The surface turbulent transport processes in the central Baltic Sea during summer are investigated using Lagrangian trajectory simulations forced by an operational ocean model and Finite Size Lyapunov Exponent (FSLE) and related diagnostics of horizontal stirring. The spatial distribution of Lagrangian Coherent Structures (LCS) and the local prevalence of attracting versus repelling LCS are presented. Coastal areas are characterised by strong stirring in contrast to the weakly stirred offshore regions. The along-trajectory absolute value of relative vorticity and total strain attest to the important role of eddies, fronts and jets in driving horizontal stirring in the ocean model. Strong asymmetry in LCS distribution between the eastern and western coast of Gotland is found and its consequences on the transport of algae blooms and pollutants are discussed.
Trophic function of any ecosystem is primarily governed by predator-prey interactions that influence the dynamics and the fate of communities. This study combines daily observations of fishery catch and gut-content analysis collected over three years and two months with an Ecopath model to characterise the trophic structure and functioning of Caranzalem Bay, a tropical estuarine bay in Goa, India. The empirical observations revealed substantial temporal variability in species composition and biomass, particularly among lower and mid-trophic groups. A balanced 44-functional-group Ecopath model was used to quantify ecosystem structure, energy flow, and trophic interactions, with relatively low internal cycling (Finn's Cycling Index, 4.37%). Primary producers provided the larger contribution to energy transfer, while detrital pathways increased overall transfer efficiency. Small-sized predators including elasmobranchs and catfishes consumed benthic prey, while large predators of the same groups preferred pelagic prey. Transient forage fish and megabenthic species showed large temporal fluctuations, with size-dependent prey preferences among predators. Notably, cannibalism, ontogenetic dietary shift, and intraguild predation added trophic pathways and complexity within the food web. In this shallow bay habitat, benthic, demersal, and pelagic distinctions were diffused owing to trophic linkages across the strata, wherein shrimps and brachyuran crabs showed strong system-wide trophic influence. With access to adjacent habitats and the open-ended nature of the bay, the motile mid- and upper-trophic-level megafauna can commute, resulting in a pulsatile nature of catch and bycatch, with a high degree of stochasticity in predator-prey encounters.
Tides profoundly influence nutrient supply and phytoplankton biomass in semi-enclosed bays by regulating water stratification and mixing; however, how tides regulate these coupled nutrient-production dynamics remain poorly understood due to the lack of high-frequency observations. Here, we conducted continuous measurements of dual water isotopes, hydrological parameters, nutrients, chlorophyll a (Chl-a), and total suspended matter (TSM) at 2-h intervals over a 24-h period during spring and neap tides in summer and winter in Zhanjiang Bay (ZJB), a typical tide-dominated semi-enclosed bay in Southern China. During the summer neap tide, weaker tidal forcing allowed stronger freshwater input (18.7%) and intensified stratification, resulting in the lower Chl-a level despite a higher terrestrial nutrient contribution, which was confined largely within the thin surface layer. During the spring tide, strong currents weakened the stratification (freshwater contribution ∼15.5%), sustaining nutrient supply to the euphotic zone without causing sediment resuspension (TSM remained low). Consequently, Chl-a was high. By contrast, in the winter spring tide, intense mixing caused severe sediment resuspension and high TSM, which, together with tidal dilution, most likely reduced light availability and thus lowered Chl-a. However, winter neap tides featured weak hydrodynamics, low TSM, and favorable light conditions, promoting phytoplankton growth (higher Chl-a level). Thus, whether tidal processes enhance or suppress phytoplankton growth depends on the balance they establish between nutrient supply and light availability. In summer, moderate mixing with weak stratification (spring tide) promotes blooms, while overly strong stratification (neap tide) suppresses them; in winter, intense mixing (spring tide) likely induces light attenuation and low Chl-a, whereas weak mixing (neap tide) favors higher Chl-a. These findings provide isotope-based insights for the ecological management of tide-dominated bays.
The identification, spatial extent, and number of upwelling centres (UCs) in eastern boundary upwelling systems (EBUS) are contingent upon the choice of sea surface temperature (SST) product, sensor class, and analytical threshold conventions. This study introduces a multi-product, cross-shore framework that applies a daily segmented-breakpoint regression to each transect of a fixed shore-normal mesh, consistently employed across an 11-year analysis (2015–2025) of the Benguela Upwelling System (BUS). Six Level-4 SST products, with native grid resolutions ranging from 0.01° to 0.25° and encompassing three sensor classes (infrared, infrared–microwave fused, and microwave-only), are processed symmetrically. A coherent two-regime cross-shore structure is identified in 87.3%–96.9% of profiles per product. Five of the six products locate the climatological inshore-to-offshore boundary at a median distance of 175–200km offshore, while the microwave-only MW_OI product consistently places this boundary further offshore at 250km, highlighting sensor class as a more critical determinant of UC structure than grid resolution. A ≥3/6 multi-product consensus robustly identifies the three central canonical BUS UCs (Lüderitz, Namaqua, Cape Columbine) across all four seasons, as well as a persistent JJA–SON UC at the Pelican Point shelf narrowing (22–23°S), which is absent from the canonical five-UC framework. However, Cape Frio and Cape Peninsula are not recovered, likely due to their proximity (∼2°) to the analysis-domain boundaries. A multivariate regression on the distances to the 200, 500, and 1000m isobaths reveals the shelf-break distance as the primary geometric determinant of UC structure (mean partial r=+0.24 for the climatological boundary distance, bootstrap-significant in 11 of 24 (product, season) regressions, pβ<0.05; r=−0.24 for the inshore gradient roughness ratio, significant in 9 of 24). All statistical inferences employ block-bootstrap standard errors adjusted for the product-specific along-coast decorrelation length. This framework reconceptualises UCs as spatio-temporally dynamic maxima of upwelling-favourable conditions along a near-continuous coastline, rather than as static point features, and is readily adaptable to other EBUSs.
Hydrodynamic processes play a pivotal role in coastal basins by governing water mass formation and material transport from the coast to open sea. Although the Mediterranean Sea is oligotrophic, regions such as the Cilician Basin, one of the major coastal basins in the northeastern Levantine Sea, suffer from eutrophication due to significant riverine and groundwater nutrient inputs. Water mass formation and the hydrodynamic structure of the wider basin modulate the effects of these inputs. Consequently, understanding the hydrodynamic structure of the basin is critical for managing eutrophication and other anthropogenic stressors. Here, we provide the first basin-scale quantification of water residence time (RT) for the Cilician Basin. We employed two basin-wide cruises in the dry and wet seasons of 2022, combined with satellite altimetry and a Lagrangian particle-tracking model to delineate the hydrographic structure and water renewal time of the Cilician Basin. The model results indicated a mean water RT of 252 days, with significant vertical variability. The shortest and longest RTs were observed during the wet and dry seasons, respectively. Satellite altimetry analysis revealed that RTs were longer during cyclonic circulation than during anticyclonic circulation of the Latakia Eddy at the southern boundary of the basin, likely due to particle trapping within the eddy cores. Overall, basin hydrography and circulation displayed strong seasonality, with mesoscale eddies regulating the interannual variability. Thus, circulation dynamics are tightly coupled with the hydrographic structure of the basin, driving the spatial and temporal patterns of water renewal.
