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Harmful Algal Blooms (HABs) are becoming an increasing problem to human health and environment (including effects on natural and cultured resources, tourism and ecosystems) all over the world. In Mexico a number of human fatalities and important economic losses have occurred in the last 30 years because of these events. There are about 70 species of planktonic and non-planktonic microalgae considered harmful in Mexican coasts. The most important toxin-producing species are the dinoflagellates Gymnodinium catenatum and Pyrodinium bahamense var. compressum, in the Mexican Pacific, and Karenia brevis in the Gulf of Mexico, and consequently the poisonings documented in Mexico are Paralytic Shellfish Poisoning (PSP) and Neurotoxic Shellfish Poisoning (NSP). Although there is evidence that Amnesic Shellfish Poisoning (ASP), Diarrhetic Shellfish Poisoning (DSP) and Ciguatera Fish Poisoning (CFP) also occur in Mexico, these problems are reported less frequently. The type of phytoplankton and epiphytic microalgae, their toxins and harmful effects as well as current methodology used to study these phenomena are presented in this paper. As an experienced group of workers, we include descriptions of monitoring and mitigation programs, our proposals for collaborative projects and perspectives on future research.

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Effect of tidal dynamics on a planktonic community in a coastal lagoon of Baja California, MexicoLuis Alberto ~o r a l e s -~a m o r a n o ' , Ramon Cajal-Medranol, Elizabeth ~r e l l a n a -c e p e d a l , Luis C. ~imenez-perez2 ABSTRACT Flfty-two water samples were collected from the surface at the entrance of a coastal lagoon for evaluation of microheterotrophic processes. The study was carried out during a penod of neap and spring tides over 14 consecutive days. Bacteria, microphytoplankton and zooplankton direct counts were made, along with fractionated respiratory activity. Nutrient, C/N ratio, total organic carbon, particulate organic nitrogen, carbon, seston, dissolved oxygen, particulate silica and chlorophyll a determinations were also made. Phytoplankton biomass was low (9.5 11g C I-') and was exceeded by bacterial biomass (89.4 11g C I-') at both spring and neap tides. Bacterial abundance was independent of tidal height and increased with intense water movement at spring tides. Autotrophic production was limited by light penetration and nutrient availability. Most of the respiratory activity in the water column (> 50 %) occurred in the 5 0.8 1tn1 size fraction. Bacterial standing stocks sustained carbon production in the water column Significant negative correlation between bacteria and nanoplankton suggests a trophic structure dependent on dissolved organic carbon inputs from marsh flora, followed by a trophic chaln composed of bacteria, nanoplankton, microzooplankton and Acartia sp. It is suggested that food chains stemming from dissolved organic carbon instead of phytoplankton may play a major role in the system since tidal dynamics enhanced bacter~al standing stocks during spring and neap tides.

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