Intense Upwelling Research Articles

Renewal of seasonally anoxic Saanich Inlet is temporally and spatially dynamic

Injection of oxygenated water into anoxic basins sets off a cascade of biogeochemical reactions and ecosystem shifts. The dynamic nature of these events can create spatial variability in the resulting water mass that strongly affects subsequent observations. Their irregularity can also make their prediction for experiment planning challenging. Our study focuses on Saanich Inlet, one of the most accessible and well-studied anoxic basins in the world. In the late summer and early fall, dense water can cross the sill into this fjord, in discrete events, bringing oxygen and nitrate to the deep waters of the inlet. We assess the potential strength of these renewal events using density measured at a bottom mooring on the sill. We find that the occurrence and potential strength of renewals is primarily controlled by tidal current speeds, which can be well predicted. However, the intensity of coastal upwelling, which brings dense water into the estuarine system, plays a significant secondary control, reducing predictability. We also demonstrate that renewals do not result in a homogeneous water mass filling the deep inlet. Instead, high frequency measurements from a profiling mooring in the centre of the inlet reveal that different densities intruding over the several-day renewal period create a complex layering of waters containing different proportions of new oxic and old anoxic waters. Finally, we show that not every instance of high density water observed over the sill results in flushing of the deepest waters inside the inlet. We hypothesize that each renewal improves the chance of a subsequent renewal in the same season by reducing the density contrast between waters entering and already inside the inlet. Consideration of the temporal and spatial complexity of these renewal dynamics is necessary to support studies using Saanich Inlet as a natural laboratory for exploring oxygen deficient systems.

Modelling the influence of submesoscale processes on phytoplankton dynamics in the northern South China Sea

Submesoscale processes in the ocean vary rapidly in both space and time, and are often difficult to capture by field observations. Their dynamical connection with marine biology remains largely unknown because of the intrinsic link between temporal and spatial variations. In May 2015, satellite chlorophyll data demonstrated high concentration patches in the edge region between mesoscale eddies, which were higher than those in the cyclonic eddy core region in the northern South China Sea (NSCS). The underlying mechanisms were examined with a high-resolution physical-biological model. By tracking Lagrangian particles in the model, this study shows that the edge region between eddies is a submesoscale frontal region that is prone to intense upwelling and downwelling motions. We identified two key submesoscale mechanisms that affect nutrient transport flux significantly, submesoscale fontal dynamics and submesoscale coherent eddies. The dynamics associated with these two mechanisms were shown to be able to inject subsurface nutrients into the upper layer, generate the high chlorophyll patch, and alter phytoplankton community structure in the NSCS. This study shows the importance of submesoscale processes on phytoplankton dynamics in the NSCS and highlights the need for high-resolution observations.

Input-export fluxes of heavy metals in the Cochin estuary, southwest coast of India.

Cochin estuary (CE) is one of the largest tropical estuaries along the southwest coast of India, sustaining rich bio-resources. Several studies enlighten the environmental changes in the CE caused by anthropogenic activities. In the present study, an attempt has been made to quantify the heavy metal (dissolved and particulate) fluxes brought by the six rivers into the CE with their exchange into the coastal ocean through the major inlet at Cochin during a steady flow period (October-November 2015). The water flux across the inlet was quantified using an acoustic doppler current profiler. The measured daily input of dissolved metals from the rivers was 2.43 × 103kg Fe, 334kg Zn, 259kg Ni, and 83kg Cr, while that of particulate metals were 85.30 × 103kg Fe, 8. 6 × 103kg Mn, 236.9kg Cr, and 111.9kg Zn. The net export of metals through the Cochin inlet (per tidal cycle) was 3.3 × 103kg Fe, 515kg Cr, 150kg Zn, and 5kg Ni in dissolved form and 3.32 × 105kg Fe, 1747kg Mn, 1636kg Cr, 1397kg Zn, and 586kg Ni in particulate form. The high concentrations of metals during ebb tides are clear indications of their contribution from the industrial conglomerates (industrial units of metallurgy, catalyst, fertilizer, and pesticides) located in the Periyar River. The significance of this study is that the export fluxes may increase further during the summer monsoon (June to September), which may impact the abundant fishery emanating in the coastal environment during the period due to intense upwelling.

