El Nivel Empresarial

One of the adverse effects of eutrophication is the serious depletion of oxygen from bottom waters, or hypoxia (Figure 4.2). Along the Swedish west coast and in the Kattegat hypoxia became apparent in the beginning of the 1980s, when oxygen concentrations in bottom waters over extensive areas in the southern Kattegat reached levels detrimental to benthic animals (species-specific effects typically starting in the range from 2 to 5 mg L-1; Vaquer-Sunyer and Duarte 2008).

The oxygen condition in the Kattegat is a balance between respiration in the sediments and bottom water and oxygen supply from photosynthesis in overlying waters and air-sea

exchange. Because the Kattegat is almost permanently stratified around 15 m (Anderson & Rydberg 1993), oxygen has to be supplied to bottom waters through advection of surface Skagerrak water penetrating below the outflowing Baltic Sea water. Average residence time of the

Kattegat bottom water is around 2 to 4 months (Gustafsson 2000, Johan Rodhe presentation to panel) with strong advective transport during winter slowing down during summer and intensifying again during autumn. These physical mechanisms result in the southern Kattegat having a natural oxygen minimum in September, but there can be strong inter-annual variations depending on the volume and degree of stratification of the bottom water. This implies that bottom waters during the low oxygen period (August-October) originate from the Skagerrak surface water in winter-spring. Due to the low and varying temperatures of Skagerrak surface water during this time of the year (2-6°C), there can be variations

in the oxygen concentrations of approximately 1 mg L-1 _{in the water mass}

supplying oxygen to the Kattegat bottom waters. Conley et al. (2007) found quantitative evidence for three factors explaining variations in the summer-autumn oxygen concentrations of the Kattegat and Belt Sea: 1) temperature, through increased metabolism and lower oxygen saturation; 2) advective bottom water transport; and 3) nitrogen input from land that enhances primary production and export of organic material from the upper mixed layer. Other studies have also concluded that there is no single factor to which all variations in oxygen concentrations can be attributed (Rasmussen et al. 2003).

Coastal areas, such as the Laholm Bay and Skälderviken, which connect to the southern Kattegat, were also severely affected by low oxygen concentrations in

Figure 4.2. Oxygen depletion near the sea bed in the southern Kattegat (blue: <4 mg l-1_{, red: <2 mg l}-1_{) during}

September in an extreme year (2002, top) and a normal recent year (2006, bottom). Source: NERI.

1980s (e.g. Rydberg et al. 1990). Hypoxia along the southern Swedish west coast is strongly linked to the conditions in the open waters of the southern Kattegat, where bottom waters, low in oxygen, are advected from the open to the coastal waters, typically during periods of easterly winds. The depletion of oxygen in the bottom layer can be further exacerbated in the shallow coastal region because the bottom water penetrates as a thin layer allowing respiration processes in the sediments and bottom waters to deplete the oxygen inventory in a thin section of the water column. Thus, hypoxia in the coastal areas of the southern Kattegat can be inter- mittent and more dynamic than the rather slow oxygen depletion and repletion processes of the open Kattegat. Higher primary production rates in the coastal zone (Carstensen et al. 2003) contribute organic matter that increases sediment

respiration, intensifying hypoxia along the southern Swedish west coast (Figure 4.2).

To the north along the Swedish west coast the coastal zone changes from shallow coastal embayments to fjords, many of these have a sill restricting the ventilation of bottom water. Long retention times of bottom waters in fjords naturally lead to hypoxia in the very deepest parts, but eutrophication has further lowered oxygen concentrations and increased the volume of hypoxia in areas such as Gullmarsfjord and Stigfjorden (Rosenberg 1990, Lindahl presentation to panel). Renewal of bottom waters typically occurs during strong wind events from north-easterly directions with infrequent major replenishments of oxygen (Erlandsson et al. 2006).

Thus, the physical characteristics of the open southern Kattegat, the coastal

embayment along the southern Swedish west coast, and the fjords on the Skagerrak coast are quite different in modulating the overall oxygen response to increased nutrient enrichment.

