HELCOM core indicator of eutrophication Clear water

How far are we from clear water? Authors Vivi Fleming-Lehtinen, Pirkko Kauppila, Hermanni Kaartokallio - Finnish Environment Institute (SYKE), Finland

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HELCOM core indicator of eutrophication Clear water

Key message The summer-time water clarity has decreased in all Baltic regions during the last century. During the last two decades, water clarity has increased in the southern Baltic sub-areas.

Figure 1. Ecological status for water clarity measured as Secchi depth, 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.

Policy relevance Water clarity reflects eutrophication through changes in phytoplankton biomass and other small organic particles. Water clarity is also affected by changes in the amount of other coloured substances, unrelated to eutrophication.

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HELCOM core indicator of eutrophication Clear water

Sub-regional assessment of water clarity

Figure 2. Ecological status for water clarity in the different sub-regions, measured as Secchi depth, during the period 20032007 Bothnian Bay The water clarity status is mostly poor or bad in the open, transitional and coastal areas. Only in one area, inner Kokkola on the Finnish coast, the status is moderate. The Quark The water clarity status in coastal-transitional waters varies from good to poor. Bothnian Sea The water transparency status is mostly poor or bad in open sea, transitional and coastal areas. Only in one area, Skärgårdsfjärden on the Swedish cost, the status is good. Archipelago Sea The water clarity status is bad. Gulf of Finland The water clarity status is poor or bad in the entire Gulf of Finland, with the exception of the Tallinn Bay that has high status. Gulf of Riga Water clarity status is bad or poor in the coastal, transitional and open waters. Northern Gotland Basin In the open sea the water clarity status is poor. In the coastal areas the status is poor or bad, in transitional waters also moderate or good (NE Stora Möja). © HELCOM 2010 www.helcom.fi

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HELCOM core indicator of eutrophication Clear water Western Gotland Basin In the open sea the water clarity status is moderate, in the coastal areas from moderate to bad. Eastern Gotland Basin In the open sea the water clarity status varies from good to high and in the coastal and transitional waters from moderate to poor. Gulf of Gdansk In the open area the water clarity status is good and in the coastal area (Vistula river mouth) moderate. Bornholm Basin In the open sea the water clarity status is good. In the northern coastal and transitional waters (Sweden) the status varies from good to bad, in the southern coast from poor to good. Arkona Basin Water clarity status is high in the open waters, variable in coastal areas. In the Zingst outlet the water transparency status is bad. Kiel and Mecklenburg Bights Water transparency status is poor or bad. Danish Straits Water transparency status is mostly poor or moderate. Only in the NW Kiel Bight the water transparency status is high / at reference. The Kattegat Water transparency status varies from moderate to bad in the Kattegat costal waters. In the open sea the status is high.

Temporal development of water clarity of the Baltic Sea since the beginning of the 20th century Decrease in summer time water transparency has been observed in all Baltic sub-regions over the last one hundred years. The decrease has been most pronounced in the northern Baltic Proper (from 9 m to 5 m) and the Gulf of Finland (from 8 m to 4 m). More recent decreases – over the past 25 years – have been most pronounced in the Western Gotland Basin, Northern Baltic Proper and the Gulf of Finland. On the other hand, in Kattegat and Eastern Gotland Basin the decreasing trend has ceased during the past 20 years and since then the water transparency has remained at about the same level. In the Arkona Sea and Bornholm Sea the water transparency has increased slightly during the last two decades. The decreased summer time water transparency in the Baltic Proper and the Gulf of Finland is at least partly a result of the increase in phytoplankton biomass. In the Gulf of Finland, Bothnian Sea, Northern Baltic Proper, Gulf of Riga, Western Gotland Basin, Northern Gotland Basin, Bornholm Sea and Arkona Sea, summertime cyanobacterial blooms have become common during the last few decades.

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HELCOM core indicator of eutrophication Clear water

Figure 3. Water transparency in June-September measured as Secchi depth (m) between years 1903 and 2009 in the open-sea sub-regions of the Baltic Sea. Secchi depth observations (m) are plotted against the year of observation and the curves fitted with non-linear smoothing (blue line) and shown with 95 % confidence intervals (light blue area). The level below 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. Another reason for the decreased water transparency is no doubt the change in the input of organic as well as inorganic substances from land. This is probably the main reason in the Bothnian Bay, where phytoplankton growth is limited by low concentrations of phosphorus and heterotrophic production is of importance. However, it is evident also in the Bothnian Sea and the Gulf of Finland and possibly in other areas.

