The modeling exercise demonstrated, that the environmental targets set for Secchi depth were stricter than those set for chlorophyll-a (Paper III, Chapter 3.4). The reason behind this discrepancy could not be solved; it may be a FRQVHTXHQFHRILQVXI¿FLHQWKLVWRULFDOLQIRUPD-tion available for the chlorophyll-a target-set-ting (HELCOM 2013b, 2014), or an effect of long-term changes in the relationships of the parameters.
In the eutrophication status assessment for the Baltic Sea, Secchi depth ES was estimated to be below the environmental target in eight of the nine sub-basins; the target was (barely) reached only in the Bothnian Bay (Paper IV, Table 3-1). Secchi depth did not differ in this sense much from the other indicators, their ES below the target in most of the sub-basins. Sec-chi depth did however express a lower ER than chlorophyll-a, DIN and DIP, in average. In all but two sub-basins, chlorophyll-a was estimat-ed to be in worse status than Secchi depth (when comparing ER).
The fact that Secchi depth ER showed small variation compared to some of the other
indica-Table 4-2. Features of the Secchi depth indicator used in the Baltic Sea.
1. Quantifiable / descriptive XപറQuantitative Descriptive
2. Complicity Explicit
XപറComposite
3. Procedure XപറMeasurable
Computable 4. Numerical response to increasing
eutrophication XപറNegative
Positive
5. Commonly observable XപറChange can be easily observed in nature by a layman Change can be observed in nature only by an expert Change cannot be seen by the bare eye
6. Commonly understandable Xപ The indicator and it’s role in the system are easily understood by layman.
The indicator and/or it’s role in the system is not simple, but can be explained to a layman in an understandable way.
RThe indicator and/or it’s role in the system is complicated and can be understood only by experts
7. Origin of change Anthropogenic dominating
XപറBoth anthopogenic and non-anthropogenic 8. Ecological targets / reference levels XപറData-based with strong certainty (1940 or earlier)
Data-based with weak certainty XപറHindcast modeling
Process modeling Expert evaluation No targets exist
9. Geographical applicability Same applicability in all geographical areas XപറApplicable in all geographical areas but role differs
Not applicable in all geographical areas
10. Documentation Xപ റScientifically documented approach and well described procedure for update
Scientifically documented approach, but update procedure is not documented to ensure repeatability
Approach not scientifically documented 11. Monitoring XപറMonitored throughout Baltic Sea
Monitored in some areas No regular monitoring
tors, was partly a methodological consequence:
in cases where an indicator responds negatively to increased eutrophication pressure, as Secchi depth does, ER behaves exponentially, instead of linearly, in relation to increasing pressure (Andersen et al. 2011). In practice, this leads to smaller variation in the Secchi depth ER com-pared to the ER of the indicators responding positively to eutrophication pressure, when ES is above or relatively close to the target (ie. ER is below or relatively close to 1). This property of the Secchi depth indicator downscaled the effect of Secchi depth when applying it togeth-er with positively responding indicators in the HEAT 3.0 tool. It subsequently overruled the
disharmony in target-setting, proposed earlier (Paper III).
In the HELCOM eutrophication status as-sessment, Secchi depth was aggregated together with chlorophyll-a, as autochthonous eutroph-LFDWLRQVHHFKDSWHUDQG¿JXUH$,QDOO of the nine sub-basins, autochthonous eutroph-ication was estimated to be below GES, and in the Bornholm Sea and the Western Gotland Basin, it was the aggregation group expressing worst status. It was not alone, however: with the exception of causal eutrophication in the Both-nian Bay, the other aggregation groups were also below GES in all sub-basins. Naturally, Secchi depth did not end up determining the overall status in any of the sub-basins.
