Hydrographic Variability Research Articles

Hydrography of the Inner Basins in Hornsund (Svalbard): Heat Advection Near Tidewater Glaciers

AbstractAtlantic‐type hydrography is becoming more dominant in the Svalbard fjords. Advected warm waters affect sea‐ice formation and melting rates of tidewater glaciers. Therefore, it is important to study hydrographic variability in fjords. Here, we use hydrographic data from Hornsund fjord to investigate the presence of warm water masses close to tidewater glaciers. Data span from May to October of 2015–2023 and cover several inner basins: Brepollen, Samarinvågen, Burgerbukta West, and Burgerbukta East, in addition to the main fjord. Average temperatures above 2C are observed during August–October in the surface–50 m layer of all the inner basins, and during September–October in the 50 m–bottom layer of all the inner basins except Burgerbukta East. The surface–50 m layer of both Burgerbukta basins is colder and fresher compared to Brepollen, despite Burgerbukta basins being closer to the fjord mouth. These spatial differences likely arise from counter‐clockwise currents that advect warm and saline shelf waters to Brepollen along the southern side of the fjord, and relatively cold and fresh mixed shelf‐fjord waters to Burgerbukta as they exit along the northern side of the fjord. Burgerbukta East is the coldest and freshest inner basin. The differences between the two Burgerbukta basins suggest a strong influence of underwater sills and site‐specific environmental processes such as glacier ablation and local circulation on the water properties. Observed differences in the water temperatures of these closely‐spaced inner basins match the spatial variability in the glacier retreat rates, and indicate the importance of ocean forcing for the ice mass loss.

Response of submarine braided channels to varying inflow hydrographs: Geomorphic experiments and stratigraphic implications

AbstractSubmarine channels on the seafloor experience a range of turbidity current‐flood hydrographs. The characteristics of the flood hydrograph influence channel dimensions, the number of channel threads (i.e. braiding intensity) and the depositional properties of sandbars within the channels. However, the morphodynamic response of submarine braided channels to different hydrograph patterns is poorly understood. In this study, six geomorphic experiments are used to test how varied flood hydrographs affect the braiding intensity and sandbar geometry within submarine channels. Three experiments represent the control group with constant density‐current inflow, and three experiments use different time‐varying hydrograph patterns. The experiments show that, as the inflow‐to‐sediment discharge ratio increases, multiple small channels coalesce into a few main channels, causing a decrease in active braiding intensity defined by the channel threads with flow. When the flow returns to the initial low flow rate, active braiding intensity increases again. These observations show that the inflow‐to‐sediment discharge ratio determines the value of active braiding intensity. Additionally, active braiding intensity is directly proportional to dimensionless sediment‐stream power and dimensionless stream power, in agreement with previously established trends for both submarine and fluvial braided channels. In contrast to active braiding intensity changes, the measured sandbar aspect ratio and compactness ratio (i.e. ratio of planform area to perimeter) of submarine braided channels is insensitive to changes in the hydrograph. However, when inflows reach peak discharge, the higher transport capacity erodes pre‐existing deposits, forming noticeable erosional surfaces, and subsequently depositing thicker sandbars during that stage. These observations imply that sandbar planform morphology is largely insensitive to hydrograph, but hydrograph variability induces a greater variability in the resultant stratigraphic packages. The experimental results predict that field‐scale stratigraphy should be dominated by strata deposited during intervals of high flow, but that these strata will also exhibit reduced lateral continuity compared to formation scenarios with constant discharge.

Consistent Seasonal Hydrography From Moorings at Northwest Greenland Glacier Fronts

AbstractGreenland's marine‐terminating glaciers connect the ice sheet to the ocean and provide a critical boundary where heat, freshwater, and nutrient exchanges take place. Buoyant freshwater runoff from inland ice sheet melt is discharged at the base of marine‐terminating glaciers, forming vigorous upwelling plumes. It is understood that subglacial plumes modify waters near glacier fronts and increase submarine glacier melt by entraining warm ambient waters at depth. However, ocean observations along Greenland's coastal margins remain biased toward summer months which limits accurate estimation of ocean forcing on glacier retreat and acceleration. Here, we fill a key observational gap in northwest Greenland by describing seasonal hydrographic variation at glacier fronts in Melville Bay using in situ observations from moorings deployed year‐round, CTDs, and profiling floats. We evaluated local and remote forcing using remote sensing and reanalysis data products alongside a high‐resolution ocean model. Analysis of the year‐round hydrographic data revealed consistent above‐sill seasonality in temperature and salinity. The warmest, saltiest waters occurred in spring (April–May) and primed glaciers for enhanced submarine melt in summer when meltwater plumes entrain deep waters. Waters were coldest and freshest in early winter (November–December) after summer melt from sea ice, glacier ice, and icebergs provided cold freshwater along the shelf. Ocean variability was greatest in the summer and fall, coincident with increased freshwater runoff and large wind events before winter sea ice formation. Results increase our mechanistic understanding of Greenland ice‐ocean interactions and enable improvements in ocean model parameterization.

