HELCOM core indicator of eutrophication
Chlorophyll-a concentrations
The status of phytoplankton, measured by
chlorophyll a
Authors
Vivi Fleming-Lehtinen, Pirkko Kauppila, Hermanni Kaartokallio - Finnish Environment Institute (SYKE), Finland
© HELCOM 2010
www.helcom.fi
Page 1
HELCOM core indicator of eutrophication
Chlorophyll-a concentrations
Key message
The status of phytoplankton is poor or bad in the north eastern sub-areas where the concentrations of chlorophylla are high.
The status of phytoplankton is moderate in the south western sub-areas where the concentrations of chlorophyll-a
are slightly elevated.
The amount of phytoplankton in the water has decreased during the last decade in the Gulf of Riga, the Western
Gotland Basin, the Arkona Sea and the Kattegat
Figure 1. Ecological status for summer time phytoplankton concentration, measured as chlorophyll a, during the period
2003-2007. The interpolated map has been produced in three steps: 1) the status of coastal assessment units has been
interpolated along the shores, 2) the status of open sea basins have been interpolated and 3) the coastal and open
interpolations have been combined using a smoothing function. The larger circles indicate the status of open sea
assessment units and the smaller circles that of the coastal assessment units.
© HELCOM 2010
www.helcom.fi
Page 2
HELCOM core indicator of eutrophication
Chlorophyll-a concentrations
Policy relevance
Phytoplankton concentration in the summer is a direct proxy of eutrophication, and to a great extent a result of increased
loading.
Sub-regional assessment of phytoplankton status
Figure 2. Ecological status for summer-time phytoplankton concentration in the open sea, transitional and coastal subareas, measured as chlorophyll a, during the period 2003-2007.
Bothnian Bay
The open sea phytoplankton status is moderate. In the coastal and transitional waters the status varies from poor to
good.
The Quark
The phytoplankton status in the coastal and transitional Quark area is good.
Bothnian Sea
The phytoplankton status in the open sea is poor. In the coastal and transitional waters the status varies from bad to high.
Archipelago Sea
The phytoplankton status is poor or moderate.
Gulf of Finland
In the open sea the phytoplankton status is bad. In the coastal and transitional zone the status is poor or bad, except for
one sight in the Finnish coast of the eastern gulf (Pyötsaari) where the status is moderate.
Gulf of Riga
© HELCOM 2010
www.helcom.fi
Page 3
HELCOM core indicator of eutrophication
Chlorophyll-a concentrations
In the open sea the phytoplankton status is bad. In the coastal and transitional waters the status is poor or bad.
Northern Gotland Basin
In the open sea the phytoplankton status is bad. The status of coastal and transitional waters is mostly poor or bad, but
also sights with status from moderate to high occur.
Western Gotland Basin
In the open sea the phytoplankton status is poor. The status of coastal and transitional waters is mostly poor or bad, but
also sights with status from moderate to high occur.
Eastern Gotland Basin
In the open sea the phytoplankton status is bad. In the coastal and transitional waters the status varies from moderate to
bad.
Gulf of Gdansk
The phytoplankton status is poor or bad.
Bornholm Basin
In the open sea the phytoplankton status is moderate. In the southern coastal and transitional waters the status varies
from moderate to bad, and in the western-north-western coast from high to bad.
Arkona Basin
In the open sea the phytoplankton status is moderate. In the coastal and transitional waters the status varies from high to
moderate.
Kiel and Mecklenburg Bights
The phytoplankton status is moderate or poor.
Danish Straits
The phytoplankton status varies a great deal form high to bad.
The Kattegat
The phytoplankton status varies a great deal form high to bad.
© HELCOM 2010
www.helcom.fi
Page 4
HELCOM core indicator of eutrophication
Chlorophyll-a concentrations
Temporal development of phytoplankton concentration in the Baltic Sea since the
1970s
Figure 3. Phytoplankton concentration measured in June-September measured as chlorophyll a (µg/l) between
years 1974 and 2007 in the open-sea sub-regions of the Baltic Sea. Chlorophyll a observations are plotted against
the year of observation and the curves fitted with non-linear smoothing (green line) and shown with 95 %
confidence intervals (light green area). The level above the target value set by the HELCOM thematic assessment on
eutrophication (HELCOM 2009) is coloured light gray. The number of observations (n) is shown on each figure.
© HELCOM 2010
www.helcom.fi
Page 5
HELCOM core indicator of eutrophication
Chlorophyll-a concentrations
Assessment
An increase in summer phytoplankton concentration was evident in most of the Baltic sub-areas from the 1970s to
the present. Only in the south western areas, the Kattegat and Arkona Sea, was the increase virtually non-existant.
In the Bornholm Sea even a decrease in summer phytoplankton concentration could be observed. It should be
born in mind, however, that the impact of man-made loading was present already in the 1970s, and thus the
target value for phytoplankton concentration has not been reached even in these areas.
The increase in phytoplankton concentration was most pronounced in the northern areas: the Gulf of Finland, the
Northern Gotland Basin, the Bothnian Bay, the Bothnian Sea, the Eastern Gotland Basin and the Gulf of Riga. In
some of these areas, especially the Gulf of Riga, the concentration has turned to a decrease.
The increase in phytoplankton concentration is to a great extent a result of increased nutrient concentrations in
the water.
A conceptual model of the relation of phytoplankton biomass to eutrophication
Phytoplankton quantity is a direct trophy of eutrophication, through the increase of nutrient concentration. The
nutrient load is in some areas added by internal nutrient loading from the bottom, excellerated by oxygen
depletion. Phytoplankton increase in turn adds to the oxygen depletion, when sedimenting to the bottom, causing
a vicious circle of eutrophication. Biotic and abiotic changes, such as climate change or changes in herbivory, also
affect the phytoplankton quantity.
