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Eutrophication, Primary Production and Vertical Export

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Eutrophication, Primary Production and Vertical Export Chapter 9 Eutrophication, primary production and vertical export 10000 1000 ) -1 y -2 s m 100 e l o mm 10 Freshwater wetlands and lakes P inputs ( Forests Paul Wassmann 1 Agroecosystems Estuaries and Norwegian College of Fishery Science coastal waters University of Tromsø 0.1 N-9037 Tromsø, NORWAY 10 100 1000 10000 10000 N inputs (mmoles m-2 y-1) [email protected] Figure 9.1: Nitrogen and phosphorous loadings to differ- ent types of ecosystems (redrawn from Eos, 1992). 9.1 Introduction The effects of global and local changes are most across the land-sea margins at such high rates that prominent at the land-sea margins where presently coastal waters and estuaries are the most fertilized population growth is greatest. For example, the ecosystems on earth (Figure 9.1). population of coastal counties of the USA has The discharge of nutrients to the coastal zone roughly doubled since 1960 (Eos, 1992). This gives increased strongly during the last centennial. The rise to increased pressure on natural resources increase in human population, the use of fertilis- and a large number of disturbances to coastal re- ers, increased intensity in agriculture, logging and gions. Presently, eutrophication of coastal wa- increased atmospheric deposition are the main ters is probably the most important environmen- cause for this intensification. However, signifi- tal effect (Gesamp, 1991). The effects of nutrient cant periods of eutrophication took place much enrichment thoroughly change coastal ecosystems earlier. Already during pre-Roman time signifi- and occur virtually worldwide. Nutrients move cant amounts of the mixed woods in the Mediter- 126 9.3. PRIMARY PRODUCTION AND VERTICAL EXPORT 127 ranean disappeared. During pre-Viking times nat- nutrient resources (Richardson & Jørgensen, ural woods disappeared in Denmark, during the 1996) Middle Ages most of the original woods from cen- tral and northern Europe. Europe developed into 2. An increase in the rate of organic carbon pro- the cultural landscape that we encounter today, duction in an ecosystem (Nixon, 1995) unrecognisable and widely different compared to the pristine state. The removal of woods and In most cases we use definition 1 and this will the introduction of agriculture had a strong im- also be the case in the present text. Thus we fo- pact on the leaching of nutrients all over Europe cus upon the supply to and the dynamics of nu- and periodically eutrophication in the Baltic and trient resources in a water body. Eutrophication North Seas must have been significant over the can entail either the process or the result. One has last 1000 years. Cultural eutrophication, intense further to distinguish between natural eutrophica- as it may be at present, is thus nothing new, but a tion that is caused by winter accumulation, pre- close and well-known companion of human civili- cipitation, vertical mixing, upwelling, river run-off sation, mainly through the introduction of agricul- and entrainment of nutrients. Climatic variability ture that paved the road for human development obviously influences and modulates the nutrient and population explosion. availability of a recipient and natural eutrophi- Coastal ecosystems can accommodate large cation thus varies over time. The natural vari- amounts of nutrients, but there is certainly the ability in eutrophication is often poorly known, danger that increased loading gives rise to in- in particular because it may be camouflaged by creases in suspended biomass, far beyond the cultural eutrophication, which is any type of nu- range of natural concentrations. For scientists trient discharge caused by anthropogenic activity, and managers alike the question arises how much e.g agriculture run-off (see Sections 3), sewage, at- nutrient discharge a specific recipient can accom- mospheric deposition (see Sections 1), changes in modate per unit time before undesirable conse- water discharge etc. quences occur. Can we determine the primary production rates where excessive biogenic matter 9.3 Primary production and is exported to the bottom water and the sedi- ment that result in bottom fauna changes and ul- vertical export: Background timately anoxia? considerations Primary production consists of new production 9.2 Eutrophication (PN) that is based on allochthonous, i.e. exter- nally supplied nutrients, and regenerated produc- The term eutrophication derives from the Greek tion (PR), which is based on autochthonous, i.e. roots eu (‘well’) and trope (‘nourishment’) and internally recycled nutrients (Dugdale & Goering, could thus be translated into well-fed, well- 1967). Hence total primary production (PT) is the nourished. With the term eutrophication we im- sum of PN and PR. The amount of carbon that ply that the ecosystem, not an individual, is well- enters the aphotic zone is entitled export produc- nourished and that nutrients or biomass are sup- tion (PE) (Figure 9.2). plied to a particular recipient. Eutrophication is The concept