Spatiotemporal distributions of dissolved N2O concentration, diffusive N2O flux and relevant functional genes along a coastal creek in southeastern China

Increased anthropogenic input of nitrogen into coastal creeks make them potential hotspots for N2O production and emission, but they are often excluded from regional and global N2O budget, and high-resolution sampling is required to characterize the strong spatiotemporal heterogeneity within the creeks. In this study, we analyzed the N2O concentration and diffusive N2O flux within a coastal creek in the Shanyutan Wetland in southeastern China in high spatial resolution across four seasons. Ancillary hydrographical variables and N2O-related functional gene abundances were also measured. Results showed that the creek was consistently oversaturated in N2O, at a seasonal average of 5.6–14.2 nmol/L, relative to the overlying atmosphere. The spatial distribution of N2O followed the gradient of nitrogenous substrate but was inversely related to the salinity gradient, and the coefficient of spatial variation of N2O flux ranged from 66.3 % to 116.5 %. Nitrite reduction (based on nirK and nirS gene abundances) and ammonia oxidation (AOA amoA and AOB amoA) appeared to outpace N2O reduction (nosZ I and nosZ II), and these were the main microbial processes that determined N2O concentration and flux. Both N2O concentration and flux were substantially higher in autumn than those in the other seasons, but that did not appear to be related to precipitation. N2O diffusive flux from the creek averaged 322.2 nmol m−2 h−1, which was over 2 times higher than the global average for lakes and reservoirs. Our results highlight that coastal creeks are strong atmospheric N2O sources with high spatiotemporal variability.

Deep ocean hydrographic variability estimated from distributed geodetic sensor arrays off northern Chile

Observations of spatio-temporal variability of the deep ocean are rare and little is known about occurrence of deep ocean mesoscale dynamics. Here, we make use of 2.5 years of time series data from three distributed sensor arrays, which acquired high-resolution temperature, pressure and sound speed data of the bottom layer offshore northern Chile. Estimating salinity and density from the direct observations enable access to the full spectrum of hydrographic variability from a multi-hourly to annual time scale and with average inter-station distances of less than 1 km. Analyses revealed interannual warming over the continental slope of 0.002 °C yr−1–0.003 °C yr−1, and could trace periodic hydrographic anomalies, likely related to coastal-trapped waves, as far as to the lower continental slope. A concurrent change in the shape of the warm anomalies and the rate of deep-sea warming that occurs with the crossing of the deep-sea trench suggests that the abyssal part of the eastern boundary current system off Chile does not extend past the deep sea trench. Furthermore, the comparison of anomaly timing and shape in between stations implies southwards flow over the mid to lower continental slope, centred closer to the trench.

Biogeochemical Response of the Water Column of Concepción Bay, Chile, to a New Regime of Atmospheric and Oceanographic Variability

Concepción Bay is a socio-economic and ecologically important embayment whose hydrographic variability has been historically regulated by wind-modulated seasonal upwelling events during spring–summer and by freshwater from precipitation and river discharges during fall–winter. This system is subject to several anthropogenic and environmental strains due to the intense port activity and the increasing occurrence of extreme natural events. This study determines a new hydrographic regime and characterizes and analyzes the biogeochemical response of the water column to changes in rainfall and upwelling patterns. Despite the intrusion of nitrate-rich upwelled waters that enhance biological productivity remains more intense during spring–summer, the system remains fertilized year-long due to the occurrence of persistent upwelling pulses during fall–winter. The hydrographic structure presented a two-layer water column that was stratified thermally in spring–summer and primarily by freshwater inputs in fall–winter. Nevertheless, the regular pattern of the rainfall has changed (a decrease in precipitation and an increased frequency of extreme rainfall events), together with recurrent upwelling-favorable wind pulses during the non-upwelling season. This new regime has altered the seasonality of the physicochemical conditions and the structure of the microplanktonic communities, with productive and sanitary implications affecting the biogeochemical status of CB.

