BERmon31 Phytoplankton as an indicator of eutrophication in coastal marine waters
Monographs of the Boreal Environment Research no. 30, p.50.URN:ISBN:978-952-11-2899-8, ISBN 978-952-11-2899-8 (PDF). The publication is available also in printed form ISBN 978-952-11-2898-1.
The tackling of coastal eutrophication requires water protection measures based on status assessments of water quality. The main purpose of this thesis was to evaluate whether it is possible both scientifically and within the terms of the European Union Water Framework Directive (WFD) to assess the status of coastal marine waters reliably by using phytoplankton biomass (ww) and chlorophyll a (Chl) as indicators of eutrophication in Finnish coastal waters. Empirical approaches were used to study whether the criteria, established for determining an indicator, are fulfilled.
The first criterion (i) was that an indicator should respond to anthropogenic stresses in a predictable manner and has low variability in its response. Summertime Chl could be predicted accurately by nutrient concentrations, but not from the external annual loads alone, because of the rapid affect of primary production and sedimentation close to the loading sources in summer. The most accurate predictions were achieved in the Archipelago Sea, where total phosphorus (TP) and total nitrogen (TN) alone accounted for 87% and 78% of the variation in Chl, respectively. In river estuaries, the TP mass-balance regression model predicted Chl most accurate when nutrients originated from point-sources, whereas land-use regression models were most accurately in cases when nutrients originated mainly from diffuse sources. The inclusion of morphometry (e.g. mean depth) into nutrient models improved accuracy of the predictions.
The second criterion (ii) was associated with the WFD. It requires that an indicator should have type-specific reference conditions, which are defined as “conditions where the values of the biological quality elements are at high ecological status”. In establishing reference conditions, the empirical approach could only be used in the outer coastal waters types, where historical observations of Secchi depth of the early 1900s are available. Most accurate prediction was achieved in the Quark. However, the average reference values in the outer coastal types are underestimated in sites near the zone of the inner coastal waters. In the inner coastal water types, reference Chl, estimated from present monitoring data, are imprecise - not only because of the less accurate estimation method - but also because the intrinsic characteristics, described for instance by morphometry, vary considerably inside these extensive inner coastal types. As for phytoplankton biomass, the reference values were less accurate than in the case of Chl, because it was possible to estimate reference conditions for biomass only by using the reconstructed Chl values, not the historical Secchi observations. An paleoecological approach was also applied to estimate reference conditions for Chl. In Laajalahti, an urban embayment off Helsinki, strongly loaded by municipal waste waters until 1986, reference conditions prevailed in the mid- and late 1800s. The recovery of the bay from pollution has delayed as a consequence of benthic release of nutrients. Laajalahti will probably not achieve the good quality objectives of the WFD on time.
The third criterion (iii) was associated with coastal management including the resources it has available. Analyses of Chl are cheap and fast to carry out compared to the analyses of phytoplankton biomass and species composition; the fact which has an effect on number of samples to be taken and thereby on the reliability of assessments. However, analyses on phytoplankton biomass and species composition provide more metrics for ecological classification, the metrics which reveal various aspects of eutrophication contrary to what Chl alone does.
Pirkko Kauppila, Finnish Environment Institute (SYKE),
tel. +358 20 490 123, email@example.com