8. Ecological implications of discharged scrubber water on the Baltic Sea ecosystem
When making risk assessments of discharges of a wastewater to the ocean it is essential to have access to good experimental ecotoxicological data in combination with expected (modelled or measured) pollutant concentrations in the recipient water. However, a third aspect that is crucial but almost completely neglected, is the need for ecological interpretation of the obtained information. What will the consequences be if a copepod species has a slightly reduced reproduction potential, or if the proportion of fish larvae with malformed hearts or spines increases? What are the effects on a larger scale, in specific food webs or entire ecosystems? These questions could be asked from a short-term human perspective - will there be consequences for marine species caught for human consumption? They could also be asked from the perspective of the marine organisms themselves. In a long-term perspective these two perspectives will of course always merge. The survival of humankind is dependent on the survival of the marine food webs.
It has been claimed in some reports that scrubber water and other wastewaters from shipping once discharged into the ocean will quickly be diluted to the point where they are harmless (Kjølholt et al. 2012, IMO; MEPC 74/INF.24 2018). Data from the present study indicate that this is far from the case. Several toxic compounds were detected in the scrubber water and with continuous discharge of scrubber water to the sea, it is likely that elevated concentrations of these can be expected along ship lanes. Long stretches of ship lanes run close to shorelines, not least in the Baltic Sea. These shallow waters are among the most productive areas of the ocean, a permanent home to numerous species of invertebrates and fish, and nursery ground for the early life stages of many pelagic fish species such as cod and herring. The shallow coastal areas in southern Baltic Proper where all ship traffic in and out of the Baltic Sea run close to land, also serve as stopover for vast amounts of migrating birds that also feed on marine species. Many coastal species like mussels, oysters, and crayfish are used for human food consumption and serve as food for other species consumed by humans. It is not unlikely that a continuous supply of PACs and other contaminants in scrubber water to these areas has an impact on these ecosystems.
It is also important to understand that the organisms in the pelagic zone of the ocean are not stationary, but often cross large areas of water, by passive dispersion driven by wind, waves, and sea currents or by active swimming. The ship lanes cut like motorways through these areas where pelagic organisms drift or swim, many of them in their most sensitive embryonic or larval stages. Many of the invertebrate species have pelagic larval stages that last for several weeks. During that time, they can be transported long distances and are at great risk of crossing shipping lanes many times. The blue mussel (Mytilus edulis), a key stone species in most Scandinavian waters, stays as larvae for up to six weeks, and genetic analyses have shown that in the Baltic Sea the mussel larvae may disperse over either the entire Baltic Proper or the entire Bothnian Sea. The only barrier that cannot be crossed is the Åland archipelago (Larsson et al. 2017). Species spending their entire life span in the pelagic, such as the copepods that serve as a staple food are running the risk of drifting over ship lanes at any part of the life cycle. Copepods serve as staple food for herring and cod and declines in copepod populations have major impact on fish stocks (Beaugrand et al. 2003, Heath and Lough 2007, Stige et al. 2011). The Baltic cod reproduce at spawning sites in the central Baltic Proper, and as the fertilised eggs and newly hatched larvae are transported to the nursery areas by the coast (Bagge et al 1994), they cannot avoid crossing the busiest ship lanes in the Baltic Sea (HELCOM 2023. HELCOM Map and Data service. Layers: HELCOM AIS Shipping density maps. https://helcom.fi/baltic-sea-trends/data-maps/; (Bagge 1993) (Fig. 12). If the water in the shipping lanes is polluted enough there is a risk that they will serve as proper barriers for pelagic marine organisms and block the natural migration between sea areas.
When IMO has taken decisions to reduce the maximum allowed sulphur content in ship fuel, certain areas have been considered to be particularly sensitive to the air emissions and therefore been designated as SECAs within the MARPOL’s Annex VI which deals with prevention of air pollution. It has been decided that the sulphur content in ship fuel in these areas could be no higher than 0.1% instead of 0.5% as the regulation is in non-SECAs. In practice, this means that the scrubber water produced in SECAs will be more polluted than water produced in non-SECAs due to a larger amount of scrubber water being discharged to the SECAs. The Baltic Sea is one of these SECAs, but the irony is that IMO also has designated the Baltic Sea as a Particularly Sensitive Sea Area (PSSA) (IMO Resolution MEPC.136(53), 2005) which is defined by IMO as “an area that needs special protection through action by IMO because of its significance for recognised ecological, socio-economic, or scientific reasons and which may be vulnerable to damage by international maritime activities” (IMO, Resolution A.982(24); IMO web page: Particularly Sensitive Sea Areas (imo.org)). So, while the IMO formulated the SECA classification aimed at protecting the terrestrial environment and human health, the PSSA classification aiming at protection of the marine environment has been ignored. Due to the brackish water conditions in the Baltic, the ecosystems are naturally sensitive. Here are fewer species which make the ecosystem functions less resilient than truly marine ecosystems where the biodiversity is much higher (Magnusson and Norén 2012). In addition, it is an enclosed sea in a highly industrialised area and even before the introduction of scrubbers it was often claimed to be one of the world’s most polluted sea areas (HELCOM 2018, Stockholm International Water Institute 2018). What the Baltic Sea ecosystems need is a removal of pollutant sources instead of introduction of new ones.
Figure 12 Shipping lanes (red lines) crossing spawning (dark blue) and nursery areas (green) for cod in the Baltic Proper. The colour intensity of the red lines indicates the number of ships per km2, and year based on 2020 HELCOM AIS data.