Interaction between phytoplankton and heterotrophic bacteria in Arctic fjords during the glacial melting season as revealed by eDNA metabarcoding.

The hydrographic variability in the fjords of Svalbard significantly influences water mass properties, causing distinct patterns of microbial diversity and community composition between surface and subsurface layers. However, surveys on the phytoplankton-associated bacterial communities, pivotal to ecosystem functioning in Arctic fjords, are limited. This study investigated the interactions between phytoplankton and heterotrophic bacterial communities in Svalbard fjord waters through comprehensive eDNA metabarcoding with 16S and 18S rRNA genes. The 16S rRNA sequencing results revealed a homogenous community composition including a few dominant heterotrophic bacteria across fjord waters, whereas 18S rRNA results suggested a spatially diverse eukaryotic plankton distribution. The relative abundances of heterotrophic bacteria showed a depth-wise distribution. By contrast, the dominant phytoplankton populations exhibited variable distributions in surface waters. In the network model, the linkage of phytoplankton (Prasinophytae and Dinophyceae) to heterotrophic bacteria, particularly Actinobacteria, suggested the direct or indirect influence of bacterial contributions on the fate of phytoplankton-derived organic matter. Our prediction of the metabolic pathways for bacterial activity related to phytoplankton-derived organic matter suggested competitive advantages and symbiotic relationships between phytoplankton and heterotrophic bacteria. Our findings provide valuable insights into the response of phytoplankton-bacterial interactions to environmental changes in Arctic fjords.

Seasonal variability of coastal pH and CO2 using an oceanographic buoy in the Canary Islands

Ocean acidification, caused by the absorption of carbon dioxide (CO2) from the atmosphere into the ocean, ranks among the most critical consequences of climate change for marine ecosystems. Most studies have examined pH and CO2 trends in the open ocean through oceanic time-series research. The analysis in coastal waters, particularly in island environments, remains relatively underexplored. This gap in our understanding is particularly important given the profound implications of these changes for coastal ecosystems and the blue economy. The present study focuses on the ongoing monitoring effort that started in March 2020 along the east coast of Gran Canaria, within the Gando Bay, by the CanOA-1 buoy. This monitoring initiative focuses on the systematic collection of multiple variables within the CO2 system, such as CO2 fugacity (fCO2), pH (in total scale, pHT), total inorganic carbon (CT), and other hydrographic variables including sea surface salinity (SSS), sea surface temperature (SST) and wind intensity and direction. Accordingly, the study allows the computation of the CO2 flux (FCO2) between the surface waters and the atmosphere. During the study period, stational (warm and cold periods) behavior was found for all the variables. The lowest SST values were recorded in March, with a range of 18.8-19.3°C, while the highest SST were observed in September and October, ranging from 24.5-24.8°C. SST exhibited an annual increase with a rate of 0.007°C yr-1. Warmer months increased SSS, while colder periods, influenced by extreme events like tropical storms, led to lower salinity (SSS=34.02). The predominant Trade Winds facilitated the arrival of deeper water, replenishing seawater. The study provided insights into atmospheric CO2. Atmospheric fCO2 averaged 415 ± 4 µatm (2020-2023). Surface water fCO2sw presented variability, with the highest values recorded in September and October, peaking at 437 µatm in September 2021. The lowest values for fCO2sw were found in February 2021 (368 µatm). From 2020 to 2023, surface water fCO2sw values displayed an increasing rate of 1.9 µatm yr-1 in the study area. The assessment of fCO2sw decomposition into thermal and non-thermal processes revealed the importance of SST on the fCO2sw. Nevertheless, in the present study, it is crucial to remark the impact of non-thermal factors on near-shallow coastal regions. Our findings highlight the influence of physical factors such as tides, and wind effect to horizontal mixing in these areas. The CT showed a mean concentration of 2113 ± 8 μmol kg-1 and pH at in-situ temperature (pHT,IS) has a mean value of 8.05 ± 0.02. The mean FCO2 from 2020 to 2023 was 0.34 ± 0.04 mmol m-2 d-1 (126 ± 13 mmol m-2 yr-1) acting as a slight CO2 source. In general, between May and December were the months when the area was a source of CO2. Extrapolating to the entire 6 km2 of Gando Bay, the region sourced 33 ± 4 Tons of CO2 yr-1.

