Secchi depth in the Baltic Sea an indicator of eutrophication

Secchi depth, a proxy of water clarity, is widely applied as an indicator of eutrophication or water quality both in open-sea- and coastal areas. In optically complex waters, such as the Baltic Sea, Secchi depth is known to respond to several components yet its performance, or possible restrictions, have not been explored. In this study, I investigated long-term changes in Secchi depth. I also explored the structure, scientific basis and use of Secchi depth as an indicator of eutrophication in the Baltic Sea. Secchi depth decreased in the open Baltic Sea during the last century (Paper I). The decrease was especially intense in the northern areas, amounting to 3.3 4.0 m (averaging 0.033 0.040 m y-1), when comparing summer time averages in 2005 2009 to those observed one hundred years earlier. The decrease was proposed to be strongly linked with documented simultaneous increase in chlorophyll-a concentration (Papers I, III). A closer look at the Finnish coastal areas, where a national monitoring program has taken place since 1970, revealed clear decreasing trends only in the Archipelago Sea accompanied by opposing trends in chlorophyll-a (Paper II). Contradictory to this, and to the development in adjacent open sea areas, Secchi depth was observed to increase in the coastal Bothnian Sea, Quark and Bothnian Bay. I suggest the increase was at least partly a consequence of decreased concentrations of dissolved iron in the surface waters near the coast. The relationship between Secchi depth and total organic carbon (TOC) was tested, but a significant relationship was not found indirectly indicating that a large part of organic carbon was colorless. Unfortunately, the long-term coastal dataset did not allow comparison to suspended inorganic matter, leaving the possible effect of potentially important coastal constituent unrevealed. The effect of the main optical constituents on light attenuation in the open sea were investigated through a bio-optical model setup, in order to resolve how the Secchi depth indicator should be applied in different parts of the Baltic Sea (Paper III). Secchi depth was shown to be highly sensitive to variation in both phytoplankton (by chlorophyll-a as proxy) and colorful dissolved organic matter (CDOM). As expected, based on the high spatial gradients in both optical constituents, the evaluation against monitoring data called for sub-basin-wise adjustments to the model outcome. Secchi depth is often applied together with other indicators, including chlorophyll-a. The modelling exercise revealed, that the environmental targets for Secchi depth, set by the Baltic Sea coastal states via their collaboration through the Baltic Marine Environment Protection Commission (HELCOM), were stricter than those set for chlorophyll-a. To facilitate future management use of the Secchi depth indicator, I made an effort to characterize it in relation to indicators in general. Secchi depth is a commonly applied and well established indicator of eutrophication and water quality in the Baltic Sea. It is technically relatively advanced: quantitative, regularly monitored, and includes ecological targets as well