Chl variations depicted by satellite observations in most regions, including El Niño signature in the tropical Pacific and Indian Oceans. In stark contrast with the trends simulated by the biogeochemical model, it also accurately captures spatial patterns of Chl trends estimated by satellite data, with a Chl increase in most extratropical regions and a Chl decrease in the center of the subtropical gyres, although the amplitude of these trends are underestimated by half. Results from our SVR reconstruction over the entire period (1979–2010) also suggest that the Interdecadal Pacific Oscillation drives a significant part of decadal Chl variations in both the tropical Pacific and Indian Oceans. Overall, this study demonstrates that non-linear statistical reconstructions can be complementary tools to in situ and satellite observations as well as conventional physical-biogeochemical numerical simulations to reconstruct and investigate Chl decadal variability.

Contrasting chlorophyll-a seasonal patterns between nearshore and offshore waters in the Bohai and Yellow Seas, China: A new analysis using improved satellite data

The seasonal cycles of sea surface Chlorophyll-a (Chl-a) concentration in the Bohai and Yellow Seas (BYS) were examined using new remotely sensed Chl-a products derived using a regional algorithm. Based on objective clustering analysis, two contrasting Chl-a seasonal patterns were delineated: 1) A nearshore pattern with a long-lasting Chl-a peak from spring to autumn, coinciding with sufficient nutrient supply and high light availability, and lower Chl-a values in winter due to the light-limitation as a consequence of low solar irradiance and high diffuse attenuation coefficient of light for the water column; 2) An offshore pattern with a strong spring bloom triggered by alleviated light limitation and abundant nutrients coinciding with high water column stability. A weak autumn peak induced by new nutrient replenishment from bottom re-suspension and horizontal transportation, a low Chl-a trough in winter mainly attributed to light-limitation, and a Chl-a trough in summer due to nutrient scarcity were observed. The linkages between the province-specific Chl-a seasonal patterns and the seasonal cycles of the corresponding environmental variables as well as other potential influencing factors are comprehensively delineated and discussed.

Foraging ecology of a marine top predator in the Eastern Tropical Pacific over 3\xa0years with different ENSO phases

The El Niño Southern Oscillation (ENSO) is a recurrent climatic pattern with important ecological consequences for seabirds due to its impacts on the abundance and distribution of food resources. We investigated the effects of ENSO phases on the foraging ecology of a marine top predator at Clarion Island in the Eastern Tropical Pacific using GPS and time-depth recorder data and regurgitates from incubating masked boobies (Sula dactylatra) during 3 consecutive years. Foraging locations were recorded in 2016 (El Niño, one female, three males), 2017 (neutral; six females, nine males), and 2018 (La Niña; eight females, ten males). Local sea surface temperature (SST) and chlorophyll-a concentration (CHL) within the birds’ foraging range were compared among the 3 years. Regurgitates were collected opportunistically from 25 and 31 incubating adults in 2017 and 2018, respectively. Average local CHL and SST were similar among years (mean SST 25 °C; mean CHL 0.10 and of 0.09 mg m−3 in January and March, respectively). Masked boobies travelled a maximum of 66 ± 34 km from the colony. The maximum trip duration was 7.7 ± 3.4 h and total distance travelled during a foraging trip was 164 ± 73 km, with no sex- or year-related differences. Masked boobies mainly caught flying fish, but their diet also included one squid and six other fish families. In contrast to previously reported changes in foraging ecology of seabirds, masked boobies at Clarion Island seemed to be unaffected during El Niño, because the local oceanography was relatively unperturbed by ENSO oscillations.

Basin-wide responses of the South China Sea environment to Super Typhoon Mangkhut (2018)

Relative to the open Northwest Pacific, the basin-scale South China Sea (SCS) is smaller and semi-enclosed, and the impacts of frequent super typhoons on the entire SCS basin have yet to be fully understood. Using multi-satellite observations and reanalysis data, this study explored biophysical responses of the upper ocean of the SCS induced by a typical super typhoon, Mangkhut (2018), and their regional differences with potential mechanisms. There were three different significant-response regions in the whole SCS, as follows: (1) In the ocean area around the typhoon path, strong vertical mixing, upwelling and cooling were induced, resulting in the surface chlorophyll-a (chl-a) concentration enhancing markedly (three-fold). Particularly, asymmetric distribution of typhoon rainfall induced asymmetric sea surface salinity change over along the path the nearshore. Diurnal peak of chl-a concentration increased obviously, and the daily growth rate of chl-a sped up considerably in non-shore areas after Mangkhut's passage. (2) In the Beibu Gulf (BBG), the peripheral winds of Mangkhut caused a change in direction of the sea surface flow field, transporting the high-temperature and high-salinity surface seawater from the southeastern area to the BBG. This induced dramatic increases in sea surface temperature, salinity and height, and a decrease in chl-a, in most areas of BBG. (3) In the southwest SCS, the southwest monsoon–induced eastward offshore upwelling jet was weakened by the opposite large-scale peripheral wind vector of Mangkhut and gradually disappeared, resulting in accumulation and enhancement of chl-a in the nearshore. In addition, Mangkhut peripheral winds also intensified (weakened), shifting the cold (warm) eddy to the north (south) and blocking the horizontal eastward transport belt of the high-concentration chl-a. In general, our present work sheds light on the new evidence that a supper typhoon can cause basin-wide anomalies in the SCS, which has broad implications for marine biophysical environment.