The role of wind, mesoscale dynamics, and coastal circulation in the interannual variability of the South Vietnam Upwelling, South China Sea – answers from a high-resolution ocean model

Abstract. The South Vietnam Upwelling (SVU) develops in the South China Sea (SCS) under the influence of southwest monsoon winds. To study the role of small spatiotemporal scales on the SVU functioning and variability, a simulation was performed over 2009–2018 with a high-resolution configuration (1 km at the coast) of the SYMPHONIE model implemented over the western region of the SCS. Its capability to represent ocean dynamics and water masses from daily to interannual scales and from coastal to regional areas is quantitatively demonstrated by comparison with available satellite data and four in situ datasets. The SVU interannual variability is examined for the three development areas already known: the southern (SCU) and northern (NCU) coastal upwelling areas and the offshore upwelling area (OFU). Our high-resolution model, together with in situ observations and high-resolution satellite data, moreover shows for the first time that upwelling develops over the Sunda Shelf off the Mekong Delta (MKU). Our results confirm for the SCU and OFU and show for the MKU the role of the mean summer intensity of wind and cyclonic circulation over the offshore area in driving the interannual variability of the upwelling intensity. They further reveal that other factors contribute to SCU and OFU variability. First, the intraseasonal wind chronology strengthens (in the case of regular wind peaks occurring throughout the summer for SCU or of stronger winds in July–August for OFU) or weakens (in the case of intermittent wind peaks for SCU) the summer average upwelling intensity. Second, the mesoscale circulation influences this intensity (multiple dipole eddies and associated eastward jets developing along the coast enhance the SCU intensity). The NCU interannual variability is less driven by the regional-scale wind (with weaker monsoon favoring stronger NCU) and more by the mesoscale circulation in the NCU area: the NCU is prevented (favored) when alongshore (offshore) currents prevail.

Extraordinarily high organic matter enrichment during the late Guadalupian in northwestern Sichuan basin, China

Shale with extraordinarily high organic matter content (EHOMC) formed in the margin of the Sichuan carbonate platform during the late Guadalupian in the eastern Paleo-Tethys. So far, not much consideration has been given to the enrichment of EHOMC. Here, integrated lithological and geochemical analyses of the top Maokou Formation black shales in the northwest Sichuan basin were undertaken to investigate the EHOMC. The black shales were divisible into three units (i.e., Units 1 to 3 in ascending order); EHOMC occurred in Unit 2. Unit 1 was deposited during the early stage of the transgression under the small-scale upwelling in an anoxic environment with lower paleo-productivity than Unit 2, considering the moderate Cd/Mo ratios and the concentration of Mo, U, V, P, Cu, and Zn. Unit 2 exhibited high Cd/Mo ratios and high concentrations of elements (Mo, U, V, P, Cu, and Zn), suggesting that the shales with EHOMC were deposited through an intense upwelling in an euxinic-sulfidic environment with high paleo-productivity. Unit 3 demonstrated moderate paleo-productivity and anoxic-suboxic conditions, while the nutrient elements were derived through increased terrestrial detrital input. Unit 2 exhibited the highest total organic matter content (TOC) (17.72–32.58 wt%); it is inferred that the enrichment of EHOMC was mainly caused by a euxinic-sulfidic environment, while high paleo-productivity was another key factor. Besides, the euxinic-sulfidic environment and high paleo-productivity in Unit 2 occurred approximately during the intense upwelling, the occurrence of the high sea-level, and the biological extinction during the end of Guadeloupe. These factors together promoted the enrichment of EHOMC in Unit 2.

Coupled changes in pH, temperature, and dissolved oxygen impact the physiology and ecology of herbivorous kelp forest grazers.

Understanding species' responses to upwelling may be especially important in light of ongoing environmental change. Upwelling frequency and intensity are expected to increase in the future, while ocean acidification and deoxygenation are expected to decrease the pH and dissolved oxygen (DO) of upwelled waters. However, the acute effects of a single upwelling event and the integrated effects of multiple upwelling events on marine organisms are poorly understood. Here, we use in situ measurements of pH, temperature, and DO to characterize the covariance of environmental conditions within upwelling-dominated kelp forest ecosystems. We then test the effects of acute (0-3days) and chronic (1-3months) upwelling on the performance of two species of kelp forest grazers, the echinoderm, Mesocentrotus franciscanus, and the gastropod, Promartynia pulligo. We exposed organisms to static conditions in a regression design to determine the shape of the relationship between upwelling and performance and provide insights into the potential effects in a variable environment. We found that respiration, grazing, growth, and net calcification decline linearly with increasing upwelling intensity for M. francicanus over both acute and chronic timescales. Promartynia pulligo exhibited decreased respiration, grazing, and net calcification with increased upwelling intensity after chronic exposure, but we did not detect an effect over acute timescales or on growth after chronic exposure. Given the highly correlated nature of pH, temperature, and DO in the California Current, our results suggest the relationship between upwelling intensity and growth in the 3-month trial could potentially be used to estimate growth integrated over long-term dynamic oceanographic conditions for M. franciscanus. Together, these results indicate current exposure to upwelling may reduce species performance and predicted future increases in upwelling frequency and intensity could affect ecosystem function by modifying the ecological roles of key species.