Observed Seasonal Evolution of the Antarctic Slope Current System off the Coast of Dronning Maud Land, East Antarctica

AbstractThe access of heat to the Antarctic ice shelf cavities is regulated by the Antarctic Slope Front, separating relatively warm offshore water masses from cold water masses on the continental slope and inside the cavity. Previous observational studies along the East Antarctic continental slope have identified the drivers and variability of the front and the associated current, but a complete description of their seasonal cycle is currently lacking. In this study, we utilize two years (2019–2020) of observations from two oceanographic moorings east of the prime meridian to further detail the slope front and current seasonality. In combination with climatological hydrography and satellite‐derived surface velocity, we identify processes that explain the hydrographic variability observed at the moorings. These processes include (a) an offshore spreading of seasonally formed Antarctic Surface Water, resulting in a lag in salinity and thermocline depth seasonality toward deeper isobaths, and (b) the crucial role of buoyancy fluxes from sea ice melt and formation for the baroclinic seasonal cycle. Finally, data from two sub‐ice‐shelf moorings below Fimbulisen show that flow at the main sill into the cavity seasonally coincides with a weaker slope current in spring/summer. The flow is directed out of the cavity in autumn/winter when the slope current is strongest. The refined description of the variability of the slope current and front contributes to a more complete understanding of processes important for ice‐shelf‐ocean interactions in East Antarctica.

Impacts of Agulhas Current meanders on intermediate water masses along the adjacent continental slope and shelf.

Variability in the Agulhas Current system is dominated by meanders, which constitute cyclonic eddies along the inshore edge of the Current on the southeast coast of South Africa. Few studies have investigated the influence of these meanders on hydrographic variability on the adjacent shelf and slope and to date only a handful have been sampled in situ. This study used available in situ data and GLORYS12v1 model output to investigate the impact of meanders on the distribution of Intermediate waters, namely Red Sea Water (RSW) and Antarctic Intermediate Water (AAIW), as well as mechanisms driving these variations. We focussed on four eddies, sampled in situ during July 1998, April 2010, January–February 2017, and July–August 2017. RSW dominated along the inshore edge of the Agulhas Current in the absence of meanders, but larger proportions of AAIW occurred in the presence of cyclonic eddies. During eddy events, the kinematic steering level was raised above the lower boundary of Intermediate waters, increasing cross-frontal mixing of waters at depths of 800–1800 m. Eddy-induced upwelling of Central and Intermediate waters onto the shelf appeared to be inhibited by bands of strong positive relative vorticity (>0.4 × 10−4 s−1), which likely promoted downwelling conditions inshore of the July 1998, April 2010, and July–August 2017 eddies. Weak positive relative vorticity (<0.2 × 10−4 s−1) inshore of the January–February 2017 eddy was associated with enhanced water mass exchange between the shelf and deeper (>1000 m) ocean. GLORYS12v1 was consistently comparable with satellite and in situ data, and simulated the overall distribution of water masses on the continental shelf and slope despite its inability to reflect the influence of river discharge in nearshore regions during austral summer. The model is thus suitable to investigate the influence of Agulhas Current meanders on the hydrography of South Africa's southeast coast.

A Comprehensive Probabilistic Flood Assessment Accounting for Hydrograph Variability of ESL Events

AbstractFlood characteristics caused by extreme sea level (ESL) events depend largely on the magnitude of peak water levels (WLs) and their temporal evolution. However, coastal flood risk is generally assessed based on only a limited number of potential peak WLs and a selection of past events or a design hydrograph. We address this gap and systematically estimate (a) spatial annual and (b) event‐based flood probabilities by comprehensively accounting for both a wide range of peak ESLs and their temporal evolution, herein referred to as hydrograph intensity. We simulate flooding at the German Baltic Sea coast with the hydrodynamic model Delft3D. We produce probabilistic flood maps, which detail flood exposed areas together with annual probability of flooding. Additionally, we show how the flood extent changes, when accounting for upper, median, and lower quantiles of hydrograph intensities. Our results demonstrate that the relevance of the intensity is site and ESL dependent. While flood extents of some ESLs of the upper and lower intensity bounds indicate no differences, others differ by up to 45%. Further, we consider two ESLs (2.24 and 2.55 m) and simulate 100 intensities for each. Compared to intensity quantiles, this results in flood extents of up to 60% difference. Hence, we find that quantiles of intensity do not cover the full range when addressing uncertainty due to hydrograph variability. We, therefore, recommend accounting for a wide range of hydrograph intensities in addition to using a wide range of ESL in future flood risk assessments.