as documented methodology. It is also easily understood by the public. On the other hand, though simple to associate, it is a composite indicator, which requires case-specific analysis before its role in the eutrophication process can be accurately defined. Finally, Secchi depth was applied in the Baltic Sea eutrophication status assessment (Paper IV), and alternative ways to apply the indicator were explored. According to the assessment 2007-2011, all open-sea areas of the Baltic Sea were severely affected by eutrophication. Due to the deteriorated status of all indicators, variation in the construction of the assessment did not affect the general outcome. Secchi depth on its own expressed deteriorated status in most areas, meeting its environmental target only in the Bothnian Bay. The strong relationship between Secchi depth and chlorophyll-a motivates the use of Secchi depth as a eutrophication indicator throughout the open Baltic Sea. The strong association to CDOM, however, presents a combination of possible additional autochthonous as well as allochthonous signals. The sensitivity of Secchi depth to the main optical constituents varies between open-sea areas, and furthermore, needs to be addressed separately in the coastal zone, where inorganic constituents are expected to be significant. Being a composite indicator, Secchi depth was found suitable for expressing eutrophication together with other indicators; relying on Secchi depth alone would introduce a risk of misinterpretations, especially when the role of water clarity in the ecosystem is not solved area-specifically. On the other hand, Secchi depth may turn to be valuable in reflecting signals not currently captured by other indicators.Näkösyvyys kertoo veden kirkkaudesta. Sitä on käytetty laajasti rehevöitymisen tilan ja vedenlaadun osoittimena (indikaattorina) sekä avomerellä että rannikonläheisillä merialueilla. Itämeren tyyppisissä, optisesti monimuotoisissa vesissä se reagoi useisiin veden ominaisuuksiin. Silti sen suorityskykyä osoittimena, tai käyttöön liittyviä rajoituksia, ei ole liiemmin selvitetty. Tässä työssä tarkastelen näkösyvyyden pitkäaikaismuutoksia Itämerellä. Tutkin myös näkösyvyys-osoittimen ominaisuuksia ja käyttömahdollisuuksia. Kuluneen vuosisadan aikana Itämeren näkösyvyys laski (Julkaisu I). Voimakkainta lasku oli pohjoisilla alueilla, yltäen 3.3 4.0 m sadassa vuodessa (keskiarvona 0.033 0.040 m v-1). Esitän, että näkösyvyyden lasku liittyy vahvasti samaan aikaan tapahtuneeseen levämäärän (a-klorofyllipitoisuus, lehtivihreän määrä) lisääntymiseen pintavedessä (Julkaisut I, III). Vuodesta 1970 alkaen jatkunut seuranta mahdollisti Suomen rannikkoalueiden näkösyvyysmuutosten lähemmän tarkastelun. Saaristomerellä todettiin selkeä laskeva suuntaus ja samanaikainen levämäärän lisääntyminen (Julkaisu II). Selkämeren, Merenkurkun ja Perämeren rannikoilla suuntaus oli päinvastainen: näkösyvyyden todettiin kasvaneen, mikä oli ristiriidassa myös näitä rannikkokaistaleita ympäröivien avomerialueiden kehityksen kanssa. Esitän että veden kirkastuminen kyseisissä vesissä on ainakin osittain seurausta liuenneiden rautayhdisteiden määrän vähenemisestä. Näkösyvyyden ja kokonaishiilen (TOC) määrän