Mixed layer depth dominates over upwelling in regulating the seasonality of ecosystem functioning in the Peruvian upwelling system

Abstract. The Peruvian upwelling system hosts a marine ecosystem with extremely high productivity. Observations show that the Peruvian upwelling system is the only eastern boundary upwelling system (EBUS) with an out-of-phase relationship between seasonal surface chlorophyll concentrations and upwelling intensity. This “seasonal paradox” triggers the following questions: (1) what are the unique characteristics of the Peruvian upwelling system, compared with other EBUSs, that lead to the out-of-phase relationship, and (2) how does the seasonal paradox influence ecosystem functioning? Using observational climatologies for four EBUSs, we diagnose that the Peruvian upwelling system is the only one to reveal that intense upwelling coincides with deep mixed layers. We then apply a coupled regional ocean circulation biogeochemical model (CROCO–BioEBUS) to assess how the interplay between mixed layers and upwelling regulates the seasonality of surface chlorophyll in the Peruvian upwelling system. Our model reproduces the “seasonal paradox” within 200 km off the Peruvian coast. We confirm previous findings regarding the main contribution of mixed layer depth to the seasonality of chlorophyll, relative to upwelling. Deep mixed layers in austral winter cause vertical dilution of phytoplankton and strong light limitation, impacting growth. The effect of advection, though second-order, is consistent with previous findings for the Peruvian system and other EBUSs, with enhanced offshore export opposing the coastal build-up of biomass. In addition, we find that the relatively colder temperatures of upwelled waters slightly dampen phytoplankton productivity and further slow the build-up of phytoplankton biomass. This impact from the combination of deep mixed layers and upwelling propagates through the ecosystem, from primary production to export and export efficiency. Our findings emphasize the crucial role of the interplay between mixed layer depth and upwelling and suggest that surface chlorophyll may increase, along with a weakened seasonal paradox, in response to shoaling mixed layers under climate change.

Response of the Low‐Level Jet to Precession and Its Implications for Proxies of the Indian Monsoon

AbstractThe long‐term variations in the South Asian monsoon have been inferred based on the variations in the ocean productivity along the western coast of the Arabian Sea. The variations in ocean productivity were previously thought to be primarily influenced by the intensity of upwelling. Here, using idealized precession experiments in fully coupled climate models, we have shown that the area as well as the region of maximum upwelling change with precession. When summer occurs at perihelion (stronger summer insolation and monsoon precipitation), the area of upwelling is narrow. In contrast, during summer at aphelion (weaker summer insolation and monsoon precipitation), upwelling occurs over a broader region. This is due to the effect of convective heating over northeastern Africa and the western equatorial Indian Ocean on the width and meridional location of the low‐level jet. Therefore, the upwelling inferred from proxies does not necessarily indicate the Indian summer monsoon strength.

A physical–statistical recipe for representation of small-scale oceanic turbulent mixing in climate models

It is well established that small-scale cross-density (diapycnal) turbulent mixing induced by breaking of overturns in the interior of the ocean plays a significant role in sustaining the deep ocean circulation and in regulating tracer budgets such as those of heat, carbon and nutrients. There has been significant progress in the fluid mechanical understanding of the physics of breaking internal waves. Connection of the microphysics of such turbulence to the larger scale dynamics, however, is significantly underdeveloped. We offer a hybrid theoretical–statistical approach, informed by observations, to make such a link. By doing so, we define a bulk flux coefficient, $\varGamma _B$ , which represents the partitioning of energy available to an ‘ocean box’ (such as a grid cell of a coarse resolution climate model), from winds, tides, and other sources, into mixing and dissipation. Here, $\varGamma _B$ depends on both the statistical distribution of turbulent patches and the flux coefficient associated with individual patches, $\varGamma _i$ . We rely on recent parametrizations of $\varGamma _i$ and the seeming universal characteristics of statistics of turbulent patches to infer $\varGamma _B$ , which is the essential quantity for representation of turbulent diffusivity in climate models. By applying our approach to climatology and global tidal estimates, we show that, on a basin scale, energetic mixing zones exhibit moderately efficient mixing that induces significant vertical density fluxes, while quiet zones (with small background turbulence levels), although highly efficient in mixing, exhibit minimal vertical fluxes. The transition between the less energetic to more energetic zones marks regions of intense upwelling and downwelling of deep waters. We suggest that such upwelling and downwelling may be stronger than previously estimated, which in turn has direct implications for the closure of the deep branch of the ocean meridional overturning circulation as well as for the associated tracer budgets.

