Interaction between the land and the sea: sources and patterns of nutrients in the scattered coastal zone of a eutrophied sea

Abstract

A long-term trophic development of three geographical transects—including a river mouth, an estuary, and an archipelago—were studied in the southern Finnish coast in the Baltic Sea. Each transect was studied to clarify how far off the coast the land-based nutrient sources (catchment factor, CF) had a decisive role in shaping the wintertime regimes of dissolved inorganic nitrogen and dissolved inorganic phosphorus and where the marine processes (marine factor, MF) start to play a major role. Generally, CF controlled the nutrient regime from the coast to the outer brink of the inner coastal area, after which MF started to dominate. The estuaries exhibited steep vertical nutrient gradients, above which the riverine input dominated the nutrient regime. The extent of the area where CF dominated the nutrient regime was therefore decisively dependent on estuarine stratification, i.e., whether the conclusions were drawn based on the surface layer data, including the riverine impact, or on the data beneath that layer, including the marine impact. This result deviates from the current consensus that the trophic regime of the sea is most directly assessed by the surface layer nutrient content. The estuarine nutrient regime is unrepresentative to that of a typical coastal water body due to the strong land-based impact on the estuary. Therefore, any generalization of the trophic condition of an estuary to represent areas farther off the coast should be done cautiously. The estuaries should also be defined as belonging to transitional waters according to the typology related to European Marine Legislation.

Legacy Effects of Eutrophication on Modern Methane Dynamics in a Boreal Estuary

“…During that time, enhanced primary production in the estuary and archipelago led to high rates of carbon accumulation in the sediments (Heiskanen and Tallberg 1999). In a recent study, Raateoja and Kauppila (2019) studied eutrophication development of three estuaries in Northern Baltic, including Pojo Bay, from the 1970s onwards. In accordance with other studies of SW Finland archipelago areas (Conley et al 2011;Jokinen et al 2018), they did not find any evidence of oligotrophication in the recent past, but note that the two large lakes in the catchment of Mustionjoki may buffer changes in coastal nutrient levels, by delaying land-to-sea nutrient transfer.…”