Multiscale spatial patterns and environmental drivers of seamount and island slope megafaunal assemblages along the Mozambique channel

Seamounts are vulnerable ecosystems targeted by fishing and potentially by future mineral exploitation. Their abundance, widespread distribution, and heterogeneity of faunal and abiotic components require integrated studies at multiscale to describe spatial patterns and identify environmental drivers needed by conservation plans. There is also a lack of knowledge on seamount benthic ecosystems in some regions, such as the Indian Ocean. These gaps, in the context of Marine Protected Areas establishment in the region, have motivated the present study focusing on the Mozambique Channel Eparses islands and flat top seamounts, along a 10-degree latitude gradient. These structures are characterized by complex volcanic and carbonate geomorphologies at multiscale and are distributed along a highly dynamic turbulent ocean circulation area with large anticyclonic eddies. For the first time, we analysed, from seabed image transects obtained by towed-camera on four seamounts, and two volcanic islands - Bassas da India and Mayotte - external slopes, and from multiscale environmental data, how benthic communities respond to this high habitat heterogeneity at regional, and local scales. This study reveals high discrepancies of benthic megafauna richness, density, and beta diversity among seamounts and among slopes of the same islands. Moreover, at similar latitude, seamounts display higher densities than island slopes. The highest densities found on a seamount of the Glorieuses archipelago are explained by strong currents and flat homogeneous geomorphology. Except on this seamount, the beta diversity is high, despite the quite limited depth range explored (84–734 m) and is the highest on island slopes and Hall Bank, driven by the diversity and hardness of the substrate. Beta diversity is mainly due to taxa turnover, with high contribution of the habitat-forming sponges and cnidarians, together with a few mobile taxa. We identified from biogeographic network analysis 12 dominant faunal assemblages, displaying a patchy distribution, with variability in composition both among and within sites. Currents and primary productivity explain ∼15% of the observed assemblage structure along the channel, while geomorphology (km scale), topography (60–500 m scale) and substrate (60-m units) explain together 24% of the faunal spatial patterns. Analysis of spatial structures along island slopes detected some small (100–200 m), medium (∼1 km) and large scale (∼2–6 km) megabenthic community structures, partly explained by topography, substrate, depth, and slope. Despite limited taxonomic identifications for this poorly sampled area, this study reveals an outstanding heterogeneity of megabenthic assemblages at multispatial scales in the Mozambique Channel seamounts and island slopes, in response to the complex hydrography and geology of the area. Further characterization of environmental drivers with greater focus at local scales including hydrographic variables are therefore needed to improve predictions of suitable habitats of vulnerable marine ecosystems.

Soil temperature and local initial conditions drive carbon and nitrogen build-up in young proglacial soils in the Tropical Andes and European Alps

Climate warming has accelerated the retreat of mountain glaciers worldwide, exposing new areas to weathering, vegetation colonization, and soil formation. In light of probable climate changes such as warming and new extremes, understanding the factors that control soil organic carbon (SOC) and nitrogen build-up is crucial to comprehend proglacial soils and ecosystem formation. To this end, we examine the evolution of SOC, nitrogen (total N and NH4+), and phosphorus (available P) along nine 120-year chronosequences of deglacierization distributed between the European Alps and Tropical Andes. Our dataset includes geochemical analyses of 188 soil samples, in situ soil temperature data for the period 2019–2022, and hydrographic variables. Although time controls proglacial soil development at all sites, our study highlights distinct pedogenesis dynamics between proglacial landscapes depending on the micro and macro environmental context. Differences in soil development were strongly driven by growing degree days (GDD), maximum soil temperature, and parent material. Notably, we identified a positive effect of GDD on SOC and N (total N and NH4+), while our results indicate a negative effect of maximum soil temperature on SOC and NH4+, suggesting that overly high temperatures reduce microbial mineralization and organic matter input to the soil matrix. We reported the presence of higher initial SOC, total N, and NH4+ in the Andean sites than in the Alps sites, suggesting enhanced soil development at the Andean locations. This comparative study suggests the relative importance of maximum temperature and initial site conditions (e.g., parent materials, glacier biomes) during proglacial pedogenesis. Our findings highlight that soil temperature modulates pedogenesis in a complex way and suggest avoiding simply associating greater soil development with higher soil temperature in proglacial landscapes.

