Paul Wassmann
Norwegian College of Fishery Science
University of Tromso
NO-9037 Tromso
Norway

Extended abstract

The complex interactions of nutrients within pelagic food webs are crucial for the determination of coastal eutrophication and thresholds of environmental sustainability. Any evaluation of the role of nutrients for environmental sustainability has to be carried out in a holistic perspective where (a) atmospheric deposition, (b) agricultural activities, (c) human settlements and business, (d) freshwater run-off, seepage and (e) coastal ecosystems dynamics are jointly considered in proportions that are adequate to their contribution. However, this is only possible in a few European regions because of lack of adequate data to evaluate the relative role of a-d. Furthermore, the traditional separation between fields in science, institutions, agencies and ministries, keeping the fields of a-d separate, prevents the necessary integrative view of the eutrophication and sustainability problem. In coastal regions most of the nutrient supply is due to atmospheric input and river-run-off. Fossil fuel (NOx) and agriculture is most often the main source for the former (ammonium emissions from husbandry). The application of fertilisers and manure in the drainage basin is often the most important source of the latter. In the grand picture, human settlements play only a local role for N and P supplies. The forcing of large-scale coastal eutrophication is in many regions caused by agricultural activities and industrial forms of food production, in particular meat production, while towns and tourism can play important, local roles.

Eutrophication effects on phytoplankton and thresholds of environmental sustainability are dependent on nutrient concentrations and nutrient composition, but also closely related to the grazing capacity of the pelagic community through top-down regulation. Increases in suspended phytoplankton are traditionally described as blooms. A bloom depends on critical nutrient concentrations, but when blooms become dense and algae sticky, critical thresholds exist where they aggregate and sink into deeper water and to the bottom. Thus eutrophication can result in increased vertical export, food supply to benthic organisms and increased oxygen consumption in bottom waters (which in the end can result in anoxia). The annual amount of primary production defines threshold intervals for vertical carbon flux and carbon loads that are of importance for the biomass accumulation in the surface layer and benthic boundary layer oxygen conditions. Theories exist that define phytoplankton blooms, primary production and vertical export in terms of retention lines, export loops and balance points. However, phytoplankton blooms are not only a function of nutrient supply and nutrient concentration, but also by key zooplankton species and top-down regulation. Depending on the top-down grazing pressure the phytoplankton concentrations in different regions can be different though having the same nutrient supply. In contradiction to common sense, the Mediterranean Sea accumulates far less phytoplankton than the Baltic or Norwegian Sea when stimulated with the same amount of nutrients. A recent in situ experiment in the P limited eastern Mediterranean Sea where phosphate was added to stimulate phytoplankton growth resulted in no increase in phytoplankton biomass (“blue water”), but rapid uptake of P by bacteria. Bacteria compete with phytoplankton and microzooplankton grazing does the rest. Again, in the North Atlantic one would expect a different scenario with “green waters”, while in the eastern Mediterranean “blue waters” prevail. This discrepancy is the basis of intensive debate. The most probable explanation is that the impact of the biota that grazes and mineralises phytoplankton has a far stronger impact in the Mediterranean than in the North Atlantic (probably due to a more active microbial food web). Thresholds for nutrient supply are thus different in different regions and ecological scenarios. The dominant phytoplankton species are different, their ecological characteristics, key zooplankton species and top-down regulation vary. Care must be taken to transfer knowledge from one region to the other. For the Mediterranean and the Black Sea regions this knowledge is limited, not easily accessible or unavailable.

There are not only thresholds for the amounts of nutrients a water body can cope with or how much phytoplankton biomass can accumulate, but there are also thresholds that determine the extent of pelagic-benthic coupling. There is a positive curvilinear relationship between primary and vertical export of biogenic matter. For example if primary production increases by 50 % due to eutrophication, vertical export can increase by 100%. This increase in vertical flux can be sufficient to induce bottom water anoxia with associated effects on bottom organisms and demersal fish. In different ecosystems the curvilinear relationship between primary production and vertical export is most likely different, but our current knowledge from the Mediterranean and Black Seas is too weak to come to any conclusion with regard to thresholds of pelagic-benthic coupling.