In the case of the Baltic Sea, where eutrophi-cation effects are severe and broadly distributed, alterations in the assessment grouping had little effect in the overall result: all sub-basins were estimated to be below GES, whether Secchi depth was grouped as an indicator of autoch-thonous or indirect eutrophication (Figure 3-5A and 3-5B). Since chlorophyll-a was generally in worst status (when comparing ER), the overall eutrophication status tended to move further below GES when removing Secchi depth from autochthonous eutrophication group (with the exception of the Gulf of Riga). As the number of eutrophication indicators in the assessment was low, and nearly half of the sub-basins had no indicator of indirect eutrophication, includ-ing Secchi depth to indirect eutrophication had WKHSRWHQWLDOWRLQFUHDVHDVVHVVPHQWFRQ¿GHQFH This is however an opportunistic approach: the choice of indicator use and grouping must be based on theoretical background. Instead of re-locating existing indicators in the assessment tool, introducing new appropriate indicators should be considered.
5 Conclusions
Water clarity has decreased in the open Baltic Sea during the last century. This was demon-strated by a 14 – 44 % decrease, revealed by a unique historical dataset of Secchi depth (Pa-per 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 compar-ing summer time averages in 2005 – 2009 to those observed one hundred years earlier. The decrease is proposed to be strongly linked with documented simultaneous decrease in chloro-phyll-a concentration (Papers I, III).
Secchi depth monitoring has been conduct-ed also in the Finnish coastal areas, since the 1970’s (Paper II). During the last 2.5 decades, clear decreasing trends were detected only in the Archipelago Sea, together with simultane-ous opposing trends in chlorophyll-a. Contra-dictory to the development in adjacent open-sea areas, Secchi depth increased in the coasts of the Bothnian Sea, Quark and Bothnian Bay.
This was proposed to be at least partly a con-sequence of decreased concentrations of dis-solved iron in the surface waters near the coast.
6WURQJVLJQL¿FDQWUHODWLRQVKLSVEHWZHHQ6HFFKL depth and TOC were rarely found– indirectly indicating that a large part of coastal organic carbon in the area was colorless.
A bio-optical model setup revealed, that Sec-chi depth in the Baltic Sea is highly sensitive to variation in the amount of phytoplankton (by chlorophyll-a as proxy) and CDOM (Paper III).
The relationship to the former motivates the use of Secchi depth as a eutrophication indica-tor. The latter connection, however, implies the presence both eutrophication- and non-eutroph-ication-related components.
Secchi depth is a commonly applied and well established indicator of eutrophication and wa-ter quality in the Baltic Sea. It is commonly understandable and easily experienced, and EHQH¿WV IURP D PRQLWRULQJ KLVWRU\ UHDFKLQJ back to pre-eutrophicated times. It is a tech-nically advanced: it is quantitative, commonly monitored, includes ecological targets and a well-documented methodology. On the other hand, although apparently simple to associate, it is composite, in that it includes signals from several elements.
It is often applied together with other indi-cators, including chlorophyll-a. The model-ling 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 Com-mission (HELCOM), were stricter than those set for chlorophyll-a.
According to the HELCOM Baltic Sea eu-trophication status assessment 2007 – 2011, all open-sea areas of the Baltic Sea were severe-ly affected by eutrophication. This was also shown by the status of the Secchi depth indi-cator, meeting the environmental target only in the Bothnian Bay. The overall eutrophication assessment was not dominated by the weak status of Secchi depth, as the other eutrophica-tion indicators were also unable to reach their environmental targets in most of the sub-basins.
Due to the deteriorated status of all indicators, alterations in the way Secchi depth was applied
in the assessment did not affect the general out-come of eutrophication status.
Secchi depth was found suitable for express-ing eutrophication in optically complex waters such as the Baltic Sea. It is able to quantify several eutrophication-related elements into RQH VLQJOH PHDVXUH UHÀHFWLQJ ERWK DOORFKW-onous and autochtDOORFKW-onous signals. In this study, we were able distinguish the shares of phyto-plankton and CDOM at a large scale. We were, however, not yet able to further identify the elements unrelated to primary production. We
would be especially interested in distinguishing the autochthonous signals, related to eg. hetero-trophic activity, from the allochtonous ones. In shallow and coastal areas, recognizing also the inorganic fraction is essential when applying the indicator. Due to these restrictions, Secchi depth should not be used alone, as the only indicator measuring eutrophication. Applied WRJHWKHUZLWKRWKHULQGLFDWRUVZLWKVXI¿FLHQW backgroung information, it adds to the certainty of an assessment.
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