The Pacific water flow branches in the eastern Chukchi Sea

The flow of Pacific-origin water across the Chukchi Sea shelf impacts the regional ecosystem in profound ways, yet the two current branches on the eastern shelf that carry the water from Bering Strait to Barrow Canyon – the Alaskan Coastal Current (ACC) and Central Channel (CC) Branch – have not been clearly distinguished or quantified. In this study we use an extensive collection of repeat hydrographic sections occupied at three locations on the Chukchi shelf, together with data from a climatology of shipboard velocity data, to accomplish this. The data were collected predominantly between 2010 and 2020 during the warm months of the year as part of the Distributed Biological Observatory and Arctic Observing Network. The mean sections show that mass is balanced for both currents at the three locations: Bering Strait, Point Hope, and Barrow Canyon. The overall mean ACC transport is 0.34 ± 0.04 Sv, and that of the CC Branch is 0.86 ± 0.11 Sv. The dominant hydrographic variability at Bering Strait is seasonal, but this becomes less evident to the north. At Barrow Canyon, the dominant hydrographic signal is associated with year-to-year variations in sea-ice melt. Farther south there is pronounced mesoscale variability: an empirical orthogonal function analysis at Bering Strait and Point Hope reveals a distinct ACC mode and CC Branch mode in hydrography and baroclinic transport, where the former is wind-driven. Finally, the northward evolution in properties of the two currents is investigated. The poleward increase in salinity of the ACC can be explained by lateral mixing alone, but solar heating together with wind mixing play a large role in the temperature evolution. This same atmospheric forcing also impacts the northward evolution of the CC Branch.

Hydrography, inorganic nutrients and chlorophyll a linked to sea ice cover in the Atlantic Water inflow region north of Svalbard

Changes in the inflow of Atlantic Water (AW) and its properties to the Arctic Ocean bring more warm water, contribute to sea ice decline, promote borealisation of marine ecosystems, and affect biological and particularly primary productivity in the Eurasian Arctic Ocean. One of the two branches of AW inflow follows the shelf break north of Svalbard, where it dominates oceanographic conditions, bringing in heat, salt, nutrients and organisms. However, the interplay with sea ice and Polar Surface Water (PSW) determines the supply of nutrients to the euphotic layer especially northeast of Svalbard where AW subducts below PSW. In an effort to build up a time series monitoring the key characteristics of the AW inflow, repeat sampling of hydrography, macronutrients (nitrate, phosphate and silicate), and chlorophyll a (chl a) was undertaken along a transect across the AW inflow at 31°E, 81.5°N since 2012 — first during late summer and in later years during early winter. Such time series are scarce but invaluable for investigating the range of variability in hydrography and nutrient concentrations. We investigate linkages between late summer hydrographic conditions and nutrient concentrations along the transect and the preceding seasonal dynamics of surface chl a and sea ice cover in the region north of Svalbard. We find large interannual variability in hydrography, nutrients and chl a, indicating varying levels of nutrient drawdown by primary producers over summer. Sea ice conditions varied considerably between the years, impacting upper ocean stratification, light availability and potential wind-driven mixing, with a strong potential for steering chl a concentration over the productive season. Early winter measurements show variable efficiency of nutrient re-supply through vertical mixing when stratification was low, related to autumn wind forcing and sea ice conditions. While this re-supply elevates nutrient levels sufficiently for primary production, it likely happens too late in the season when light levels are already low, limiting the potential for autumn blooms. Such multidisciplinary observations provide insight into the interplay between physical, chemical and biological drivers in the marine environment and are key to understanding ongoing and future changes, especially at this entrance to the central Arctic Ocean.