Some marine environments favour organisms that, from a human perspective, are considered as wasteful dead ends of the food chain. These organisms, mainly gelatinous plankton, compete with fish for zooplankton and thereby channel marine production away from fish predators. This was demonstrated by the dramatic shift from fish to gelatinous predators in the Black Sea, and the associated decline in fishery harvest. Most scientists have suggested that a recent outbreak of the gelatinous predator Leidy’s comb jellyfish that resulted in the collapse of the Black Sea food web was caused by its introduction in ballast water. While there is little doubt that Leidy’s comb jellyfish was introduced from north American waters, it is noteworthy that there have been several outbreaks of the indigenous jelly fish Aurelia aurita in the decades prior to this recent devastating outbreak of Leidy’s comb jellyfish. The most probable hypothesis explaining why gelatinous organisms may out-compete fish is that gelatinous planktivores improve their competitive ability against fish along with decreased visibility (e.g. though eutrophication) in the water column. This is because tactile feeding in gelatinous organisms does not depend on vision while feeding in fish generally does. Recent research has shown that visual predation is much more effective than tactile predation unless vision is limited. There is a surprisingly high correlation between fish abundance and water transparency for a 32-year Black Sea time-series. The results suggest that interaction strength of visual predators, and thereby marine pelagic food web structure, is strongly influenced by light and optical properties of the water column.

Light thus influences the competitive relations between major functional groups of marine ecosystems. Anthropogenic or naturally induced changes in light intensity and water transparency might lead to a simultaneous change in a large number of interaction strengths in pelagic food webs. This is likely to affect marine food web structure. It is perceivable that reduced visual predation will stimulate growth of tactile gelatinous predators through reduced exploitative competition. The decrease in light caused by eutrophication may result in decreases in fish stocks and fish catches despite the greater availability of food. The role of nutrients for environmental sustainability and economically important resources such as fish may also be governed by changes in light availability, which militate against fish above certain thresholds. Again, thresholds seem to exist that, if exceeded, give rise to ecosystem changes. In the worst case points of no return are reached where a ecosystem does not fall back to its original state when eutrophication pressure ceases.

Eutrophication and thresholds of environmental sustainability may remain unexplained unless integrated approaches to drainage basin nutrient inputs and coastal eutrophication are taken into account in the decision taking. Further, if phytoplankton growth and accumulation is only understood by bottom-up regulation, the thresholds for excessive phytoplankton blooms caused by eutrophication cannot be determined. Top-down regulations of plankton communities, in particular the role of functional key species, and cascading effects through the food web of basically different ecosystems have to be evaluated. In order to address the role of nutrients for environmental sustainability we have to improve the co-operation between traditionally separated fields of science, all contributing and determining the eutrophication of rivers and coastal ecosystems: (a) agriculture and land use, (b) point source emissions, i.e. sewage discharge from urban areas and industry, (c) atmospheric deposition and (d) groundwater seepage. A few national and European initiatives have been taken to apply such an approach, but few attempts have been made to couple these processes in an integrated manner in the Mediterranean and Black Seas. Environmental quality and sustainability in the coastal zone is directly connected with agricultural, industrial, logging, tiling and other human activities in the drainage basin, as reflected by eutrophication. Under the influence of man, nature has turned into culture and it is useless to ask if we wish to live in an undisturbed ecosystem or not. We can only ask what quality our coastal ecosystems should have and which are the thresholds that limit sustainability.

While agriculture is strongly subsidised, overproduction of food a problem and claims to reduce nutrient discharge from agriculture and husbandry inadequate in all European countries, we continue to suffer eutrophication-induced losses in biodiversity, ecosystem degradation, reduced (non-subsidised) coastal fisheries and aquaculture and harmful algal blooms. Environmental sustainability can be improved, subsidies reduced and money earned by integrating traditionally separated sectors. Science, agencies and ministries have to co-operate more thoroughly. Calculations of the substantial economical and environmental losses that are caused by lack of integration have to be produced.