muutoksia testattiin myös suhteessa toisiinsa, ilman näyttöä merkitsevästä riippuvuudesta minkä tulkitsin johtuvan siitä, että ainakin osa veteen liuenneesta hiilestä on väritöntä. Kiintoaineen suhteen vertailua ei ikävä kyllä ollut mahdollista tehdä, joten sen merkitystä näkösyvyyden muutoksiin rannikolla ei pystytty tutkimaan. Pohdin Itämeren avomerialueiden tärkeimpien valon vaimenemiseen vaikuttavien ainesosien vaikutusta näkösyvyyteen bio-optisen mallijärjestelyn avulla (Julkaisu III). Tämä auttoi selvittämään kuinka näkösyvyysosoitinta tulisi soveltaa Itämeren eri osissa. Näkösyvyys osoittautui olevan herkkä sekä levämäärän (lehtivihreän kautta tulkittuna) että humusaineiden (CDOM) vaihtelulle. Herkkyys vaihteli alueellisesti siinä määrin, että mallin tuloksia jouduttiin sovittamaan merialuekohtaisesti. Näkösyvyyttä hyödynnetään tilanarvioissa usein yhdessä muiden osoittimien, kuten levämäärän, kanssa. Mallinnuksen seurauksena päädyin esittämään, että näkösyvyydelle kansainvälisesti, Itämeren Suojelukomission (HELCOM) toimesta asetetut ympäristön hyvän tilan tavoitetasot ovat levämäärälle asetettuja tavoitteita kunnianhimoisemmat. Tukeakseni näkösyvyysosoittimen tulevaa käyttöä, tein arvion sen ominaisuuksista suhteessa osoittimiin yleensä. Näkösyvyys on jo laajasti käyttöönotettu rehevöitymisen ja vedenlaadun osoitin Itämerellä. Se on teknisesti kehittynyt: määrällinen (kvantitatiivinen), säännöllisesti seurattu (monitoroitu), menetelmiltään todennettu osoitin, jolle on kyetty määrittämään hyvän tilan tavoitetasot. Se on myös helposti ymmärrettävä ja käytännönläheinen. Vaikka se on toiminnallisesti yksinkertainen, on se rehevöitymiseen liittyvien syy-seuraussuhteiden osalta monimutkainen, ja edellyttää siltä osin aluekohtaisen analyysin ennen käyttöönottoa. Tutkin lopuksi näkösyvyyden käyttöä Itämerenlaajuisessa rehevöitymisen tilanarviossa (Julkaisu IV), kokeillen vaihtoehtoisia tapoja yhdistellä sitä muiden osoittimien kanssa. Vuosille 2007-2011 määritetyn Itämeren tilanarvion perusteella kaikki avomerialueet olivat rehevöityneitä. Erikseen jokaisen rehevöitymisen tilan osoittimen kautta tulkittuna tulos oli useimmilla alueilla sama, joten niiden uudelleenryhmittelyllä ei ollut vaikutusta kokonaistilanarvioon. Yksin näkösyvyyden kautta arvioituna rehevöitymisen tila oli huono useimmilla avomerialueilla, Perämerta lukuun ottamatta. Näkösyvyyden voimakas riippuvuus levämäärään kannustaa hyödyntämään sitä rehevöitymisen tilan osoittimena kautta Itämeren. Humusaineilla, joista merkittävä osa on maalta peräisin, on lisäksi vaikutusta vedenkirkkauteen tämä tekee osoittimesta herkän myös rehevöitymisen ulkopuolisille muutoksille. Tämä herkkyys vaihtelee alueellisesti, ja se tulee ottaa huomioon ja suhteuttaa olosuhteisiin niin rannikoilla kuin avomerellä. Näkösyvyys on parhaimmillaan ympäristön tilanarvioissa yhdessä muiden osoittimien kanssa. Luottaminen yksinomaan tämän syy-seuraussuhteiltaan monimuotoisen osoittimen viestiin altistaa virhetulkinnoille, erityisesti mikäli vedenkirkkauden syitä ja riippuvuuksia ei ole selvitetty aluekohtaisesti. Toisaalta, yhdessä muiden osoittimien kanssa näkösyys saattaa tunnistaa signaaleja jotka eivät vaikuta muihin osoittimiin