English

Mechanisms of the interannual variability of upwelling onset dates on the Petite-Cote of Senegal are investigated using daily NEMO model data outputs and NOAA ocean surface temperature observations (OISST). We first determined the phenology (onset dates, end dates, duration and occupied area of the upwelling) over the Petite-Côte located in south of the Senegalese coast. Our results show that upwelling in the Petite-Côte starts on average on 03-December (with a standard deviation of 13 days) and ends on average on 13-June (with a standard deviation of 11 days). Upwelling lasts, on average, 6 months in the region. The maximum upwelling intensity is noted on 15-May and the maximum occupied area exhibits a February-May plateau. We then performed a composite analysis based on the earliness of the upwelling setup dates to understand the mechanisms involved. Our results show that for the earliest upwelling years, we note a strengthening of the vertical velocities 3 weeks before the onset of the upwelling but we note especially an abnormal intensification of the coastal jet. The latter seems to be the initiator of the early onset of the upwelling in the region. These reinforcements are associated with a significant decrease in surface temperature (SST) and an intensification of northerly trade winds.   Key words: Upwelling, phenology, Senegalese coast, petite-côte.

Characterization of hydrological features and plankton during neap tides in a subtropical lagoon south of the California Current

ABSTRACT This study aimed to characterize their Spatio-temporal distribution of hydrological variables and plankton during neap tides in Bahía Magdalena, a subtropical lagoon south of the California Current. We compare data from surface and bottom waters of stations located inside the lagoon with those near the bay's mouth and between surface and bottom. The availability of nutrients and the chlorophyll-a are associated primarily with the intense upwelling activity in spring. Due to direct marine influence, this relationship was more significant at the inlet than inside the lagoon, especially at the bottom. During the intense upwellings marine influence is maintained even during neap tides. The wind probably causes the homogeneous distribution in most variables within the bay's surface; this may also contribute to the re-suspension of nutrients due to vertical water column mixing. Abiotic variables were not linearly related with zooplankton biovolume and the density of diatoms and dinoflagellates. Phytoplankton was composed mainly by the micro fraction and diatoms during spring and nanoplankton in the warmest season of the year; however, the strong upwellings at the spring's end caused a HAB composed for diatoms. Chlorophyll-a concentration and diatoms were more abundant inside the lagoon and could explain the lower nutrient concentration of that zone. Zooplankton biovolume seasonality differed from other years, with higher values during winter and a significant decreasing trend throughout the year, resulting from the phytoplankton variability. Upwelling and wind variability explains the high biological productivity observed over most of the year.

Trends in temperature, salinity and productivity in the Vienna Basin (Austria) during the early and middle Miocene, based on foraminiferal ecology

The Neogene Vienna Basin (VB) is a major hydrocarbon province with a long history of exploration accumulating extensive stratigraphic and structural information from numerous seismic and drilling programs. Based on the quantitative analysis of hundreds of foraminiferal samples from 52 drillings, we present the first continuous reconstruction of paleoenvironmental evolution of the VB from the early Miocene to the middle Miocene spanning 6.4 million years. Our analyses comprise reconstructions of sea surface temperature (SST), bottom water temperature (BWT), salinity, trophic levels, stress indicators, mode of life, feeding preferences and diversity indices (Fisher α, dominance and equitability) based on the transfer function for foraminifers of Hohenegger (2005, Palaeogeogr. Palaeoclimatol. Palaeoecol. 217, 115–130) and applied statistics (e.g. box plots and ternary diagrams). Bottom water temperatures indicate a cooling during the early and middle Badenian (Langhian), which seemingly contradicts the global warming of the Middle Miocene Climatic Optimum (MMCO) and a subsequent warming, which contrasts the expected trend following the cooling of the Middle Miocene Climatic Transition. Both trends are discussed as result of bathymetric evolution of the VB and intense upwelling during the early and middle Badenian. Repetitive alternations of Orbulina-rich assemblages, indicating a warm and stratified waterbody, with globigerinid-dominated assemblages, indicating upwelling and high productivity, are discussed in the light of a yet undescribed waxing and waning of upwelling intensity on a decadal to millennial scale. The applied formula for SST-calculations seems to underestimate real temperatures. Nevertheless, it reveals a distinct warming of about 3 °C from the early to the middle Miocene. In contrast to previous interpretations, we document normal marine conditions from the Ottnangian to the Sarmatian (middle Burdigalian to Serravallian) with a slight increase in salinity during the late Sarmatian.

