Harmful algal blooms may occur when phytoplankton amass in a restricted area but also include blooms where certain toxic species constitute just a small part of the total phytoplankton. Whilst most species of the microorganisms involved are benign, several produce toxins given the right conditions, and both the high density of a bloom and its potential toxicity can make the surrounding area hazardous to marine life.
Blooms can trigger a cascading effect through marine ecosystems whereby those which house poisonous species (not all HABs are toxic) cause toxins to be accumulated through the food web to affect seabirds, marine mammals, and sometimes humans. Additionally, a dense build-up of algae may damage gills of fish and affect organisms that feed by filtering water. Dense blooms may sink to the sea floor resulting in low oxygen conditions or anoxia (total oxygen depletion) when the bloom is decomposed, leading to the mortality of fish and other creatures. Since farmed fish often are enclosed in aquaculture pens they may also be particularly vulnerable, as they cannot swim away from a bloom.
As a global issue that can lead to significant economic, societal, and human health problems, studying HABs is of importance to marine science and ICES. Increasing incidences of HABs worldwide, and the knock-on effects for ecosystems, have necessitated the expansion of the relevant science and research. The work of ICES Working Group on Harmful Algal Blooms Dynamics (WGHABD), whose scope is global, is therefore of increasing value.
According to the latest WGHABD report, harmful algal blooms are being observed in new geographical regions – the result of changing environmental and oceanographic conditions related to climate change, such as warming temperatures as well as through the spreading of organisms via ballast water as a result of shipping.
“In the seas surrounding Sweden we get requests about HABs more or less year around with a peak in summer. The frequencies of HABs are quite variable, some years almost none are reported while other years are bloom years” explains WGHABD chair Bengt Karlson. “In spring it may be fish killing algae or algae that produce toxins that accumulate in shellfish, mainly in the Kattegat and the Skagerrak. In summer cyanobacteria is the main problem in the Baltic Sea. In autumn algae that produce toxins that accumulate in shellfish are common in the Kattegat and the Skagerrak. Sometimes there are surface accumulations of algae that are non-harmful but a nuisance during autumn. Coastal cyanobacteria blooms are fairly common in the Baltic proper and in the Gulf of Bothnia in autumn.”
“HABs include many different organisms. They behave differently. Some cyanobacteria (blue green algae) have resting stages that overwinters on the seafloor or in the water column. Most cyanobacteria disintegrate and nutrients are recycled. Many dinoflagellates that produce biotoxins that may accumulate in shellfish, have resting stages that rest on the sea floor. Other harmful algae simply overwinter in small numbers in the water column.”
The Baltic Sea, an area which has been under particular focus in terms of HABs and related eutrophication (where man-made nutrient run-off causes algal build-up) experienced a large harmful algal bloom in summer 2014. The organisms involved were cyanobacteria – for example. Nodularia spumigena (toxic) and Aphanizomenon and Dolichospermum spp. (non toxic).
“The extent, intensity, and duration are probably the largest since the years 2005 and 2006,” adds Karlson. “Whether 2014 is a record year is not known. Also, for example, 1999 was a large bloom year. SMHI (Swedish Meteorological and Hydrological Institute) will summarize the results from this summer in early September.”
“It is unclear how much the summer blooms contribute to the total eutrophication of the Baltic Sea. It may be the case that the primary production year around based on anthropogenic inputs of nutrients have a larger impact.”
Theme Session H at this year’s ICES Annual Science Conference ‘Harmful Algal Blooms in Aquaculture and Fisheries ecosystems: prediction and societal effects’ will take place on the Thursday and Friday (18-19 September) and will include presentations on HAB monitoring tools and methodologies, environmental drivers and bloom characteristics, forecasting alert systems, and user-driven remote sensing information services in aquaculture.
Algal blooms near the islands of Bornhom, July 2014; © Swedish Coast Guard