Data
Summer chlorophyll a concentration status 2003-2007 (Excel file)
© HELCOM 2010
www.helcom.fi
Page 6
HELCOM core indicator of eutrophication
Chlorophyll-a concentrations
Technical data
Data source: The HELCOM countries have provided average status concentrations for 2003-2007. The open sea
chlorophyll-a concentration measurements were made during research and monitoring cruises of the HELCOM
contracting parties. The data is kept at the database of the Finnish Environment Insitute SYKE, where the contact person
is Vivi Fleming-Lehtinen.
Description of data: The unit of the measurements is µg/l (microgrammes per liter).
Geographical coverage: All regions of the Baltic Sea.
Temporal coverage: From 1979 to 2007.
Methodology and frequency of data collection: Measurements have been made on irregular research cruises and during
monitoring cruises.
Methodology of data analyses: The status of the Baltic Sea according to the described indicator has been classified using
the multi-metric indicator-based HELCOM Eutrophication Assessment Tool (HEAT). Each area was assessed using
information on reference conditions (RefCon) and acceptable deviation from reference condition (AcDev) combined with
national monitoring data from the period 2003–2007. The basic assessment principle is RefCon ± AcDev = EutroQO, where
the latter is a "eutrophication quality objective" (or target) corresponding to the boundary between good and moderate
ecological status. When the actual status data (average for 2003-2007) exceeds the EutroQO or target, the areas in
question is regarded as affected by eutrophication.
The Ecological Quality Ratio (EQR) is a dimensionless measure of the observed value (AcStat) of an indicator compared
with the reference value (RefCon). The ratio is equal to 1.00 if actual status is better than or equal to reference conditions
and approaches 0.00 as deviation from reference conditions becomes large. The value of EQR is used to assign a quality
class to the observed status. The classes in descending order of quality are RefCon, High, Good, Moderate, Poor, Bad. The
central definition of the quality classes is given by the value of acceptable deviation (AcDev).
The RefCons and AcDev values for the chlorophyll-a concentration assessment were first defined by a group of national
experts from the HELCOM Contracting Parties for the HELCOM thematic assessment on eutrophication (HELCOM 2009).
The first assessment was based on identifying the status for the period 2001-2006, including data from coastal areas. This
assessment covers the period 2003-2007.
For a complete explanation of the methodology used, please see Andersen et al (2010) and thematic integrated
assessment on eutrophication of the Baltic Sea (HELCOM 2009).
The months of June, July, August and September were chosen to represent the period of abundant occurrence of
cyanobacteria.
Reference conditions for the open Baltic sub regions were determined by combining information obtained from the
means of the data collected between 1979 and 1980, through modeling work (Schernewsky & Neumann) and using
scientific judgement. In coastal and transitional sights also other statistical methods were used. The target value was set
at a 50% deviation from the reference conditions as calculated for the HELCOM integrated thematic assessment on
eutrophication of the Baltic Sea (HELCOM 2009).
The eutrophication status maps were produced by spatially interpolating the values for the areas listed in the data table.
ArcGIS 9.3.1 was used to interpolate the open and coastal areas. The coastal areas interpolation was delimited by a 6
nautical miles buffer along the coastline. The result was then joined to the open sea areas and the final map was
processed to add a smoother transition between coast and open sea areas.
Strength and weaknesses of data: Practically the method is unchanged, although the reliability of the laboratory analyses
may vary over time. The temporal and spatial coverage of the data is not even. In addition, timing of the measurements in
relation to the cyanobacterial biomass maximum may have an effect on the results, especially if the amount of data is
low.
Reliability, accuracy, robustness, uncertainty (at data level): Interpretation of the data ought to be done over long time
periods (minimum of five years).
Further work required (for data level and indicator level): The indicator will be updated annually with data collected
from as many temporal and spatial points as possible in each of the sub-areas.
© HELCOM 2010
www.helcom.fi
Page 7
HELCOM core indicator of eutrophication
Chlorophyll-a concentrations
References
Andersen, J.H., P. Axe, H. Backer, J. Carstensen, U. Claussen, V. Fleming-Lehtinen, M. Järvinen, H. Kaartokallio, S.
Knuuttila, S. Korpinen, M. Laamanen, E. Lysiak-Pastuszak, G. Martin, F. Møhlenberg, C. Murray, G. Nausch, A.
Norkko, & A. Villnäs. 2010. Getting the measure of eutrophication in the Baltic Sea: towards improved assessment
principles and methods. Biogeochemistry. DOI: 10.1007/s10533-010-9508-4.
Fleming-Lehtinen, V., Laamanen, M., Kuosa, H., Haahti, H. and Olsonen, R. 2008. Long-term development of inorganic
nutrients and chlorophyll a in the open northern Baltic Sea. Ambio 37:86-92.
HELCOM 2007. HELCOM Baltic Sea Action Plan. http://www.helcom.fi/BSAP/en_GB/intro/
HELCOM 2009. Eutrophication in the Baltic Sea. An integrated thematic assessment of the effects of nutrient
enrichment in the Baltic Sea region. Baltic Sea Environment Proceedings No. 115B.
Schernewski, G. and Neumann, T. 2005. The trophic state of the Baltic Sea a century ago: a model simulation study.
Journal of Marine Systems 53:109-124.
Suikkanen, S., Laamanen, M & Huttunen, M. 2007. Long-term changes in summer phytoplankton communities of the
open northern Baltic Sea. Estuarine, Coastal and Shelf Science 71:580-592.
Wasmund, N., Uhlig, S. 2003. Phytoplankton trends in the Baltic Sea. ICES Journal of Marine Science 60:177-186.
© HELCOM 2010
www.helcom.fi
Page 8