Concentration and condition of American lobster postlarvae in small‐scale convergences

AbstractInvertebrate larvae are often abundant in the surface ocean, which plays a key role in their dispersal and connectivity. Pelagic microhabitats characterized by small‐scale hydrographic variability are complex and ubiquitous in the coastal ocean, but their study is challenging, and they have been largely neglected in meroplankton ecology. Surface convergences, i.e., surface microhabitats featuring convergent horizontal currents, may aggregate the last larval stage of the American lobster and could provide shelter and food for Stage IV postlarvae and thus enhance their condition. We tested these hypotheses by conducting a series of cruises in the southwestern Gulf of Maine in summer 2021, sampling 15 paired sets of potential convergences and off‐convergence unstructured habitat. We measured postlarval abundance, surface hydrography, acoustic backscatter, and circulation. Experiments and image analysis compared condition, color, and morphology of postlarvae sampled inside and outside potential convergences. Potential convergences varied in near‐surface hydrographic patterns, with most displaying consistency among two transects and diverse patterns in salinity and temperature (e.g., across‐convergence gradients with equal or different signs). While the highest postlarval abundances were found in convergences, abundance patterns on and off convergences were not consistent, and another analysis indicated higher abundance in convergences than in a 7‐year untargeted surface ocean data set. Experiments indicated no survivorship differences among convergence and non‐convergence individuals at two temperatures, while image analyses revealed differences in color and size. Physical measurements and qualitative neuston community analyses indicated substantial heterogeneity among potential convergences. Our results reinforce that small‐scale heterogeneities are highly variable but important to the ecology of meroplankton, including the pelagic and neustonic habitats where lobster postlarvae are abundant.

Importance of nanophytoplankton biomass during summer 2019 in a retreating marine-terminating glacier-fjord system, Marian Cove, West Antarctica (62°S)

The biogeochemical dynamics of fjords around Antarctica are strongly influenced by cryospheric, climatic, and oceanographic processes that occur on a seasonal scale. Furthermore, as global climate change continues, there is a growing awareness of the impact of ocean warming on glacier melting, which is expected to affect the composition of phytoplankton community structure in West Antarctica’s nearshore marine areas. In this study, we describe the role of hydrographic forcing on the short-term summer variability of the phytoplankton community in Marian Cove, an Antarctic glacial fjord (62°S). Phytoplankton and hydrographic variables were measured at five stations along the Marian Cove during summer 2019 (January–February). The highest concentrations of microphytoplankton biomass were found in the outer area of the fjord, whereas nanophytoplankton biomass displayed continued dominance during most of the summer period in Marian Cove. Hydrographic assessment showed that freshwater inputs from the glacier influenced the surface layer of the fjord, modulating phytoplankton biomass, which was dominated by nanodiatoms (Minidiscus sp., Thalassiosira spp.) and nanophytoflagellates (Cryptomonas spp., Phaeocystis sp.). Concurrent measurement of phytoplankton biomass and environmental conditions during December 2018–January 2019 indicated that a period of weak southeastern winds generated vertical stability, which led to the development of a major peak of microphytoplankton biomass in the outer cove, driven by warm, allochthonous, oceanic, nutrient-rich waters. High carbon biomass dominated by nanodiatoms and nanophytoflagellates was observed in cold, fresh, and low-light subsurface waters of the cove. Our results highlight the effects of a warming ocean, which may favor the summer resurgence of nanodiatom and nanophytoflagellate communities in Antarctic fjords due to increased glacial meltwater inputs.

Western South Atlantic seasonal variability recorded in a mid-deglacial bivalve from the outer Uruguayan continental shelf

The oceanographic dynamics on the continental shelf off southeastern South America are primarily controlled by the southward-flowing warm Brazil Current, converging with the northward-directed cold Malvinas (Falkland) Current, and interacting with the continental discharge of the Plata River. The seasonally reversing regional wind field together with the seasonal cycle of riverine discharge, determines which of these three components provides the dominant forcing. The Uruguayan shelf is thus located in a transitional zone that extends from the region influenced by the Brazil-Malvinas Confluence (BMC) in the open ocean to the Subtropical Shelf Front (STSF) on the continental shelf. Understanding how the resulting oceanic seasonal variability responded to different climatic boundary conditions may shed light on its future behavior. This study presents the first reconstruction of mid-deglacial seasonal hydrographic variability on the continental shelf off southeastern South America in seasonal resolution based on stable oxygen and carbon isotopes (δ18O, δ13C) from a thick-walled shell of a long-living bivalve. The mid-deglacial (14.3 cal ka BP; Bølling-Allerød interstadial) Retrotapes exalbidus bivalve shows a mean δ18O of 3.27 ± 0.42‰ (2.50 ± 0.42‰ when corrected for changes in global ice volume) and a seasonal δ18O amplitude of 1.69‰ for raw isotopic excursions. Moreover, the δ13C exhibits abrupt negative peaks coincident with more negative δ18O values that indicate seasons of elevated freshwater discharge. Finally, the growth rate of the bivalve suggests that the specimen was closer to the metabolically optimum than modern individual of this species from southern South America. Combining biogeographic and ecologic information with these isotopic data, the results point to colder waters and a slightly lower mid-deglacial seasonal amplitude in temperature compared with modern conditions at this shelf site. Because of the northward-displaced Plata River mouth during deglacial times, negative δ13C peaks are expected to reflect an influence of non-point freshwater sources in the form of small fluvial distributaries along the paleo-coast. Most of this signal may, however, be driven by seasonal metabolic effects associated with low ambient water temperatures related to a shallow-water environment located closer to the respective paleo-coastline due to the low sea level at those times.