Integration of data for nowcasting of harmful algal blooms

Harmful algal blooms (HABs) are a significant and potentially expanding problem around the world. Resource management and public health protection require sufficient information to reduce the impacts of HABs by response strategies and through warnings and advisories. To be effective, these programs can best be served by an integration of improved detection methods with both evolving monitoring systems and new communications capabilities. Data sets are typically collected from a variety of sources, these can be considered as several types: point data, such as water samples; transects, such as from shipboard continuous sampling; and synoptic, such as from satellite imagery. Generation of a field of the HAB distribution requires all of these sampling approaches. This means that the data sets need to be interpreted and analyzed with each other to create the field or distribution of the HAB. The HAB field is also a necessary input into models that forecast blooms. Several systems have developed strategies that demonstrate these approaches. These range from data sets collected at key sites, such as swimming beaches, to automated collection systems, to integration of interpreted satellite data. Improved data collection, particularly in speed and cost, will be one of the advances of the next few years. Methods to improve creation of the HAB field from the variety of data types will be necessary for routine nowcasting and forecasting of HABs

Factors regulating the coastal nutrient filter in the Baltic Sea

The coastal zone of the Baltic Sea is diverse with strong regional differences in the physico-chemical setting. This diversity is also reflected in the importance of different biogeochemical processes altering nutrient and organic matter fluxes on the passage from land to sea. This review investigates the most important processes for removal of nutrients and organic matter, and the factors that regulate the efficiency of the coastal filter. Nitrogen removal through denitrification is high in lagoons receiving large inputs of nitrate and organic matter. Phosphorus burial is high in archipelagos with substantial sedimentation, but the stability of different burial forms varies across the Baltic Sea. Organic matter processes are tightly linked to the nitrogen and phosphorus cycles. Moreover, these processes are strongly modulated depending on composition of vegetation and fauna. Managing coastal ecosystems to improve the effectiveness of the coastal filter can reduce eutrophication in the open Baltic Sea.peerReviewe

Eutrophication in marine waters: harmonization of MSFD methodological standards at EU level

The Marine Strategy Framework Directive (MSFD) establishes the framework for the protection, conservation and sustainable use of the marine environment at the Union level. Because of its potential negative effects on the marine water quality, eutrophication is one of the criteria assessed under MSFD. This report presents the results of the joint work between JRC and a network of Member States (MS) eutrophication designated experts to assess the level of harmonization in Eutrophication methodological standards and threshold definition at regional and EU level. The information compiled at regional and national level showed that although methodologies are defined already for all the criteria, the degree of harmonization of methodological approaches across MS and regions is in some cases very low. In addition further developments are needed for some regions to agree on common indicators and threshold values. Based on these results the report highlights existing gaps and proposes recommendations to improve the eutrophication assessment framework at EU level.JRC.D.2-Water and Marine Resource

Secchi depth calculations in BALTSEM

Secchi depth measurements have been carried out for over 100 years in the Baltic Sea and the changes in Secchi depth give indications of the development of phytoplankton biomass in response to eutrophication. In the implementation of the ecosystem approach to Baltic Sea management, indicators based on Secchi depth are unique in that targets representing a good environmental status can be obtained from actual observations, whereas most other indicators lack observational evidence of a reference state representing conditions before substantial eutrophication. In the on‐going revision of the HELCOM Baltic Sea Action Plan, new targets on e.g., Secchi depth have been developed (HELCOM2012). The following step is to use modeling to find nutrient inputs to the Baltic Sea, so called Maximum Allowable Inputs, that result in ecosystem changes so that eventually the good environmental status indicated by the targets is reached. This modeling effort is carried out using the coupled physical‐biogeochemical model BALTSEM developed at BNI. The BALTSEM model resolves the Baltic Sea horizontally with 13 sub‐basins, but each of these with high vertical resolution. The biogeochemical model includes inorganic and bioavailable organic nitrogen, phosphorus and silica, three phytoplankton groups, zooplankton and oxygen. Benthic nutrient regeneration and retention are modeled in addition. This report describes a statistical post‐processing algorithm to calculate Secchi depth from BALTSEM results to provide additional accuracy and confidence of Secchi depth estimates compared to the simplistic intrinsic transparency calculations within the BALTSEM model. The additional quality in the Secchi depth calculation results isof major importance for the results of the calculation of the Maximum Allowable Inputs

Secchi depth calculations in BALTSEM

Secchi depth measurements have been carried out for over 100 years in the Baltic Sea and the changes in Secchi depth give indications of the development of phytoplankton biomass in response to eutrophication. In the implementation of the ecosystem approach to Baltic Sea management, indicators based on Secchi depth are unique in that targets representing a good environmental status can be obtained from actual observations, whereas most other indicators lack observational evidence of a reference state representing conditions before substantial eutrophication. In the on‐going revision of the HELCOM Baltic Sea Action Plan, new targets on e.g., Secchi depth have been developed (HELCOM2012). The following step is to use modeling to find nutrient inputs to the Baltic Sea, so called Maximum Allowable Inputs, that result in ecosystem changes so that eventually the good environmental status indicated by the targets is reached. This modeling effort is carried out using the coupled physical‐biogeochemical model BALTSEM developed at BNI. The BALTSEM model resolves the Baltic Sea horizontally with 13 sub‐basins, but each of these with high vertical resolution. The biogeochemical model includes inorganic and bioavailable organic nitrogen, phosphorus and silica, three phytoplankton groups, zooplankton and oxygen. Benthic nutrient regeneration and retention are modeled in addition. This report describes a statistical post‐processing algorithm to calculate Secchi depth from BALTSEM results to provide additional accuracy and confidence of Secchi depth estimates compared to the simplistic intrinsic transparency calculations within the BALTSEM model. The additional quality in the Secchi depth calculation results isof major importance for the results of the calculation of the Maximum Allowable Inputs

