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Deep-Water Oxygen Conditions in the Bothnian Sea

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Deep-Water Oxygen Conditions in the Bothnian Sea Boreal environment research 18: 235–249 © 2013 issn 1239-6095 (print) issn 1797-2469 (online) helsinki 28 June 2013 Deep-water oxygen conditions in the Bothnian sea mika raateoja Finnish Environment Institute (SYKE), Marine Research Centre, P.O. Box 140, FI-00251 Helsinki Received 17 July 2011, final version received 25 Sep. 2012, accepted 24 Apr. 2012 raateoja, m. 2013: Deep-water oxygen conditions in the Bothnian sea. Boreal Env. Res. 18: 235–249. The hydrography of the deep waters of the Bothnian Sea (BS) underwent considerable changes during the most recent two decades. Most importantly, slightly worsened deep- water oxygen (O2) conditions followed a gradual increase in the water-column density gra- dient. Up to the present time, the lowest measured O2 concentrations fell within the range 3.5–4.5 ml l–1, 1.4 ml l–1 being used in this work as the upper limit for hypoxia. I investi- gated the long-term deep-water characteristics of the BS to determine how possible it is for an hypoxic event to occur there in the near future. It appears that there is only a remote probability of this, unless profound changes take place in the hydrographic regime of the BS and/or in the anthropogenic load to the BS. The key factor behind this conclusion is the hydrodynamic regime of the Åland Sea and the ridge formations around it. This area acts as a buffering/filtration mechanism that prevents the saline and O2-poor deep waters of the Northern Baltic Proper from entering the BS without considerable mixing and dilution. A discussion is presented of the consequences of this conclusion, taking into account the past adverse development in the Gulf of Finland, with its internal loading of phosphorus and its accelerated eutrophication. With respect to the Finnish coastal area of the BS, the stable O2 conditions and the favourable hydrographic setup in the near-coastal area suggest that any O2 problems will most probably be concentrated in the inner-archipelago areas. Introduction Sea. The Baltic Sea has an intrinsic tendency to experience O2 problems due to its natural The spreading of hypoxia in coastal and estua- vertical salinity stratification, and hence, den- rine marine environments is currently a ubiqui- sity gradients that effectively restrict the vertical tous problem. Coastal systems that experience transport of O2. These salinity anomaly layers, episodic hypoxic events or permanent hypoxia called haloclines, have played a pivotal role in have greatly increased in numbers (Selman et the spreading of a hypoxic environment in the al. 2008). The conspicuous factor behind this Baltic Sea; indeed, O2 problems have persisted adverse development is eutrophication, which there for as long as the present geographical boosts the flow of organic matter to the ben- coverage of the halocline system has prevailed, thic system, ultimately degrading the oxygen that is, from the commencement of the Litto- (O2) conditions there. This development has also rina Sea stage at approximately eight millennia taken place in the Baltic Sea. BP. (Zillen et al. 2008). From this perspective, However, eutrophication is not solely to anthropogenic eutrophication has had little time blame for the current poor O2 status of the Baltic to leave its fingerprints in the O2 regime of the Editor in charge of this article: Kai Myrberg 236 Raateoja • Boreal env. res. Vol. 18 of Bothnia is not serious, the increasing trends Northern Quark in the nutrient levels should be seen as warning signs for the future”. A closer look into the BS US2 US7 system is indeed topical. US5B Eutrophication is a process that, if given a chance, follows the cause-and-effect cascade MS11 of increased allochthonous load of nutrients increased autochthonous load of organic matter SR5 SR9 61°N, 23°E anoxia increased autochthonous load of nutrients (the internal loading of phosphorus). Southern Quark There exists a strong feedback path from the F64 end-product, the internally-loaded phosphorus (P), adding the autochthonous component to Southern Åland Sill LL7 eutrophication. This feedback mechanism has the potential to strongly accelerate the eutrophication LL19 process. I wanted to clarify whether this cascade, which has already commenced and is still on- going in the GOF, could be a potentially realis- Fig. 1. map of the study area. stations sr5, Us2, tic scheme for the BS. In this article, I present and Us5B were chosen as the main data sources for a long-term hydrographic dataset collected at the study. stations sr9, ms11, and Us7 were used representative monitoring stations in the BS. I to describe the oxygen conditions in the Finnish near- address the