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HELCOM core indicator of eutrophication Clear water In the beginning of the 20th century water transparency was considerably lower in the Gulf of Riga than in the other sub-regions investigated. Kattegat is part of the transitional area between the high salinity North Sea and the brackish water Baltic Sea and water exchange in the area is frequent, which may be related to variations in water transparency.

A conceptual model of the relation of water clarity to eutrophication Water clarity is affected by light attenuation in the water, caused by dead and living organisms, dissolved coloured substances and inorganic particles. Eutrophication increases the attenuation, through nutrients increasing the amount of living organisms. Turbid waters affect the ecosystem through decrease in light availability below the surface.

Data Secchi depth in 2003-2007 (Excel file)

Technical data Data source: The HELCOM countries have provided status concentrations for 2003-2007. The open sea Secchi depth measurements were made during research and monitoring cruises of the HELCOM Contracting Parties. The data is kept at the database of the Finnish Environment Institute SYKE, where the contact person is Vivi FlemingLehtinen. Description of data: The measurement unit is meter. The original purpose of the data was to give an indication of water clarity over long-time periods. Geographical coverage: All regions of the Baltic Sea.

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HELCOM core indicator of eutrophication Clear water

Figure 4. Water transparency observations made between 1903 and 2009 in the sub-regions (BB = Bothnian Bay, BS = Bothnian Sea, GoF = Gulf of Finland, NBP = Northern Baltic Proper, GR = Gulf of Riga, WGB = Western Gotland Basin, EGB = Eastern Gotland Basin, Ark = Arkona Basin and Bor = Bornholm Basin).

Temporal coverage: From 1903 to 2007 except for the period from the 1940’s to the beginning of the 1970’s when observations are scarce due to the second world war. Methodology and frequency of data collection: Data were mainly collected for the HELCOM COMBINE and national monitoring programmes. 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,

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HELCOM core indicator of eutrophication Clear water 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 water transparency assessment were first defined by a group of national experts from the HELCOM Contracting Parties for the HELCOM thematic assessment on eutrophication (HELCOM 2009a). 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 integrated thematic assessment on eutrophication of the Baltic Sea (HELCOM 2009). Months of June, July, August and September were chosen to represent the period of abundant occurrence of cyanobacteria. Data from the beginning of the century that was produced using a 0.6 m diameter Secchi disk and a water viewer was corrected according to Launiainen et al. (1989). The Secchi depth data from each sub-region was plotted against the observation year and a non-linear smoothing curve was fitted to the plot. This technique estimates the local fit of the curve. The 95% confidence intervals of the curve were estimated on the basis of standard error of ± 2 of the curve estimation. Reference conditions for the Baltic sub regions were obtained by combining information achieved from the means of the data collected between 1905 and 1910 and the value of the smoothing curve before 1940. The target value was set at a 25% 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: Secchi depth is one of the few parameters for which there is data from a long time period. Practically the method is unchanged. Technically Secchi depth measurement is simple, cheap and easy to do. The temporal and spatial coverage of the data is not even, and data is lacking from certain time periods, such as that from the 1940’s to the beginning of the 1970s. 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. In order to resolve the significance of Secchi depth as a eutrophication parameter, the roles of the parameters affecting water transparency are be thoroughly investigated.

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. HELCOM 2009a. 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. Launiainen, J., Vainio, J., Voipio, A., Pokki, J. & Niemimaa, J. (1989): Näkösyvyyden vaihteluista ja muuttumisesta pohjoisella Itämerellä (Long-term changes in the secchi depth in the northern Baltic Sea). – XIV Geofysiikan päivät. Geofysiikan seura. Helsinki, 117-121. (In Finnish, English summary) Sandén, P. & Håkansson, B. (1996): Long-term trends in Secchi depth in the Baltic Sea. – Limnol. Oceanogr. 41:346351.

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