Miguel Angel Ahumada-Sempoal

The Gulf of Tehuantepec is one of the most productive regions in the northeastern tropical Pacific; however, in-situ studies of phytoplankton biomass during the rainy season remain scarce. Under stratified conditions, surface chlorophyll-a (Chl-a) may not adequately represent water-column Chl-a because a substantial fraction of it can occur below the surface. This study examined the horizontal and vertical distribution of Chl-a and evaluated how mixed-layer depth (MLD), thermocline depth (TD), and water-column structure influenced Chl-a patterns and the position of the deep chlorophyll maximum (DCM) in the Gulf of Tehuantepec and adjacent offshore waters during June 2010. Chl-a concentrations were determined from water samples collected at 0, 10, 20, 30, 50, and 75 m, together with CTD profiles obtained across continental shelf and offshore stations during an oceanographic survey. Surface Chl-a showed a marked coastal-offshore gradient, with higher concentrations over the continental shelf and oligotrophic conditions offshore. However, depth-integrated Chl-a did not differ significantly between the shelf and offshore stations, indicating that surface Chl-a alone did not adequately describe water-column Chl-a under stratified conditions. Vertical Chl-a profiles differed significantly between the continental shelf and offshore waters. DCMs were mostly located within the upper 40 m. Over the continental shelf, DCMs occurred mainly within the MLD, whereas offshore DCMs occurred either within or below the thermocline, with below-thermocline cases being slightly more frequent. Depth-integrated Chl-a was significantly related to both MLD and TD, whereas DCM depth was significantly related only to TD. Overall, the vertical placement of the thermocline was a key hydrographic feature structuring Chl-a distribution during rainy-season stratification.

Five high chlorophyll-a concentration events, > 2.3 mg m-3, developed from December 2011 to March 2012, are analyzed in the context of the observed vertical stratification variability created by the interaction of wind and the coastal mesoscale circulation at La Boquilla, a point in the Western Margin of the Gulf of Tehuantepec. The analysis is based on records of temperature at 5 and 24 m depth, water velocity profiles, wind velocity at a nearby meteorological station, Level 3 MODIS-Aqua daily chlorophyll-a concentration, and daily ASCAT winds. Most of the high chlorophyll-a concentration events occurred when mesoscale cyclonic circulation was close to La Boquilla, under westerly wind stress τ > 0.02 N m-2. Each high chlorophyll-a event succeeded the dissipation of a thermal front and the development of a two-layer exchange between the western margin and the central gulf. The highest chl-a concentration occurred when upwelling and mixing modified the stratification. The modest high chl-a concentration occurred when mixing alone modified the stratification.

Since 2014, the global land and sea surface temperature has scaled 0.23 °C above the decadal average (2009–2018). Reports indicate that Mediterranean Sea temperatures have been rising at faster rates than in the global ocean. Oceanographic time series of physical and biogeochemical data collected from an onboard and a multisensor mooring array in the northwestern Mediterranean Sea (Blanes submarine canyon, Balearic Sea) during 2009–2018 revealed an abrupt temperature rising since 2014, in line with regional and global warming. Since 2014, the oligotrophic conditions of the water column have intensified, with temperature increasing 0.61 °C on the surface and 0.47 °C in the whole water column in continental shelf waters. Water transparency has increased due to a decrease in turbidity anomaly of −0.1 FTU. Since 2013, inshore chlorophyll a concentration remained below the average (−0.15 mg·l⁻¹) and silicates showed a declining trend. The mixed layer depth showed deepening in winter and remained steady in summer. The net surface heat fluxes did not show any trend linked to the local warming, probably due to the influence of incoming offshore waters produced by the interaction between the Northern Current and the submarine canyon. Present regional and global water heating pattern is increasing the stress of highly diverse coastal ecosystems at unprecedented levels, as reported by the literature. The strengthening of the oligotrophic conditions in the study area may also apply as a cautionary warning to similar coastal ecosystems around the world following the global warming trend.