Variability of the ocean mixed layer depth and the upwelling activity in the Cape Bojador, Morocco

This study examines the seasonality of the patterns of the ocean mixed layer depth (MLD) with the coastal upwelling system, using a model data from marine Copernicus with a resolution of 0.083° × 0.083°. For that, we examined wind forcing, ocean circulation, sea surface temperature, sea surface salinity, sea surface chlorophyll-a, and mixed layer depth seasonality from 2007 to 2017 in the Cape Bojador south of the Moroccan Atlantic coast. We observed that in the fall season the wind speed was low, and the maximum wind speed is in the summer season. Furthermore, we found that coastal areas are related to a generally shallower MLD all the year in the Cape. We also proved that the source of the upwelling is between 25 and 26°N. However, we observed that the sea surface chlorophyll concentration in the same region is minimum, while in the region between 24.5 and 22°N, the CHL-a is maximum. We suggested that the biological richness in Cape Bojador, due to the activity of upwelling, derives to the south because of the northward ocean filament. Despite the different physical and biological parameters used to detect upwelling activity, we found that upwelling detection based on sea surface temperature is the best method to describe the upwelling activity in this region.

Morphological Variation of the Sea Silverside Odontesthes regia in Regions with Dissimilar Upwelling Intensity along the Humboldt Current System

We evaluated the influence of areas with dissimilar upwelling intensity along the Humboldt Current System on the morphological variation of the economically important sea silverside Odontesthes regia by using geometric morphometric (GM) and meristic data of populations sampled off Northern Peru, Central Peru, Southern Peru-Northern Chile, and Central-Southern Chile (CSCH). Multivariate analyses of variance, a UPGMA tree, and discriminant analyses of meristic counts separated CSCH individuals, which had slightly higher numbers of gill rakers and radius of the anal fin. Permutation tests and canonical variate analyses of GM data distinguished all populations and highlighted deformations in the head and fins. Variations in GM and meristic analyses were significantly correlated with values of the sea surface temperature and surface chlorophyll-a concentrations. Morphological differences among populations might be associated with the spatial coastal upwelling dynamic of the Humboldt Current System, which highlights the role that this system plays in relation to the phenotypic variation of fish.

Spatial Variations of Subsurface Chlorophyll Maxima During Thermal Stratification in a Large, Deep Subtropical Reservoir

AbstractSubsurface chlorophyll maxima (SCM) are found in stratified lakes, reservoirs, and oceans. Spatial variations of the SCM magnitude, depth, and thickness during stratification and related factors were examined in a large, deep subtropical reservoir, Lake Qiandaohu. Significant spatial differences in the SCM parameters were found throughout the lake as thermal stratification developed in the late spring of 2014. SCM depth and thickness were positively correlated with euphotic depth and mixed layer depth but negatively correlated with epilimnetic nutrient concentrations. SCM magnitude was negatively correlated with euphotic depth and mixed layer depth and positively correlated with epilimnetic nutrient concentrations. Seasonal variations in SCM differed among subregions of the lake, which were related to seasonal differences in environmental parameters. SCM (averaging 66.2 mg/m2 in spring and 96.4 mg/m2 in summer) accounted for about 82% of the total areal chlorophyll of the water column (Tchl), and the SCM magnitude was better correlated with Tchl than the surface chlorophyll concentration. This study contributes to understanding of factors causing spatial variations in of SCM in the deep subtropical lakes.