of new production is of utmost im- not a clearly defined term and there are various portance for understanding natural and eutrophi- definition such as: cated ecosystems because the fraction f = PN /PT represents the upper limit of organic matter and 1. The process of changing the nutritional sta- energy which can be removed or extracted from tus of a given water body by increasing the the surface waters of the system without de- 128 CHAPTER 9. EUTROPHICATION, PRIMARY PRODUCTION AND VERTICAL EXPORT Given the practical difficulties in estimating P Total primary production N for lengthy periods of time, sediment traps can be used to estimate PN.PE estimates as measured by e sediment traps come close to PN, but are always n o New Regenerated z smaller because it comprises only the particulate production tic production ho fraction and some transformation takes place from p u E ammonium to nitrate even in the upper layers. Calculations of the productivity index f by ap- plying PE give, therefore, rise to underestimates Export production (Wassmann, 1993). As a consequence, the term (Sedimentation) e = PE/PT can be applied and used as an approx- imation of f. In boreal, coastal areas where steady Figure 9.2: New and regenerated production are based on (a) the supply of the limiting (allochthonous) nutri- state, if at all, cannot be assumed for intervals of ents from the aphotic zone, by advection, run-off or from less than the length of the productive period, e is the atmosphere (straight arrows) and (b) the recycled (au- meaningful as a base for estimating f for lengthy tochthonous) nutrients in the euphotic zone (circular ar- periods only (e.g. > 6 months). Therefore, the rows), respectively. New and regenerated production com- prise total primary production. Export production is the term < e >, representing e for lengthy periods of amount of sinking organic carbon at the bottom of the eu- time will be applied. photic zone. 9.4 Nutrient supply, primary stroying the long-term integrity of pelagic sys- production, retention and tems (Vezina & Platt, 1987; Iverson, 1990; Legen- vertical export dre, 1990). PN represents thus the biomass that has to be handled by an eutrophicated recipient Increased supply of nutrients to the euphotic zone (e.g. mineralization, accumulation, harvestable gives rise to increased production of algae which biomass and export of biomass to adjacent recip- sooner or later sink to deeper water and the sed- ients). Given the importance of PN for the over- iment, resulting in increased sediment-water ex- all cycling of organic matter, considerable empha- change rates, at times in mass mortality of macro- sis has recently been given to estimating PN in fauna and fish eggs (Rosenberg & Loo, 1988; Mor- coastal (e.g. Wassmann, 1990b) as well as oceanic rison et al., 1991) and finally in anoxia (Rosenberg, environments (e.g. Knauer et al. 1990). New 1985; Graf, 1987). During the last decades wide- production represents the carrying capacity of a spread occurrence of low oxygen concentrations marine ecosystem. New production represents the or anoxia in bottom waters, decreased catches of maximum production capacity of an ecosystem or fish and blooms of toxic algae threatening aqua- the harvestable production. New production is culture as well as stocks of wild animals have been a critical component of marine primary produc- reported with increasing frequency (Rosenberg & tion that limits the supply of food to the ben- Loo, 1988). These changes seem to be caused by thos, zooplankton, fish, and extensive aquaculture increased inputs of nutrients to aquatic areas from as well as the removal rate of atmospheric CO2 sewage, agricultural run-off and atmospheric fall- by the marine biota. New production estimates out, giving rise to various degrees of eutrophica- are of great interest for understanding eutrophi- tion of fresh-water as well as marine, coastal envi- cation. Increased new production results in addi- ronments (e.g. Wulff et al., 1990). tional biomass that the ecosystem has to deal with Figure 9.3 shows the principle processes of at- in terms of grazing, vertical export to the bottom mospheric CO2 uptake and release, primary pro- and pelagic and benthic degradation. duction, suspended biomass and vertical export to 9.4. NUTRIENT SUPPLY, PRIMARY PRODUCTION, RETENTION AND EXPORT 129 the bottom. Seawater takes up CO2 from the at- mosphere that is either taken up by phytoplankton or released again to the atmosphere. The phyto- plankton uptake of CO2 is caused by primary pro- duction and first and foremost dependent on pho- tosynthetic active radiation and nutrients. In ad- dition it is influenced by the residence time of phy- toplankton in the euphotic layer (determined by vertical mixing and stratification). Phytoplank- ton accumulates in the upper layers if grazing and degradation rates are lower than primary produc- tion, i.e. a bloom takes shape. A part of the CO2 suspended biomass, consisting of phytoplankton cells and detritus will inevitably escape grazing CO2 Phytoplankton and degradation and sink into the aphotic zone Euphotic and further to the bottom.
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