Barrier Effect of the Pearl River Estuarine Plume on Wind‐Induced Coastal Upwelling of Nutrients

AbstractCoastal upwelling caused by winds seasonally occurs in summer in the Northern South China Sea. The alongshore prevailing southwest winds, induced by East Asia Summer Monsoon, exert a uniform Coriolis force to drive the surface water offshore, which in turn pushes the deeper coastal water upward. The common view is that coastal upwelling brings up nutrients from deep water. The presence of a buoyance layer such as the riverine or estuarine plumes damps the Ekman effect and hence upwelling. However, the semi‐quantitative analysis of reduced upwelled nitrate amount due to plume (barrier) effect during this interactive process is rare. To examine the dynamics of water masses and nutrient upward transport process in the coastal waters in summer, vertical profiles of temperature, salinity, and nitrate were taken along the transects from the Pearl River estuary to 150 km offshore in July cruise of 2002 in the northern South China Sea. The results show a three‐layer structure resulting from the three water masses under the impact of a large buoyant plume: high‐nutrient plume water in the surface layer, undetectable‐nutrient offshore surface water in the middle layer, and low‐nutrient deep water in the bottom layer. This structure indicates that this middle layer was the offshore surface water entrained underneath the estuarine plume and hence, weakened the intensity of upwelling. Upwelled nitrate amount is reduced by ∼30%–35% from the deep water to the surface layer due to the middle layer based on our estimate. The finding suggests that the presence of estuarine plumes plays an important role as a barrier to weaken upwelling of deep water and subsequently reduce the upward transport of nutrients by wind‐induced coastal upwelling and therefore, such the barrier effect should be considered in estimating budgets of nutrients and subsequent carbon fixation in coastal waters.

Effect of Intensity and Mode of Artificial Upwelling on Particle Flux and Carbon Export

Reduction of anthropogenic CO2 emissions alone will not sufficiently restrict global warming and enable the 1.5°C goal of the Paris agreement to be met. To effectively counteract climate change, measures to actively remove carbon dioxide from the atmosphere are required. Artificial upwelling has been proposed as one such carbon dioxide removal technique. By fueling primary productivity in the surface ocean with nutrient-rich deep water, it could potentially enhance downward fluxes of particulate organic carbon (POC) and carbon sequestration. In this study we investigated the effect of different intensities of artificial upwelling combined with two upwelling modes (recurring additions vs. one singular addition) on POC export, sinking matter stoichiometry and remineralization depth. We carried out a 39 day-long mesocosm experiment in the subtropical North Atlantic, where we fertilized oligotrophic surface waters with different amounts of deep water. The total nutrient inputs ranged from 1.6 to 11.0 μmol NO3– L–1. We found that on the one hand POC export under artificial upwelling more than doubled, and the molar C:N ratios of sinking organic matter increased from values around Redfield (6.6) to ∼8–13, which is beneficial for potential carbon dioxide removal. On the other hand, sinking matter was remineralized at faster rates and showed lower sinking velocities, which led to shallower remineralization depths. Particle properties were more favorable for deep carbon export in the recurring upwelling mode, while in the singular mode the C:N increase of sinking matter was more pronounced. In both upwelling modes roughly half of the produced organic carbon was retained in the water column until the end of the experiment. This suggests that the plankton communities were still in the process of adjustment, possibly due to the different response times of producers and consumers. There is thus a need for studies with longer experimental durations to quantify the responses of fully adjusted communities. Finally, our results revealed that artificial upwelling affects a variety of sinking particle properties, and that the intensity and mode with which it is applied control the strength of the effects.

