NUTRIENTS - Marine Pollution: What Everyone Needs to Know - page 31

are improving as a result of environmental legislation in the
1970s. An interactive map is available at http://www.wri.org/
project/eutrophication/map .
What are Harmful Algal Blooms (HABs)?
Some phytoplankton species, usually dinoflagellates, pro-
duce toxins that can impair respiratory, nervous, and other
functions and even cause the death of fish, shellfish, sea-
birds, and mammals. HABs have been called red tides or
brown tides because of water discoloration when they occur,
though many kinds of harmful algae do not discolor the
water. Their economic impacts can be severe if shellfish
harvest and fishing are closed. Reports of HABs have been
increasing worldwide, and some correlations have been
shown with N inputs. There are examples from around the
world where increases in nutrient loading have been linked
to the development of large blooms with toxic effects. Not
only has the frequency of reports of HABs been increasing,
new toxin-producing species have been discovered to cause
problems. However, attempts to relate trends in HABs to
nutrients are difficult because of variability in geographical
regions and over time. They are also complicated by other
factors, including increased monitoring and reporting and
the influence of climate change. Evidence of a link in one
region should not be considered evidence of a general link-
age of HABs to nutrients everywhere. Possible causes for
the apparent expansion of HABs include natural dispersal
of species by currents and storms and dispersal through
human activities such as shipping, shellfish translocation,
and eutrophication. There are also more scientists out there
looking for HABs. Some aspects of the global expansion of
HABs could also be due to improved detection of HABs in
places where toxic species have always been present and
were not stimulated by the human activities.