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.

PRESENT LEVELS
In 2001 the average phytoplankton biomass varied from 35 to 405 µg C l^-1 at the 17 Danish stations sampled in 2001 and with a long-term sampling record of more than five years. In 12 out of 17 areas the average phytoplankton biomasses in 2001 were lower (8–61%) than the long-term averages (Table 2.5).

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.
Because diatoms dominate the spring bloom, of which a large fraction sediment to the bottom, they are important for the transfer of organic matter of pelagic origin to the benthic fauna. In addition, the input of organic matter to the water mass below the pycnocline will affect the subsequent development in bottom water oxygen conditions.

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
Several potentially toxic and harmful species of algae are common minor components of the phytoplankton communities in Danish waters. In some years, however, they form conspicuous blooms that may have severe effects on other organisms or on the tourist and fishing industries (Table 2.6). Blooms of some species, e.g. Chrysochromulina polylepis, have occurred only once, while blooms of other species are recurring, e.g. Karenia mikimotoi and Chattonella sp. The latter was found in high concentrations for the first time in 1998.

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.

Table 2.5
Year 2001 average phytoplankton biomasses and biomasses relative to long–term averages for the given time periods at all Danish stations with quantitative phytoplankton sampling and a history of more than five years of sampling. Figures show percentage increase or decrease in biomass relative to long-term average.

Geographical area	Period	2001 biomass µg C l-1	2001 relative to long- term average
Sønderho, east	1990–2000	147	37
Ringkøbing Fjord	1989–2000	45	8
Nissum Bredning	1985–2000	117	25
Løgstør Bredning	1985–2000	133	4
Skive Fjord	1985–2000	255	10
Ålborg Bugt	1989–2000	67	43
Hevring Bugt	1989–2000	147	15
Århus Bugt	1989–2000	113	10
Mariager Fjord	1989–1996	230	25
Horsens Fjord	1989–2000	138	13
Vejle Fjord	1989–2000	176	14
Kolding Fjord	1989–2000	405	10
Little Belt, northern part	1989–2000	112	20
Gniben	1979–2000	41	61
Little Belt, southern part	1989–2000	121	14
Roskilde Fjord	1992–2000	44	40
The Sound, northern part	1979–2000	35	31

Table 2.6
Blooms of potentially toxic phytoplankton species or exceptional blooms in Danish waters. Years with major blooms are given, and the most significant ones marked in bold. Months indicate the most frequent time of year for blooms. Maximum abundance and effects refer to the most extreme episodes registered.
1 = Gyrodinium aureolum
2 = Maximum biomass given was total biomass of all cyanobacteria including Aphanizomenon sp. and Anabaena sp. registered in the national monitoring programme. Locally, much higher biomasses may have occured in accumulations along the shore.

Species	Year	Month(s)	Geo-graphical area	Maximum abun-dance or biomass	Effects
Chattonella spp.	1998, 2000, 2001	Jan-May	Århus Bugt, Danish west coast, Skagerrak, Kattegat	23 mill. l-1	Fish kills
Chryso-chromulina spp.	1988, 1992	May	Kattegat, Belt Sea	100 mill. l-1	Fish kills, dead benthic inverte-brates, macrophytes affected
Dictyocha speculum	1983, 1999	May-Jun	Little Belt, Alssund	25 mill. l-1	Fish kills in maricultures associated with bloom
Gymno-dinium chloro-phorum	1999	Aug-Dec	Nissum Bredning, W Kattegat ,Skagerrak	15 mill. l-1	None reported
Karenia mikimotoi 1	1968, 1981, 1997	Aug-Oct	Danish west coast, Kattegat, Belt Sea	8.6 mill. l-1	Dead benthic inverte-brates
Nodularia spumigena 2	1975, 1976, 1992, 1994, 1995, 1997, 1999, 2001, 2002, 2003	Jul-Aug	Århus Bugt, W Kattegat, Belt Sea, W Baltic	869 µg C l-1	Dogs dying after swimming in water
Phaeocystis sp.	Yearly 2000	May-Jun	Danish west coast		Foam on beaches
Proro-centrum minimum	1983, 1984, 1987, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1999	Jul-Oct	Most Danish estuaries and coastal areas	53 mill. l-1	None reported
Pseudo-nitzschia spp.	1992, 1999, 2000	May-Nov	Kattegat, Belt Sea SW Kattegat Danish west coast	346 µg C l-1	None reported

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