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 1.2.15 HARMFUL ALGAL BLOOMS AND THE FACTORS CONTROLLING
ALGAL GROWTH
1.2.15.1 Introduction
There has been increasing interest in the occurrence of nuisance and harmful algal
growth in freshwater environments during the decade ending 2000. This follows a long interest
in marine systems related to red tides and shellfish toxicity. Today numerous sites of impairment
have been identified in freshwater systems as well as numerous taxonomic groups of harmful
algae and numerous toxins that can be produced. The circumstances leading to the excessive
growth of algae and plants are understood with some confidence but the factors leading to the
growth of specific types of algae are still being investigated. There is very little known about the
factors leading to the production of toxins by such algal growths. The purpose of this section is
to summarize the body of knowledge associated with this topic as well as the factors
contributing to algal growth and potential control measures.
Excessive growth of these plants in an aquatic environment is often termed a "bloom" or
an "algal bloom". Some authors have also referred to them as "water blooms". Such blooms are
subjectively identified when growth has proceeded to an extent that the presence of the vast
numbers of these cells is obvious to unaided vision. However, these blooms can also occur at
depths that make them invisible, or at concentrations that do not make them visible but
nevertheless have an adverse impact on the resource. Blooms can be harmless, or they can be a
nuisance because of esthetics or smell. At their worst, a bloom can eventually consume
dissolved oxygen during decomposition or produce powerful toxins dangerous to wildlife and
humans. A harmful algal bloom (HAB) is usually associated with adverse effects on water
quality including anoxia and toxin production. Anderson and Garrison (1997) provide an
extended general description of what constitutes a HAB. "HABs are 'blooms' of microscopic
and macroscopic algae that cause harm in a multitude of ways." The entire volume associated
with their reference is devoted to HABs and is an excellent place to begin a study of HABs
(Limnology and Oceanography 42[5, part 2] 1009-1305).
Most of the contemporary interest in HABs is directed toward certain well-known algal
types. The following text is intended to provide a brief acquaintance with the common algae and
the specific ones often responsible for HABs. For additional motivation to read the introductory
sections, the reader is directed to the later section on the history of algal toxic poisonings.
1.2.15.2 Background
What do the algae include? The algae technically include all single-celled plant organisms. In
practice their highly variable growth form separates certain macroscopic algae (the kelps and
other giant algae, for example) from the microscopic forms that compose the large number of
species in many aquatic environments. The microscopic forms can be found living as single cells
such as many diatoms like Navicula spp., green algae such as Cosmarium spp. or
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