INTERPRETACION Y ANALISIS DE LOS RESULTADOS

By the mid 1980s, nutrient inputs to the Kattegat and Skagerrak are believed to have increased by factors of 5-6 for nitrogen and 8 for phosphorus from the beginning of the 20th _{century (Conley et al. 2007, R. Rosenberg presentation to}

panel). As discussed above, significant strides have been made to reduce loadings to alleviate the undesirable effects of eutrophication in inland and marine waters and to improve air quality. The conceptual framework underpinning present

nutrient management plans is that ecosystems will return to their original state once the nutrient pressure is released. This managerial framework is, however,

challenged by emerging ecological theory that suggests that ecosystems respond in a non-linear manner to changing pressures leading to the existence of regime shifts between alternative stable states (see Section 5.3.2). In this section, we will investigate to what extent the marine ecosystems along the Swedish west coast are responding to decreasing nutrient inputs in a predictable manner.

5.2.1 Nutrient concentrations and ratios

Nutrient concentrations have decreased in both coastal and open waters in recent years as a response to reduced inputs from the land and atmosphere and advection (SEPA 2008a, Ærtebjerg 2007, Carstensen et al. 2006). The difference in the timing of nitrogen versus phosphorus reductions has led to changes in the N/P ratio that are most pronounced in the coastal areas, albeit also observable in the open waters (SEPA 2008a, Ærtebjerg 2007). In recent years the N/P ratio has

consistently been below the Redfield ratio of 16 on a molar basis (SEPA 2008a), typically around 5 for the open Kattegat (Ærtebjerg 2007), except in the Skagerrak where intrusions of N-rich water from the Jutland Coastal Current during summer months can raise this ratio. During the productive season (March-September) the open waters are potentially N-limited 100% of the time, whereas inorganic P is not always depleted from surface water, suggesting P limitation 80% of the period (Ærtebjerg 2007). The reduced concentrations of inorganic nitrogen and

phosphorus have changed the nutrient ratios in favour of higher silicate availability, suggesting that silicate is potentially limiting only in estuaries with large N and P discharges. In general, nutrient levels have declined in response to reductions in nutrient inputs as anticipated, and further reductions will increase the periods of both N and P limitation and alleviate potential silicate limitation even further.

5.2.2 Phytoplankton

Despite decreasing nutrient levels in recent years, there is no univocal response for phytoplankton. Chlorophyll levels in the open waters of the Kattegat have

remained at an almost constant level of about 2 µg L-1 _{according to Ærtebjerg}

(2007), whereas SEPA (2008a) found declines analysing phytoplankton biomass from a single station in the Kattegat, mainly due to reduced concentrations of dinoflagellates and nanoflagellates. The anticipated effects of nutrient reductions on phytoplankton biomass are therefore not consistently documented. Reduced pools of nutrients should conceptually lead to reduced phytoplankton biomass, but increasing turn-over rates of nutrients, reduced grazing of phytoplankton, and extended growing season could explain the lack of a clear phytoplankton biomass response.

Reduced inorganic nutrient levels should especially lead to reduced spring blooms and production, but given the strong dynamics of this phenomenon and infrequent sampling during spring, quantitative evidence for this hypothesis is not

straightforward. Assuming that the magnitude of the spring bloom has been reduced, then production should be relatively larger during the summer period. Seasonal patterns of primary production could indicate a shift in primary production from new production in spring to regenerated production during summer (Rydberg et al. 2006). That could explain the constant mean annual chlorophyll level that is observed. Increases in temperature will increase the turnover rate of nutrients in the surface layer, and mean surface water temperature in the Kattegat has increased by ~0.5 °C from the 1990s to the 2000s (Ærtebjerg 2007). Climate change may also have led to earlier development of the spring bloom that forms once the water column stabilizes, alleviating light as the limiting factor in the surface layer. An extended productive period will result in higher annual means. It should be noted that McQuatters-Gallop et al. (2007) found no decline in chlorophyll levels in the coastal North Sea following reductions in river nutrient loads and nutrient concentrations, which they attribute to the sea becoming warmer and clearer.