Ecological Vulnerability of Coral Reef Ecosystem in Wakatobi National Park During Indian Ocean Dipole Event

This research examines coral reefs vulnerability which threatening its existences and functions by climate change. The ecological vulnerability in Wakatobi (Wangi-wangi, Kaledupa, Tomia, and Binongko) was assessed during Indian Ocean Dipole (IOD) event in 2016. Climate exposure was determined using sea surface temperature, chlorophyll-a concentration, and wind speed magnitude; sensitivity was determined using coral susceptibility, fish susceptibility, and macroalgae primary productivity rate; then adaptive capacity was developed by hard coral cover, coral size distribution, coral richness, fish biomass, herbivore diversity, and herbivore grazing relative to algal production. The values of Exposure, Sensitivity, and Adaptive capacity in Wakatobi were 0.93±0.02, 0.42±0.18, and 0.44±0.10, respectively. Site specific vulnerability scores ranged from 0.52 to 1.60 (mean 0.92±0.26). Binongko was the least vulnerable than other islands. Tomia was observed as the least adaptive capacity and Wangi-wangi was the most bleaching incidents. These results could help coral reefs monitoring priority during the event and then when the event is gone by focusing on the marked islands and sites. Sites that were observed as more vulnerable is urgently need a management strategy to overcome the vulnerability status in the future, such as increasing site adaptability.

Temporal and Spatial Variations of Monsoonal Upwelling along the South West and East Coasts of India

The implicit/explicit impact of global warming or the anthropogenic climate change on the coastal upwelling processes affects the marine ecosystem significantly and continuous monitoring of the coastal upwelling along the coastal regions is essential. The Ekman transport due to alongshore winds and Ekman pumping due to wind stress curl play important roles on the coastal upwelling, especially along the southern part of Indian peninsula. Upwelling Indexes based on wind and SST are computed in the study to demarcate the upwelling features along the coasts. Since upwelling leaves its trail on surface features, long time series of satellite-derived wind, Sea Surface Temperature (SST), Sea level anomaly (SLA) and Surface Chlorophyll-a concentration (Chl-a) are analyzed to understand the upwelling along the coastal regions of south India. The present study reveals a decrease in the intensity of upwelling along the western coast (Chl-a ∼ 0.2 mg/m3 dec−1, Upwelling Index based on wind () and Vertical Velocity of Ekman Pumping (VVEP) is approximately 0.0012 m2/s dec−1 and 0.120 cm/day dec−1 respectively) and a slight increase all along the eastern coast of southern India during summer monsoon (Chl-a along 11.3°N and 13.9°N is 1 mg/m3 dec−1, and VVEP along 10.5°N is 0.0023 m2/s dec−1 and 0.2 cm/day dec−1 respectively).

Spatially heterogeneous trends in nearshore and offshore chlorophyll a concentrations in lakes Michigan and Huron (1998–2013)

Abstract Nutrient abatement programmes have been successfully implemented around the globe to reduce nutrient loading into aquatic ecosystems. Concurrently, the worldwide spread of invasive filter feeders, such as dreissenid mussels, may alter nutrient dynamics in invaded systems by sequestering nutrients away from pelagic zones and reducing primary production in offshore areas. Such is the case in the Laurentian Great Lakes of North America, where decades of nutrient abatement and the establishment of dreissenid mussels have seemingly resulted in more oligotrophic conditions and altered spatial patterns of nutrient availability and primary production. Recent studies have focused on whole lake trends in primary production despite spatial differences in tributary inputs, bathymetry, and other environmental conditions that can affect primary production in nearshore areas. Thus, we hypothesised that trends in nearshore chlorophyll concentrations in different areas may diverge in a manner consistent with spatial differences in nutrient input. To evaluate these differences in surface chlorophyll responses, we assessed temporal trends in four different areas of Lake Michigan and three different areas of Lake Huron. We hypothesised that in lakes Huron and Michigan, nearshore zones have experienced slower declines of chlorophyll concentrations relative to offshore zones. To assess this hypothesis, we estimated temporal trends of surface water chlorophyll concentrations (a proxy for primary production) from satellite imagery from 1998 to 2013. We calculated average surface chlorophyll concentrations for 10‐m depth intervals ranging from the shore (0–10 m) to offshore (&gt;90 m) during representative months of May, July, and September. We then analysed these data to determine if long‐term trends in surface chlorophyll varied by season, proximity to the shoreline, and water depth. The rates of annual change in chlorophyll concentrations in nearshore areas were markedly different to offshore trends in both lakes. Chlorophyll concentrations declined overtime in offshore areas, but nearshore chlorophyll concentrations were either stable (in May) or increased (in July and September) throughout the time series. Differences in chlorophyll concentrations among areas were prominent in Lake Michigan. While differences between the northern and southern basin have been previously documented, trends in chlorophyll concentrations also differed between the eastern and western sides of Lake Michigan. Despite similar bathymetry and geographic features to Lake Michigan, regional trends were not observed in Lake Huron. The results of this study are generally consistent with the nearshore shunt hypothesis, which predicts that dreissenid filtering, nutrient re‐suspension, and continued nutrient loading from tributaries can cause an increase in primary production in nearshore areas during periods of offshore oligotrophication. Thus, the localised effects of nutrient abatement programmes in a given lake will be influenced by complex interactions between lake bathymetry and the presence of non‐native filter feeding organisms.