The functional composition of nearshore fish communities demonstrated by trait analysis: Response to environmental gradients

There is limited understanding on nearshore fish community functioning in relation to changing environmental gradients. To address this, we investigated the trait composition of fish communities in five regions along the Portuguese coast during 4 years. Northern regions were characterized by higher Chlorophyll-α and small-sized species with short lifespans. The southernmost region was characterized by warmer waters, more intense upwelling events and typical warm-water species. Higher species richness and functional redundancy at the southernmost region suggested that in case of potential species loss, community functioning would be less affected in this region, and thus, a certain level of community resilience is maintained. The central region presented lower functional redundancy and high functional richness, which indicated the presence of rare species with rare traits, suggesting a more vulnerable community. This information on the nearshore fish community distribution may be particularly important towards managing coastal ecosystems in response to changing environmental conditions.

Oligocene–Miocene dinoflagellate cysts from the San Gregorio Formation, La Purísima area, Baja California Sur, Mexico

Dinoflagellate cysts from the LB-5 core in the San Gregorio Formation (SGF) from the La Purísima area, Baja California Sur, Mexico, limit the age of the studied succession to Oligocene–Miocene (28–20 Ma). Our results allow correlation of this core with the type locality and another well-dated sections of the SGF in the region, namely the nearby La Purísima section, with an unequivocal Oligocene–Miocene boundary (O/M – 23 Ma). Lithology and dinoflagellate cyst data indicate mainly marine sedimentary environments, with a transgression from continental at the base to upper bathyal (200–500 m) in the O/M level (∼243 m), and a regression towards the top of the unit. Dinoflagellate cysts are virtually absent in some intervals, probably due to oxidation during diagenesis. The abundance of heterotrophic taxa during intervals with high cyst concentrations indicates that palynological preservation was adequate for quantitative analyses in the rest of the samples. Samples with dinoflagellate cyst concentrations >2000 cysts/gram of sediment (c/g sed), the abundance of heterotrophic taxa, and the presence of phosphoritic layers and diatomites indicate high biogenic productivity during the deposition of the SGF. Intense upwelling conditions in the area are probably associated with the cold events Mi-1 (∼23 Ma) near the O/M and Mi-1a (∼21.7 Ma) at ca. 170 m depth. Quantitative dinoflagellate cyst data in the LB-5 core suggest lower productivity (mean = 203 c/g sed) in the Oligocene than during the Miocene (mean = 848 c/g sed) interval. The absolute abundances of the Miocene intervals indicate similar dinoflagellate cyst concentrations in the LB-5 core and modern samples from the Pescadero Basin in the southern Gulf of California.

The El Niño Southern Oscillation (ENSO) Effect on Upwelling in The North Maluku Sea

The north Maluku Sea has a high wind speed in the southeast season caused an increase in the Ekman transport at the coastal area so affects in rising of chlorophyll-a and the Sea Surface Temperature (SST) is cooling, who indicates the upwelling phenomenon. Upwelling in the North Maluku Sea is a type of coastal Upwelling where it occurs on the coast with a depth of more than 1500 m. The intensity of Upwelling is influenced by climate variability, one of which is El Niño Southern Oscillation (ENSO). By using remote sensing is chlorophyll-a data, SST, wind obtained from satellite imagery, and the value Ekman Mass Transport (EMT) with method composite formula. This research aims to know the influence of climate variability ENSO against the phenomenon of upwelling that occurred in the North Maluku Sea in the southeast season. Effect of climate variability ENSO at the time of El Niño (La Niña), chlorophyll-a concentration has positive (negative) anomalies, negative (positive) SST anomaly and wind conditions thus causing EMT energy to increase (decrease).

Variability of Dissolved Oxygen in the Bottom Layer of the Southern Senegalese Shelf