4.3.1 Status and trends

Hypoxia in the open southern Kattegat and Öresund has become more prevalent since the 1970s when the first regular monitoring programs were established, and there are no signs of recovery despite reduced inputs of nutrients over the last 10- 15 years (Conley et al. 2007). Extensive areas (~100-500 km2_{) are exposed to}

severe hypoxia (<2 mg l-1_{) in most recent years (2003-2006) (Ærtebjerg 2007), but}

sizes of these areas are much lower than in the catastrophic year of 2002 when >2000 km2 of the Kattegat and Öresund were exposed (Figure 4.4). The 2002 event was indeed an unfortunate combination of the factors leading to hypoxia in the open waters of the Kattegat: high temperatures, an almost complete stagnation of bottom waters, and high inputs of nitrogen (HELCOM 2003). However, with projected increases in temperature and precipitation due to climate change it is likely that the physical setting and conditions for this event may reoccur. The status and trends of hypoxia in the coastal embayments along the southern Swedish west coast is similar to the open Kattegat with some variation related to

how far into these embayments the hypoxia bottom water penetrates. The deep parts of the Öresund becomes hypoxic almost every year, while the shallower parts of the Öresund, Skälderviken and Laholm Bay only become hypoxic in the worst years.

Oxygen concentrations in the Swedish fjords on the Skagerrak coast have also decreased over longer time-scales, with a tendency for some recovery in recent year (Erlandsson et al. 2006; Figure 4.3). In the Gullmarfjorden, annual means were around 3.5 ml L-1 up to around mid 1970s when oxygen levels started

decreasing, reaching a level of about 2.0 ml L-1 _{in the late 1990s. These levels have}

then improved to about 3.0 ml L-1 _{in the most recent years. This recent improve-}

ment can be attributed to a combination of reduced primary production (Odd Lindahl presentation to panel) and the relatively favourable climatic conditions (expressed as lower NAO index) in recent years, which increased the salinity and density of the Skagerrak water replenishing the bottom waters of the

Gullmarfjorden.

4.3.2 Organic matter supplies and metabolism

The fate of organic matter produced in the surface layer can follow different pathways: remineralization in the surface layer, incorporation into higher trophic levels, or export to the bottom waters and sediments. Experimental studies using sediment traps have estimated annual sedimentation in the southern Kattegat to be 63 g C m-2 _yr-1 _{(Olesen and Lundsgaard 1995). Carstensen et al. (2003) estimated}

the annual sedimentation for the entire Kattegat to be 55 g C m-2 _yr-1 _{in an}

empirical model study, corresponding to 47% of the primary production. There are, 0 1 2 3 4 5 6 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010 o xy g en (m l l -1 )

Figure 4.3. Annual mean oxygen concentrations in the Gullmarfjorden with observations weighted according to their month of sampling (SMHI data).

however, differences in the methods used to estimate the sedimentation rate as well as interannual variation. Therefore, this value should be interpreted with caution. Another approach to assess sedimentation rates is to apply an empirical

relationship between measured primary production and sedimentation rate (Wassmann 1990). This relationship shows a progressively increasing export of production from the plankton that when applied to the primary production data from the Gullmarfjorden suggests that organic sedimentation rates may have increased four-fold since the 1950s (~25 g C m-2 yr-1) to the 1990s (~100 g C m-2 yr-1_{), even though primary production increased only three-fold (Lindahl 2002;}

Figure 4-4). The fraction of primary production that is exported from the surface layer similarly increased from ca. 30% to over 50%, although the extrapolation of the relationship from Wassmann (1990) to high primary production rates by virtue is uncertain. These studies indicate that sedimentation rates probably range from 50 g C m-2 _yr-1 _{in the open waters to 100 g C m}-2 _yr-1 _{in the coastal regions. These}

findings of different primary production rates in inshore compared to offshore waters are also supported by Rydberg et al. 2006.

Seabed oxygen consumption rates are estimated to range from 10 to 20 mmol O2

m-2 _day-1 _{from a variety of experimental and modelling studies (Rasmussen et al.}

2003 and references therein). Converting sedimentation rates from the literature, assuming all sedimenting organic material is respired, gives somewhat higher values (~20 mmol O2 m-2 day-1), suggesting that up to 50% of the sedimented

organic matter may be buried. However, these carbon and oxygen budgets are Figure 4.4. Estimated sedimentation in the Gullmar Fjord 1950 to 2007

uncertain. More investigations quantifying these rates over time and space would be useful for describing the effects of eutrophication on oxygen conditions.