Mutualisms between reef-building corals and endosymbiotic dinoflagellates are particularly sensitive to environmental stress, yet the ecosystems they construct have endured major oscillations in global climate. During the winter of 2008, an extreme cold-water event occurred in the Gulf of California that bleached corals in the genus Pocillopora harbouring a thermally ‘sensitive’ symbiont, designated Symbiodinium C1b-c , while colonies possessing Symbiodinium D1 were mostly unaffected. Certain bleached colonies recovered quickly while others suffered partial or complete mortality. In most colonies, no appreciable change was observed in the identity of the original symbiont, indicating that these partnerships are stable. During the initial phases of recovery, a third species of symbiont B1 _Aiptasia , genetically identical to that harboured by the invasive anemone, Aiptasia sp., grew opportunistically and was visible as light-yellow patches on the branch tips of several colonies. However, this symbiont did not persist and was displaced in all cases by C1b-c several months later. Colonies with D1 were abundant at inshore habitats along the continental eastern Pacific, where seasonal turbidity is high relative to offshore islands. Environmental conditions of the central and southern coasts of Mexico were not sufficient to explain the exclusivity of D1 Pocillopora in these regions. It is possible that mass mortalities associated with major thermal disturbances during the 1997–1998 El Niño Southern Oscillation eliminated C1b-c holobionts from these locations. The differential loss of Pocillopora holobionts in response to thermal stress suggests that natural selection on existing variation can cause rapid and significant shifts in the frequency of particular coral–algal partnerships. However, coral populations may take decades to recover following episodes of severe selection, thereby raising considerable uncertainty about the long-term viability of these communities.

Difficulties in scaling up theoretical and experimental results have raised controversy over the consequences of biodiversity loss for the functioning of natural ecosystems. Using a global survey of reef fish assemblages, we show that in contrast to previous theoretical and experimental studies, ecosystem functioning (as measured by standing biomass) scales in a non-saturating manner with biodiversity (as measured by species and functional richness) in this ecosystem. Our field study also shows a significant and negative interaction between human population density and biodiversity on ecosystem functioning (i.e., for the same human density there were larger reductions in standing biomass at more diverse reefs). Human effects were found to be related to fishing, coastal development, and land use stressors, and currently affect over 75% of the world's coral reefs. Our results indicate that the consequences of biodiversity loss in coral reefs have been considerably underestimated based on existing knowledge and that reef fish assemblages, particularly the most diverse, are greatly vulnerable to the expansion and intensity of anthropogenic stressors in coastal areas. The increasing intensity of human disturbance worldwide is triggering unprecedented biodiversity losses, which is raising concerns over whether ecosystems will work and continue delivering goods and services to humanity. In contrast to previous experimental studies, which describe saturating relationships between ecosystem functioning and biodiversity, we show that in reef fish systems, functioning (as standing biomass) accelerates with the addition of new species. This non-saturating relationship implies unique contributions of species to the functioning of this ecosystem and indicates that the consequences of losing biodiversity are significantly greater than previously anticipated. We also demonstrate a negative effect of human density on reef fish functioning such that for the same number of people the loss of standing biomass is significantly larger in more diverse ecosystems. Unfortunately, human effects can arise through multiple stressors (such as fishing, coastal development, and land use) and are widespread and likely to worsen, as some 75% of the world's coral reefs are currently nearby human settlements and because almost all countries with coral reefs are expected to double their populations within the next 50 to 100 years. Our results call for both further investigation of the impact of biodiversity on ecosystem functioning and strategies to manage and prevent the increasing intensity and expansion of anthropogenic stressors in coastal areas.

