Tracing pollution and recovery using sediments in an urban estuary, northern Baltic Sea: are we far from ecological reference conditions?

Abstract

One of the primary challenges of the Water Framework Directive (WFD) of the European Union is to provide a guide for the recovery of surface waters from pollution. However, few studies deal with reference conditions according to the WFD in coastal waters. Using the urbanised Laajalahti Bay (Helsinki, Finland) as an example, reference conditions and pollution history were defined using the stratigraphy of diatoms, sediment geochemistry, stable isotopes, sedimentary pigments, long-term monitoring results of water quality, and nutrient-loading. Principal components analysis was used to generate a multidimensional index of water quality on the basis of the sediment data. We distinguished 5 phases: (1) a pre-industrial phase (before ca. 1815); (2) a phase of slight human disturbance (ca. 1815 to 1900); (3) an onset of anthropogenic impact (ca. 1900 to 1955); (4) a severe pollution phase (ca. 1955 to 1975); (5) basin recovery and a phase of internal loading (from ca. 1975 onwards). Phase 2 was used to define reference conditions. Phase 1 was not used as it occurred before the formation of the semi-enclosed bay by post-glacial isostatic land-uplift. There was good agreement between the sedimentary record and the water-quality data during Phase 4. Despite an improvement in water quality after the local municipal treatment plant closed in 1986, Laajalahti Bay is still far from reference conditions due to internal loading.

Historical Land Use, Nitrogen, and Coastal Eutrophication: A Paleoecological Perspective

“…The most severe hypoxic events coincide with land clearance, agricultural expansion, land drainage, and the reduction of denitrification in the drainage area. Analysis of ecosystem indicators from areas draining into Laajalahti Bay in Finland, a part of the north Baltic Sea, records the history of pollution, beginning with preindustrial time (prior to 1815), followed by decreasing water quality with the increase in humans from around 1900 to 1955 and acute pollution from about 1955 to 1975 (Kauppila et al 2005). The accelerated decrease in water quality was caused by increasing agriculture and in some estuaries also the discharge of waste water.…”

Historical records of coastal eutrophication-induced hypoxia

“…Cores from Charlotte Harbor showed similar pigment increases as well as BSi increases since 1950 . Peaks in chlorophyll a, its breakdown products and diatoxanthin, corresponding to high nutrient loads, were reported from Laajalahti Bay (Kauppila et al, 2005).…”

Long‐term trends in eutrophication and nutrients in the coastal zone

“…Laajalahti Bay shows the increase in planktonic diatoms that would be expected as a response to eutrophication. The high planktonic production, indicated by the very low Secchi depth in Laajalahti Bay (0.5-1 m, Weckström et al 2004), is maintained by internal loading (Kauppila et al 2005).…”

“…This embayment does not represent the conditions typical of many other inner coastal waters of southern Finland. The high internal P loading from the sediments due to the historical sewage inputs means Laajalahti Bay is N limited, while a number of other embayments in the low-salinity Gulf of Finland are typically P limited (Kauppila et al 2005). The internal loading has resulted in Laajalahti Bay still not approaching reference conditions despite closure of the municipal sewage treatment plant 18 yr ago.…”

“…Benthic diatoms are not part of the biological elements of the WFD in coastal and transitional waters, possibly reflecting the lack of information about benthic systems and eutrophication in a management context. In the case of Laajalahti Bay it may be prohibitively expensive to achieve good ecological status, and the directive may set less stringent environmental objectives (Kauppila et al 2005). This indicates the need for historical information at a local scale to allow appropriate management.…”