Phytoplankton and harmful algal blooms
Phytoplankton are the base of pelagic food webs in aquatic systems. In addition, sedimentation of phytoplankton provides an essential nutritional input to the benthic fauna. With generation times ranging from <1 day to a few days phytoplankton respond rapidly to changes in nutrient concentrations. Therefore, phytoplankton have been included in the Danish monitoring programme since 1979 as an indicator of the eutrophication status. Phytoplankton are quantified as carbon biomass determined from microscopy or indirectly as the concentration of chlorophyll a, a pigment found in all autotrophic phytoplankton organisms.
No data exists on phytoplankton biomass or chlorophyll a concentrations under pristine conditions. Therefore present levels are generally compared with background concentrations in offshore areas. For Danish waters the offshore Skagerrak chlorophyll a concentration of 1.25 µg l-1 can be applied. However, this concentration represents the maximum concentrations for the growing season (spring-late summer) (OSPAR 2001). The OSPAR assessment criteria are not operational, and the Danish assessment has therefore been based on mean chlorophyll a concentrations in March-October.
In 1999 and 2000 diatoms generally dominated the phytoplankton carbon biomass with important but somewhat smaller contributions from dinoflagellates (especially in 1999) and other organisms (Figure 2.19). This pattern of dominance has, however, changed over time, in particular in open sea areas. Here diatoms accounted for <20% up to 40% of the total biomass from 1979 until 1998 while dinoflagellate contribution to biomass increased from <23% during 1979 to 1985 to 28–65% after 1986. The increasing importance of dinoflagellates has been accompanied by reduced contributions from other groups, mainly nanoflagellates.
In estuaries and coastal waters diatoms prominently dominated the phytoplankton
(35–78%) since sampling was initiated in 1989 (Figure
2.19). The contributions from dinoflagellates have varied between
16% and 46% and the importance of other groups, mainly nanoflagellates,
increased from 1989 to 1998.
The biomass of diatoms has decreased significantly over the past 20 years in the open Kattegat and Belt Sea. During 1995–2000 the average biomass of diatoms in these areas was 50% of the biomass in the early 1980s. In estuaries and coastal areas no long-term trend in diatom biomass was found (Figure 2.20).
Primary production in the open sea areas has shown an overall decline with major year-to-year variations from 1977 to 1997 (Figure 2.21). The subsequent increase in primary production during 1998–2001 may be due to a reduction in the number of monitoring stations and changes in the sampling strategy in 1998. During 1993-2001 primary production was significantly correlated with runoff, wind and temperature. The primary production index adjusted to changes in climatic conditions shows a lower production during the 1990s than during the 1980s. Despite the apparent decrease in primary production from the 1980s to the 1990s, the annual primary production in the Kattegat area has increased 2- to 3-fold from the 1950s to 1984–1993, apparently as a result of eutrophication (Richardson & Heilmann 1995).
In estuaries and coastal areas primary production has decreased from 1980 to 1997 and subsequently increased during 1998-2001 (Figure 2.22). For the period 1993-2001 primary production was significantly correlated with runoff, irradiance and temperature. Index values adjusted for variations in climatic conditions showed a very consistent decline after 1993. This decline in primary production was presumably due to reduced phosphorus loading to the estuaries through the establishment of sewage treatment plants in the late 1980s and early 1990s and subsequent reduction in the nitrogen load both from point and diffuse sources.
Chlorophyll a concentrations have decreased in the open sea areas since the 1980s (Figure 2.21). However, mainly during the 1980s the year-to-year variations have been substantial and possibly related to the lack of standardised sampling strategies and methods for analyses prior to 1989. For the period 1990-2001 chlorophyll a concentrations correlated significantly with primarily irradiance during early spring and autumn. Chlorophyll a index values adjusted for variations in climatic conditions were very variable prior to 1990 and showed a general slight in-crease for the period 1990-2001. Since 1980 the concentrations of chlorophyll a in Danish open sea areas (Hansen et al. 2000) have been >50% above background concentrations (1.25 µg chl a l-1) given by OSPAR (2001).
Chlorophyll a concentrations have decreased in estuaries and coastal waters since the mid 1980s (Figure 2.22). The concentrations of chlorophyll a correlated significantly with runoff during 1993–2001. When adjusted for variations in climatic conditions, chlorophyll a index values showed a very consistent decline from 1993 to 2001.
In open sea areas the Secchi depth, a measure of water transparency, has increased since the mid-1980s (Figure 2.21). The Secchi depth adjusted for variations in climatic conditions has increased despite a similar increase in chlorophyll concentration. In estuaries and coastal areas the decline in chlorophyll concentrations since the mid-1980s has been accompanied by in-creased Secchi depth (Figure 2.22).
EXCEPTIONAL AND HARMFUL ALGAL BLOOMS
Potentially harmful species are registered and quantified in the national
monitoring programme. In addition, the commercial bivalves fishermen and
the mussel industry are undertaking monitoring of toxic phytoplankton
and algal toxins in bivalve shellfish in all areas where shellfish are
harvested. Areas may be closed for fishing of shellfish if toxic algae
are found in concentrations above given limits (Bjergskov
et al. 2001) or if algal toxins are detected in shellfish in concentrations
above the limit for human consumption. Most registrations of shellfish
containing algal toxins in concentrations above limits are from the 1980s
and the early 1990s and from the eastern coast of Jutland (Figure
2.23) where the fished shellfish amounts to only one third of the
catches in the Limfjorden in northern Jutland.
Danish Environmental Protection Agency & National Environmental Research Institute • updated: