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Coastal areas are of particular ecological and economic importance as they provide direct and indirect goods and services to society, both biological indicators and water quality assessment are of importance when monitoring these environments. Hydrographical parameters, nutrients, chlorophyll-a, and macroalgae biomass were measured, and the TRIX index was obtained, at three sites of La Paz Bay subjected to different types of anthropogenic influence: San Juan de la Costa (SJC), Casa del Marino (CM), and El Tecolote (TE), during two months of 2017 and four months of 2018. Also, macroalgae blooms at these sites were monitored by sampling the biomass of key species. Temperature showed the typical seasonal variation, from 21 °C in winter to 28 °C in summer, while salinity showed little variation (35–37 psu). Chlorophyll-a varied in parallel with changes in water temperature (0.3–2.4 μ g L−1), while nutrients varied depending on the season and site. The TRIX index showed significant differences between seasons, with the highest values in the warmer months (6.0) and the lowest in November (2.7); the sites studied ranged from mesotrophic to oligotrophic. Four macroalgae species accounted for over 60% of the biomass during the sampling period: Ulva ohnoi (16.1 kg), Spyridia filamentosa (6.5 kg), Acanthophora spicifera (4.2 kg), and Ulva sp. 1 (2.7 kg). Ulva, Gracilaria, and Hypnea species showed a positive significant correlation with nutrients and the TRIX index. A high macroalgae biomass may be related to a mesotrophic state, showing its value for determining the health status of coastal ecosystems and issuing proper management recommendations.

Ulvacean blooms are generally characterized by one or more Ulva species, some of which are common to blooms across a broad geographic range. In tropical environments the identified stimuli that induce reproductive development of Ulva are restricted to temperature, salinity, dehydration and fragmentation. Culture studies have been prolific in describing the reproduction of Ulva species, but its reproductive changes in natural environment have not been described yet. Hence, seasonal changes were described in reproductive stages and their relationship with biomass and environmental factors of Ulva species at four macroalgal blooms in a subtropical bay. Eight Ulva species were found: U. acanthophora, U. clathrata, U. flexuosa, U. intestinalis, U. lactuca, U. lobata, U. nematoidea and U. rigida. Reproductive stage and biomass varied according to site and season. Five species showed four reproductive stages (vegetative, thallus with fully differentiated zooids in formation and empty cells after zooids release); for the remaining species only vegetative thalli were found. Ulva rigida showed the highest biomass values, followed by U. acanthophora, following a seasonal pattern.

The Gulf of California (GC) is an unique large ecosystem characterized by its rich biodiversity, high biological productivity and endemism of marine life. However, as many other large ecosystems worldwide, it is subject to diverse anthropogenic pressures (overfishing, climate change, losses of biodiversity and habitats, and pollution). We reviewed over 150 studies dealing with contaminants in water, sediments and representative organisms from the GC, and here we discuss the main issues associated to the presence of metals, metalloids, persistent organic pollutants (POPs, including polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), brominated diphenyl ethers (deca tri-a) (PBDEs), and several other pesticides), plastics, nutrients and algal blooms. The GC ecosystems have been subject to a wide range of pollution sources. Nevertheless, the pollution levels remain relatively low to moderate depending on the location and contaminant type. Contamination hotspots are found i) for metals and metalloids, in sites where mining spills have occurred and ii) for nutrients and pesticides, in wetlands that receive discharges from intensive agricultural and shrimp farming. We also identified sites where harmful algal blooms (HABs) have been observed. However, numerous coastal environments in GC, affected by pollution sources and events have yet been poorly studied. More detailed, extensive and comprehensive studies on the pollution levels and trends, transfer and toxic effects are still needed.

In La Paz Bay conspicuous macroalgal blooms of Ulva spp. are recurrent in the waterfront of the city; studies relate its growth to seasonality and nutrient enrichment but their relative influence is not known. We use the δ15N to discern amongst nitrogen sources at three sites with different substratum and anthropogenic activities. Ulva blooms were monitored monthly at San Juan de la Costa (SJC), Casa del Marino (CM) and El Tecolote (TE). Species presence varied between sites and months. At SJC Ulva displayed the highest signal of δ15N associated with fishing products and local mining wastewaters (19.5‰). CM showed isotopic values related to sewage waters (13‰). The δ15N of species at TE were the lowest. We found higher isotopic signatures than in other tropical/subtropical regions, associated with nutrient rich water masses. Ulva species can be used to trace and discern amongst different sources of nitrogen from natural or anthropogenic sources.

The Gulf of California (GC) is a unique large marine ecosystem characterized by its rich biodiversity, high biological productivity and endemism of marine life, which are the basis of Mexico's most important fisheries. However, as many other large ecosystems worldwide, it is subject to diverse anthropogenic pressures (overfishing, losses of biodiversity and habitats, pollution, climate variability and climate change (CC)). We reviewed over 180 studies dealing with the impacts of CC and climate variability in the GC region. Main issues on the impact of ENSO events, acidification, sea level rise, and water availability are discussed for the GC in a global context. Impacts of ENSO events on the GC are geographically heterogeneous, upwelling tends to be suppressed, and this in turn influences the distribution and abundance of marine mammals, fishes, shrimps, benthic species, seaweeds and phytoplankton. ENSO events strongly impact small pelagic fishes, key components of the GC ecosystem, thus affecting fisheries in general as well as non-harvested species such as marine mammals and sea birds. Probably the most well-known phenomenon associated to ENSO impact in the GC is coral bleaching, a process by which corals expel most of their microalgal endosymbionts to become pale or white (bleached). Beside shifts in the distributional limits of benthic organisms, invasions of alien species are also occuring. Harmful algal blooms (HABs) are common and recurrent in the GC. Fourteen HAB-forming species have been recorded, with Gymnodinium catenatum being the most frequent. HABs are inhibited by El Nino and promoted by La Nina. During El Nino events the size and biomass of macroalgal blooms tend to decrease. Finally, several aspects that require further research are proposed.

Macroalgae blooms of Gracilaria vermiculophylla, Hypnea spinella and Spyridia filamentosa have been found in coastal lagoons in the SE Gulf of California. Agriculture, livestock, shrimp and poultry farms and sewage contribute anthropogenic nitrogen to the systems. The delta(15)N of these sources, water column and macroalgae were studied in order to identify the N supply for macroalgae blooms. delta(15)N of three species of macroalgae (4.3-13.6 per thousand) were enriched compared to the water column (delta(15)N-NO(3)(-) 3.7-6.8 per thousand), probably because of fractioning from the macroalgae. delta(15)N of POM (1.4-10.3 per thousand) was similar to the water column but the relationship was unclear. Depending on the site, macroalgae showed different delta(15)N values since some sites receive more or less influence from one given source of the associated watershed, which is reflected in the different delta(15)N values of the macroalgae of the same system and in the relative contributions of the sources.