Interannual variability of surface bio-optical characteristics in the frontal zones of the Indian sector of the Southern Ocean during austral summer

Surface bio-optical constituents (Chl-a, light-absorption coefficient of phytoplankton (aph (443)), detritus (ad(443)), coloured dissolved organic matter (aCDOM(443))), and hydrographic variables were studied across the fronts along 57.5°E during 2011, 2013 and 2015 in the Indian sector of Southern Ocean to understand their interannual variability and phytoplankton pigment package effect. Latitudinal increases of Chl-a, aph(443), and aCDOM(443) were observed in 2011 and 2013. Mean Chl-a was higher in 2013 than in 2015 and 2011. The Chl-a specific absorption (a*ph(443)) latitudinally decreased implying ‘pigment package effect’ with increasing latitude. Interannually increase of aph blue (443)/red (675) ratio suggested higher non-photosynthetic absorption during 2013 and 2015 than in 2011. The relative contributions of bio-optical constituents to total absorption (at) showed interannual decrease in aph(443) and increase in aCDOM(443) and ad(443) inferring phytoplankton growth and decay phases. The robust aph(443)-Chl-a relationship during 2011 was used to derive aph(443) for other years, which holds good for in situ and satellite-derived counterparts. aCDOM(443) positively correlated with Chl-a and negatively with CDOM-spectral slope (S300-500), suggesting autochthonous sources. Satellite-retrieved Chl-a, aph(443) showed significant correlation (p < 0.001) with in situ data. This study provides baseline bio-optical data for understanding phytoplankton size-class, and improvement of bio-optical algorithms in this relatively inaccessible polar region.

Variations of temperature, salinity and oxygen of the Baltic Sea for the period 1950 to 2020

Variations of temperature, salinity and oxygen of the Baltic Sea on interannual to decadal timescales were studied for the period from 1950 to 2020. Both observational data and the output of a numerical circulation model of the Baltic Sea were analyzed. In addition, we investigated the influence of atmospheric parameters and river runoff on the observed hydrographic variations. Variability of sea surface temperature (SST) closely follows that of air temperature in the Baltic on all timescales examined. Interannual variations of SST are significantly correlated with the North Atlantic Oscillation in most parts of the sea in winter. The entire water column of the Baltic Sea has warmed over the period 1950 to 2020. The trend is strongest in the surface layer, which has warmed by 0.3–0.4°C decade−1, noticeably stronger since the mid-1980s. In the remaining water column, characterized by permanent salinity stratification in the Baltic Sea, warming trends are slightly weaker. A decadal variability is striking in surface salinity, which is highly correlated with river runoff into the Baltic Sea. Long-term trends over the period 1950–2020 show a noticeable freshening of the upper layer in the whole Baltic Sea and a significant salinity increase below the halocline in some regions. A decadal variability was also identified in the deep layer of the Baltic Sea. This can be associated with variations in saltwater import from the North Sea, which in turn are influenced by river runoff: fewer strong saltwater inflows were observed in periods of enhanced river runoff. Furthermore, our results suggest that changes in wind speed have an impact on water exchange with the North Sea. Interannual variations of surface oxygen are strongly anti-correlated with those of SST. Likewise, the positive SST trends are accompanied by a decrease in surface oxygen. In greater depths of the Baltic Sea, oxygen decrease is stronger, which is partly related to the observed increase of the vertical salinity gradient.