An Introduction to Harmful Algae

Abstract

In the open sea, primary production is almost totally based on photosynthesisby pelagic unicellular or colonial microalgae, collectively known as phytoplankton.Benthic algae are important primary producers only in extremelyshallow water where sunlight sufficient for photosynthesis penetrates to thebottom. Thus, phytoplankton are the basis of aquatic food chains.Tens to hundreds of species of phytoplankton belonging to different taxonomicunits usually coexist in natural assemblages.Phytoplankton are microscopic,ranging in size from less than 1 μm to more than 100 μm, with generationtimes of no more than a few days. Thus, phytoplankton populationsexhibit large temporal variations in response to abiotic factors such as light,temperature, nutrients, and water movement, and biotic factors such as grazing,competition, parasitism, and microbial attack. Normally, the standingcrop of phytoplankton remains low because these loss factors generally balancerapid intrinsic rates of growth through cell division.Occasionally, increases in one or a few species can overcome losses, suchthat a given species can dominate phytoplankton assemblages and causeblooms lasting for several weeks or more. Such blooms are due to combinationsof favorable phytoplankton growth, increased physical concentration byhydrographic or meteorological processes, and reduced losses due to factorssuch as viruses, sedimentation, and grazing.Some phytoplankton blooms can cause adverse effects. These include oxygendepletion, reduced water quality aesthetics, clogging of fish gills, or toxicity.Blooms of such Harmful Algae (HA) cause Harmful Algal Blooms (HABs).Of the approximately 5,000 known species of phytoplankton, only some 300species form HABs that are deleterious to aquatic ecosystems in one way oranother, and only about 80 of these species are known to be toxin producers.Some phytoplankton toxins can be accumulated and/or transported in foodchains to higher trophic levels where they contaminate shellfish,making themunsuitable for human consumption, or poison upper-level consumers, includingfish, seabirds, marine mammals, and humans. The economic effects ofsuch blooms, including losses to fisheries, tourism, monitoring, and healthcare can be substantial. In Europe, such losses annually approach 862 millionEuros, and in the USA 82 million dollars (see Chap. 30).Harmful algae have been the subjects of scientific and societal interest forcenturies.Because blooms of toxic dinoflagellates were known to occasionallydiscolor water red or brownish-red, they were, and still are, known as ``redtides.''Water discoloration was noted for the lower Nile in the Bible, and Darwinmade microscopic observations of discolored water during the voyage ofthe HMS Beagle. However, the frequency of HABs, and the locations affectedmay be increasing worldwide. In recent years, increases in the numbers of HAspecies able to produce toxins have been detected, and new toxins continue tobe chemically characterized.It is often assumed that phytoplankton toxins evolved to deter their zooplanktongrazers. However, most phytoplankton species, including manytoxin producers, appear to be routinely grazed by many zooplankters in naturalmixed phytoplankton assemblages. Other HA toxins appear to beinvolved primarily in allelopathy, being released in the dissolved state into seawater and causing deleterious effects on other competitor phytoplanktonspecies. Some HA toxins may be secondary metabolites that are only coincidentallytoxic. Thus, the role of phytoplankton toxins in the ecology of thealgae that produce them remains unclear.HA toxin levels can vary depending on concentrations of nutrients in thewater such as nitrogen and phosphorus. In some cases, HA intracellular toxinlevels increase in cells grown under unbalanced nutrient conditions. This maybe because toxins are the molecules that algal cells use to store or retainsparse nutrients, or because cells under nutrient stress transfer nitrogen fromchlorophyll molecules to toxin molecules, causing reductions in rates of celldivision but building up toxin levels in the remaining non-dividing cells.Alternatively, some HA species may produce higher amounts of toxins undernutrient-stressed conditions, thereby more effectively reducing losses to grazersand/or by releasing greater amounts of allelochemical substances to neutralizeco-occurring phytoplankton species that are competitors for sparsenutrients.Unbalanced nitrogen and phosphorus conditions are often recurrent incoastal waters due to increased anthropogenic discharges of a given nutrient,relative to others. Thus, it is possible that even if HABs have not increased inoccurrence, the deleterious effects of these blooms may have increased theirimpacts due to increased toxicity due to unbalanced or anthropogenicallyaltered nutrient ratios.Despite increased research activity, the last major organized publishedsynthesis of HA ecology was the volume originating from the NATO workshopon harmful algal blooms held in Bermuda in 1996 (Physiological Ecologyof Harmful Algal Blooms; Springer-Verlag, 1998).Although the reviews inVI Prefacethe volume from the Bermuda meeting were excellent and comprehensive forthe time, they are now almost a decade old and somewhat dated by recentdevelopments. Accordingly, we were approached by Springer-Verlag with arequest to compile an updated synthesis of HA ecology, organized primarilyaround processes and questions, rather than organisms. Thus, we invited aglobal assemblage of active HA researchers to contribute to the chapters inthis volume, and many of these same specialists had also contributed to theprevious Bermuda meeting volume. All chapters in this volume were peerreviewed.We hope that this volume will complement other recent reviews andsyntheses in Harmful Algae and other journals, and in international HA meetingvolumes to identify gaps in our present understanding of HA ecology andsuggest areas for additional research.