AbstractThe observation station “Melax” was deployed in 2015 on the wide and shallow south Senegalese shelf to study the ocean dynamics, air‐sea interactions, and dissolved oxygen (DO) cycle. Data from February 2015 to August 2016 were used to study the main physical processes affecting the variability of DO in the bottom layer (∼30 m depth) on time scales ranging from tidal to seasonal. Between November and May, wind‐driven upwelling provides phytoplankton enrichment of the surface layers and brings cold, salty, and depleted DO on the shelf. Water properties at Melax vary depending on the source waters located at the shelf edge. The DO concentration changes between the shelf edge and Melax are broadly consistent with the inferred respiration rates estimated in previous studies. In contrast, the monsoon season (July–October) is characterized by weak westerly winds and northward currents. Bottom waters are warmer, fresher, and more oxygenated. The slower circulation in this period allows a stronger decoupling between the water properties of the waters observed at Melax and those of the source waters. Stratification strengthening near the bottom layer inhibits vertical mixing and induces strong high‐frequency variability in properties caused by internal tide‐generated waves. Intense upwelling events can deepen the mixed layer and intermittently transform the bottom layer waters (locally or remotely). Relaxation events associated with current reversals significantly modify their properties. Coastal trapped waves constitute a distant forcing that can act year‐round, impacting both shelf waters and source regions.

Seasonal upwelling reduces herbivore control of tropical rocky intertidal algal communities.

Communities are shaped by a variety of ecological and environmental processes, each acting at different spatial scales. Seminal research on rocky shores highlighted the effects of consumers as local determinants of primary productivity and community assembly. However, it is now clear that the species interactions shaping communities at local scales are themselves regulated by large-scale oceanographic processes that generate regional variation in resource availability. Upwelling events deliver nutrient-rich water to coastal ecosystems, influencing primary productivity and algae-herbivore interactions. Despite the potential for upwelling to alter top-down control by herbivores, we know relatively little about the coupling between oceanographic processes and herbivory on tropical rocky shores, where herbivore effects on producers are considered to be strong and nutrient levels are considered to be limiting. By replicating seasonal molluscan herbivore exclusion experiments across three regions exposed to varying intensity of seasonal upwelling, separated by hundreds of kilometers along Panama's Pacific coast, we examine large-scale environmental determinants of consumer effects and community structure on tropical rocky shores. At sites experiencing seasonal upwelling, grazers strongly limited macroalgal cover when upwelling was absent, leading to dominance by crustose algae. As nutrients increased and surface water cooled during upwelling events, increases in primary productivity temporarily weakened herbivory, allowing foliose, turf and filamentous algae to replace crusts. Meanwhile, grazer effects were persistently strong at sites without seasonal upwelling. Our results confirm that herbivores are key determinants of tropical algal cover, and that the mollusk grazing guild can control initial stages of macroalgal succession. However, our focus on regional oceanographic conditions revealed that bottom-up processes regulate top-down control on tropical shorelines. This study expands on the extensive body of work highlighting the influence of upwelling on local ecological processes by demonstrating that nutrient subsidies delivered by upwelling events can weaken herbivory in tropical rocky shores.

Strong solar influence on multi-decadal periodic productivity changes in the central-western Bay of Bengal

Oceanic productivity is a vital component of the global carbon cycle. The productivity in the tropical oceans is strongly modulated by the monsoon winds. The monsoon winds are driven by the temperature. Therefore, the oceanic productivity is most likely to be influenced by the increasing global warming. However, the projected response of oceanic productivity to global warming is ambiguous. The past records can help to better constrain the future response of oceanic productivity to different warming scenarios. Here, we reconstruct decadal monsoon and productivity changes from the central-western Bay of Bengal, covering the entire Holocene. We use both foraminiferal (relative abundance of Globigerina bulloides, stable oxygen isotopic ratio of surface dwelling Globigerinoides ruber) and geochemical [calcium carbonate (CaCO3), organic carbon (Corg), organic carbon/total nitrogen (Corg/TN)] proxies to reconstruct productivity and associated parameters. All the proxy data suggests a large variation in productivity and carbon burial during the Holocene. A very intense upwelling, concomitant with a phase of increased precipitation and CaCO3 burial is inferred during the beginning of Greenlandian (~12,000 - ~10,000 yrs BP). The intense upwelling, however, did not result in higher organic carbon burial. The beginning of Northgrippian (~8000–7000 yr BP) was a significantly wet phase followed by gradual decrease in precipitation. Incidentally, the reduced upwelling during the Northgrippian was synchronous with a large increase in organic carbon burial. The opposite trend in upwelling intensity and %Corg during the Meghalayan as well, suggests decoupling of upwelling induced productivity and organic carbon burial in the western Bay of Bengal. Towards the late Meghalayan (after ~1500 yr BP), we report a relatively enhanced precipitation with continual increase in CaCO3 but with reduced Corg burial. Further, these long-term variations were accompanied by significant short-term periodic changes, closely resembling Gleissberg cycle as well as Seuss cycle, suggesting a possible link with the solar variability.