A retrospective view of the development of the Gulf of Bothnia ecosystem

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Long-term temporal and spatial trends in eutrophication status of the Baltic Sea

Much of the Baltic Sea is currently classified as ‘affected by eutrophication’. The causes for this are twofold. First, current levels of nutrient inputs (nitrogen and phosphorus) from human activities exceed the natural processing capacity with an accumulation of nutrients in the Baltic Sea over the last 50–100 years. Secondly, the Baltic Sea is naturally susceptible to nutrient enrichment due to a combination of long retention times and stratification restricting ventilation of deep waters. Here, based on a unique data set collated from research activities and long-term monitoring programs, we report on the temporal and spatial trends of eutrophication status for the open Baltic Sea over a 112-year period using the HELCOM Eutrophication Assessment Tool (HEAT 3.0). Further, we analyse variation in the confidence of the eutrophication status assessment based on a systematic quantitative approach using coefficients of variation in the observations. The classifications in our assessment indicate that the first signs of eutrophication emerged in the mid-1950s and the central parts of the Baltic Sea changed from being unaffected by eutrophication to being affected. We document improvements in eutrophication status that are direct consequences of long-term efforts to reduce the inputs of nutrients. The reductions in both nitrogen and phosphorus loads have led to large-scale alleviation of eutrophication and to a healthier Baltic Sea. Reduced confidence in our assessment is seen more recently due to reductions in the scope of monitoring programs. Our study sets a baseline for implementation of the ecosystem-based management strategies and policies currently in place including the EU Marine Strategy Framework Directives and the HELCOM Baltic Sea Action Plan.publishedVersio

Long-term temporal and spatial trends in eutrophication status of the Baltic Sea

Much of the Baltic Sea is currently classified as 'affected by eutrophication'. The causes for this are twofold. First, current levels of nutrient inputs (nitrogen and phosphorus) from human activities exceed the natural processing capacity with an accumulation of nutrients in the Baltic Sea over the last 50-100 years. Secondly, the Baltic Sea is naturally susceptible to nutrient enrichment due to a combination of long retention times and stratification restricting ventilation of deep waters. Here, based on a unique data set collated from research activities and long-term monitoring programs, we report on the temporal and spatial trends of eutrophication status for the open Baltic Sea over a 112-year period using the HELCOM Eutrophication Assessment Tool (HEAT 3.0). Further, we analyse variation in the confidence of the eutrophication status assessment based on a systematic quantitative approach using coefficients of variation in the observations. The classifications in our assessment indicate that the first signs of eutrophication emerged in the mid-1950s and the central parts of the Baltic Sea changed from being unaffected by eutrophication to being affected. We document improvements in eutrophication status that are direct consequences of long-term efforts to reduce the inputs of nutrients. The reductions in both nitrogen and phosphorus loads have led to large-scale alleviation of eutrophication and to a healthier Baltic Sea. Reduced confidence in our assessment is seen more recently due to reductions in the scope of monitoring programs. Our study sets a baseline for implementation of the ecosystem-based management strategies and policies currently in place including the EU Marine Strategy Framework Directives and the HELCOM Baltic Sea Action Plan.Peer reviewe