Marine biophysical models can be used to explore the displacement of individuals in and between submarine canyons. Mostly, the studies focus on the shallow hydrodynamics in or around a single canyon. In the northwestern Mediterranean Sea, knowledge of the deep-sea circulation and its spatial variability in three contiguous submarine canyons is limited. We used a Lagrangian framework with three-dimensional velocity fields from two versions of the Regional Ocean Modeling System (ROMS) to study the deep-bottom connectivity between submarine canyons and to compare their influence on the particle transport. From a biological point of view, the particles represented eggs and larvae spawned by the deep-sea commercial shrimp Aristeus antennatus along the continental slope in summer. The passive particles mainly followed a southwest drift along the continental slope and drifted less than 200 km considering a pelagic larval duration (PLD) of 31 d. Two of the submarine canyons were connected by more than 27 % of particles if they were released at sea bottom depths above 600 m. The vertical advection of particles depended on the depth where particles were released and the circulation influenced by the morphology of each submarine canyon. Therefore, the impact of contiguous submarine canyons on particle transport should be studied on a case-by-case basis and not be generalized. Because the flows were strongly influenced by the bottom topography, the hydrodynamic model with finer bathymetric resolution data, a less smoothed bottom topography, and finer sigma-layer resolution near the bottom should give more accurate simulations of near-bottom passive drift. Those results propose that the physical model parameterization and discretization have to be considered for improving connectivity studies of deep-sea species.

Marine biophysical models can be used to explore the displacement of individuals in and between submarine canyons. Mostly, the studies focus on the shallow hydrodynamics in or around a single canyon. In the northwestern Mediterranean Sea, the knowledge on the deep-sea circulation and its spatial variability in three contiguous submarine canyons is limited. We used a Lagrangian framework with three-dimensional velocity fields from two hydrodynamics models to study the deep bottom connectivity between submarine canyons and to compare their influences on the particle transport. The particles represented eggs and larvae spawned by the deep-sea commercial shrimp Aristeus antennatus along the continental slope in summer. The passive particles mainly followed a southwest drift along the slope and drifted less than 200 km within 31 days. Two of the sub-marine canyons were connected by more than 27 % particles if they were released at sea bottom depths above 600 m. The vertical displacement of particles was depending on the submarine canyons, the depth and the can-yon wall where particles were released and it encouraged the analyses of the particle transport by canyons in-stead of generalizing the dynamics. In the two hydrodynamic models tested in this study, passive drift simulation differed depending on topography. Despite being run on a coarser grid, the hydrodynamic model using finer bathymetric resolution data and adjusted to the topography seemed to better model the passive drift of particles. Those results promote that the physical model parameterization has to be considered for improving the transport studies of deep-sea species.

There are many marine protected areas (MPAs) containing coral reef aggregations in the eastern Pacific region. However, the connectivity of corals between MPAs is still poorly known, especially in the Marine Conservation Corridor of the Eastern Tropical Pacific (MCCETP). Here, we assess the potential connectivity of corals across equatorial eastern Pacific MPAs through a Lagrangian particle-tracking algorithm coupled offline with an ocean-circulation numerical model. Connectivity metrics and graph theory were used to analyze the networks and highlight those MPAs that are critical for maintaining the connectivity of corals across the region. Our results show that the equatorial eastern Pacific MPAs form a relatively well-connected network, at least 40% of coral larvae released per year end up within the boundaries of an MPA. MPAs like Malpelo and Gorgona islands included in the MCCETP were found to be critical for connectivity of corals because of their high betweenness centrality and potential role as stepping-stones between coastal MPAs and offshore MPAs such as the Galapagos Islands. Two pelagic larval duration (PLD) scenarios (40 and 130 days) indicate a quasi-unidirectional larval flow from coastal MPAs toward oceanic MPAs, where the only resilient MPAs (Coiba and Malpelo islands) depend mostly on subsidiary recruitment from MPAs located along the coast of Costa Rica, Panama and Colombia. In the two PLD scenarios, Cocos Island maintains a very low resilience potential. Our results indicate the imperative need to include coastal MPAs in the MCCETP network initiative, since connectivity and resilience of coral reefs in the equatorial eastern Pacific region rely heavily on coastal MPAs.