following questions: (1) how have coastal zone of the Bs. the cascade of stations sr5, F64, and LL19 reflected the hydrographic variation the near-bottom O2 conditions in the BS recently between the nBP and the Bs. the station ll7 in the developed? (2) what are the external factors that GoF served as the basis for comparison for the deep control the O2 regime of the BS? and (3) what can water nutrient conditions. we expect to happen in the near future? Baltic Sea. Today, hypoxic environments con- The Bothnian Sea sistently cover large offshore areas in the Baltic Proper, while the western parts of the Gulf of The BS is the southern basin of the Gulf of Finland (GOF) can be classified as seasonally Bothnia, which is the semi-enclosed northern hypoxic (HELCOM 2009b); furthermore, the extension of the Baltic Sea. In the Baltic Sea, the hypoxic environment has recently been spread- BS is the largest basin after the Gotland Basin, ing in the coastal areas (Conley et al. 2011). with an area of about one-sixth of the Baltic Sea The offshore Bothnian Sea (BS), on the other area and a water volume of about one-fifth of hand, has not been further eutrophied in terms the Baltic Sea volume (Leppäranta and Myrberg of dissolved inorganic nutrients since the early 2009). The average depth of the BS is 66 m as 1990s (Fleming-Lehtinen et al. 2008, HELCOM compared with 54 m for the Baltic Sea; the BS 2009b). Even so, alarming signals have emerged contains a central plane with large areas deeper during this period, e.g., the chlorophyll a levels than 100, or even 200 m (Fig. 1). and cyanobacterial biomasses have steadily The southern Åland Sill, the southern Quark, increased there since the mid-1990s (Fleming- and the shallow and scattered Archipelago Sea Lehtinen et al. 2008, Jaanus et al. 2011). The isolate the BS effectively from the Northern Finnish coastal area of the BS suffers from the Baltic Proper (NBP). Consequently, the hydro- allochthonous nutrient load, and the inner coastal graphical forcing of the NBP on the BS is area experiences episodic hypoxic events (Lun- limited, albeit constantly present. The northern dberg et al. 2009). Lundberg and co-workers Quark, which topographically separates the BS stated that “… even if eutrophication in the Gulf from the Bothnian Bay, does not restrict the Boreal env. res. Vol. 18 • Deep-water oxygen conditions in the Bothnian Sea 237 southerly flow of fresher water to the BS. This Material and methods flow and the direct river runoff into the BS together create a pronounced fresh water impact Stations on the BS. Together these topographic features result in hydrological properties for the BS that The main stations used in this study (SR5, US2, are markedly different from those in the NBP. and US5B) are located in the central plain of the The pronounced river runoff into the Gulf of BS at water depths > 100 m (Fig. 1). These sta- Bothnia leads to a short water residence time on tions are situated in the sediment-accumulation the Baltic Sea scale, that is, 5 to 6.5 years (Myr- areas that are the first to experience 2O problems. berg and Andrejev 2006). In this deep area, a consistent albeit faint halo- The BS freezes over, at least partly, every cline represents a quasi-stable hydrographic bar- winter. The probability of freezing is highest in rier for the vertical flow of 2O to deeper waters, the Finnish coastal area and the areas next to the emphasizing the role of the O2 supply via hori- northern Quark that freeze every winter, becom- zontal advection (Table 1). US2 is located in the ing smaller in the southern half of the offshore northwestern part of the BS, and is hence subject area; this remains ice-free during mild winters, to the impact of the southward surface current of i.e., about one-third of the winters (Feistel et al. less saline water from the Bothnian Bay. SR5 is 2008). located in the southern part of the central plain, Every summer, a thermocline develops in the and its deep-water hydrographical characteristics BS and this acts as a barrier for vertical mixing are strongly influenced by the hydrographical at a depth ranging from 10 to 20 m depending on forcing of the Åland Sea (ÅS). the year and the season. It separates the surface The near-coastal stations of the BS (SR9, water mass with temperatures of 16–18 °C at its MS11, and US7) are located in the Finnish outer maximum from the deep waters at 2–5 °C. coastal area of the BS. The water depth is < 50 m, The BS has a cyclonic surface water circula- and the virtually non-existing vertical density tion, which leads to a less saline surface layer on gradient in winter indicates that the halocline its deeper western side, and a more saline surface does not extend to these shallow areas (Table 1). layer on its shallower eastern side.
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