Grazing in the open waters of the Kattegat and Skagerrak is entirely pelagic due to the permanent stratification, whereas filter feeders are also potential grazers of phytoplankton in the coastal zone. Based on established grazing rate-to-biomass relationships, the mesozooplankton is potentially capable of controlling the average phytoplankton biomass in the summer period, but, due to their relatively long reproduction times, the mesozooplankton is incapable of promptly responding to bloom situations by increases in biomass. Another issue is the palatability of the phytoplankton. Due to the rather turbulent environments in the Kattegat and Skagerrak, the phytoplankton community is dominated by larger species, typically diatoms such as Skeletonema spp. and Rhizosolenia spp. and dinoflagellates such as Ceratium spp. Changes in phytoplankton communities and reduced grazing are another factor that could explain why phytoplankton biomass apparently has not declined with nutrient concentrations. Another explanation for the constant

chlorophyll levels is increased advective transports of large cyanobacteria blooms from the Baltic Sea in recent years. Particularly in 2006, large quantities of

Nodularia spumigena spread from the Baltic Proper into the Öresund and the

Kattegat.

The frequency of phytoplankton blooms, particularly harmful algae blooms or HABs is believed to have increased with eutrophication (Hallegraeff 1993, 2003), and for the Kattegat it has been documented that years with higher nutrient inputs and concentrations are likely to have more summer blooms (Carstensen et al. 2004). These findings suggest that the bloom frequency along the Swedish west coast should decrease as a response to the measures taken, but this has not yet been documented. It should, however, be acknowledged that only very few of the blooms on the Swedish west coast are considered harmful and the biomass of the blooms seldom reach levels that can be characterised as a nuisance.

5.2.3 Phytobenthos

Macroalgae and angiosperms are expected to increase their depth distribution with improving light conditions. Such improvements could therefore only be partially expected at present, because consistent reductions in phytoplankton biomass have not been consistently observed. Changes in the macroalgae community along the Swedish west coast have been reported (SEPA 2008a) with significantly decreasing depth distributions for two species only (Halidrys and Dilsea). The response of the entire macroalgal community has not been analysed. The experience from the coastal Danish monitoring program suggests little improvement in macroalgae and eelgrass depth distribution in some but not all coastal areas, despite significant declines in chlorophyll and improved light conditions (Ærtebjerg 2007). Rask et al. (1999) also reported improvements for eelgrass in the dry year of 1996 in which N inputs were less than one-half that for an average year. These results indicate a potential slow recovery, where colonisation of new suitable habitats takes

considerable time. It is believed that these results can be projected to the Swedish west coast as well, and that a slow gradual recovery may have started. However, the recovery time is not known.

5.2.4 Dissolved Oxygen

Oxygen conditions in the bottom waters have not improved despite reduced nutrient inputs and concentrations as described in Section 4.3. Increasing temperatures experienced in the region, perhaps related to climate change, counteract the anticipated improvements through reducing the supply of oxygen and increasing the metabolism. Another, perhaps even more important, factor is the reduced capacity of permanently removing nutrients in the sediment. Periods of low oxygen, particularly anoxia, reduces the nitrification-denitrification pathway of removing nitrogen. Changing the benthic community from deep-burrowing

macrofaunal organisms to hypoxia-tolerant species affects the sediments ability to remove nutrients through burial and denitrification (Diaz and Rosenberg 2008). The reduced ability to remove nutrients provides a nutrient feedback to the water

column that may sustain continued effects of eutrophication, i.e. a vicious cycle of hypoxia (Vahtera et al. 2007).

However, it should be acknowledged that nutrient inputs do have an effect on oxygen conditions (Conley et al. 2007) and that present oxygen conditions would likely have been worse if nutrient inputs had not already been reduced, but there is a need for further reductions to counteract the effects of global warming. Accor- ding to the empirical relationships in Conley et al. (2007), a 1 °C temperature increase would require a compensating nitrogen reduction of 20 kt of nitrogen. Conley et al. (2007) also suggested that regime shifts may have occurred and proposed that reoccurring large events of hypoxia may have a cascading effect in decreasing the oxygen concentrations. Similar consequences have been observed in Chesapeake Bay and the Gulf of Mexico (Conley et al. in press). Thus, for the oxygen conditions along the Swedish west coast not to deteriorate even further, additional measures to reduce nitrogen inputs must be taken to prevent regime shifts and counteract temperature increases.

Because dissolved oxygen conditions are the most important regulatory factor for benthic animal communities, it is not surprising that little recovery of these communities from regional eutrophication has been observed along the Swedish west coast (Rosenberg & Nilsson 2005). Nonetheless, benthic community recovery has been observed where direct organic loading has been abated or when physical conditions allow reoxygenation of basins (Rosenberg et al. 2002). Recovery may be delayed by the recruitment of larvae of deeply burrowing, long-lived species that characterize the healthy community. However, if these populations can re- establish they will advance the reversal of eutrophication by increasing denitrification and the sequestration of P in the sediments.

5.3. Other Significant Drivers Affecting