The state of Oaxaca in southern Mexico harbors the highest faunistic and floristic biodiversity of the region; however, to date, research effort has focused on terrestrial flora and fauna. Despite this bias, there is a large amount of dispersed information regarding the coastal and marine biodiversity of Oaxaca. The present study aimed to: 1) update and synthesize existing information to improve understanding of coastal and marine biodiversity; and 2) provide a baseline for future biodiversity studies in Oaxaca. The review of 198 references about Oaxaca's flora and fauna produced 2, 157 species records, from 15 taxonomic groups (Division, Phyla or Class). Nevertheless, knowledge about the fauna and flora of Oaxaca remains scarce compared to other regions, such as the Gulf of California. Additionally, floristic and faunistic knowledge is heterogeneous among the taxonomic groups; for instance, 242 (11 .2 °/o) species represent macroalgae, three invertebrate groups (annelids, crustaceans and mollusks) represent about 44.0% (949 species) of all species recorded, while the vertebrates are represented by 762 species (35.3°/o). Similarly, many invertebrate groups recorded in adjacent regions have not yet been recorded on the Oaxaca coast, including some platyhelminthes, rotifers, nematodes, oligochaetes, sipunculids, echiurans, tardigrades, pycnogonids, some crustaceans, brachiopods, chaetognaths, ascidians and cephalochordates. The study of the marine flora and fauna is far from finished; additional effort is required to complete the marine biodiversity inventory of Oaxaca.

Global warming and ocean acidification affect coral calcification. Nevertheless, there is not enough information regarding the growth parameters of the main reef-building coral species in marginal growth areas such as the Pacific coast of southern Mexico. In order to fill this gap, coral growth parameters of 8 hermatypic coral species (massive species: Porites panamensis, Porites lobata, Pavona gigantea, and Pavona varians; branching species: Pocillopora meandrina, Pocillopora damicornis, Pocillopora verrucosa, and Pocillopora capitata) were estimated in 2 areas of the southern Mexican Pacific. Branching coral species had a higher calcification rate (2.99–5.23 g CaCO3 cm–2 yr–1) than massive species (0.34–1.13 g CaCO3 cm–2 yr–1). A significant relation between sea surface temperature (SST) and skeletal density was observed in all massive coral species. Also, 2 massive species (P. gigantea and P. varians) showed a significant relation between SST and calcification rate. Upwelling in the Gulf of Tehuantepec transports deep water with low pH and low aragonite saturation, and may be affecting the calcification rate of stony corals in the studied area.

In May 2023, oceanic and atmospheric anomalies indicated El Niño conditions in the eastern Pacific, followed by coral bleaching in coral communities and reefs of Huatulco. We conducted surveys and sampled coral reef communities from late June to mid–August of 2023 to evaluate the intensity and extent of the changes associated with the warming event. From January of 2023, Huatulco experienced positive sea surface temperature (SST) anomalies; however, beginning in June, the high-temperature anomalies became extreme (>31 °C; ~2 °C above historical records). These high temperatures resulted in extensive coral bleaching in middle–late June and mortality from middle–late July (>50–93%). In addition, the area experienced significant reductions in echinoderm abundance and fish biomass. In 2023, severe bleaching affected coral systems in the Central Mexican Pacific, Gulf of Mexico, and Mexican Caribbean, making this the most devastating marine heatwave event, simultaneously impacting coral reefs across Mexico’s Pacific and Atlantic coasts.

This investigation reports, for the first time, results of CO₂ system variables in the Gulf of Tehuantepec, located in the Mexican tropical Pacific. We quantified the post‐Tehuano concentration of dissolved inorganic carbon (DIC) and pH (April 2013). These values were used to calculate pCO2, aragonite saturation (Ω_Ar), and air‐sea CO₂ fluxes (FCO₂). The intense vertical stratification was found to contribute to the biogeochemical processes in surface waters (<70 m). However, in post‐Tehuano conditions, high pCO2 (∼1000 µatm) and DIC concentrations (2200 µmol kg⁻¹), as well as low Ω_Ar (∼1.1) and pH (∼7.5), remain in surface waters for a few days after Tehuano winds have weakened. We identified four oceanographic areas: (a) a highly mixed region due to previous Tehuano events; (b) coastal upwelling in the western region; (c) mesoscale eddies; (d) a poleward surface coastal current. The first three promoted the influence of Subtropical Subsurface Water in the chemistry of surface waters, whereas the coastal current contributed to the horizontal advection of DIC. The calculated CO₂ fluxes ranged from −2.3 mmol m⁻² d⁻¹ in areas with stratified waters to over 25 mmol m⁻² d⁻¹ for mixed areas. Positive values indicate an ocean‐to‐atmosphere flux. Our findings suggest that the Gulf of Tehuantepec is a major source of CO₂ into the atmosphere.