Eutrophication and Oligotrophication angiosperms that directly or indirectly support aquatic food webs In freshwaters, phosphorus historically has been least abundant among the nutrients needed in large quantity (macronutrients) by primary producers (Hecky and Kilham, 1988; Wetzel, 2001) Thus, it often has been the first element to become limiting to primary productivity in many freshwater systems, with nitrogen being secondary in importance 10 000 P Inputs (mmol m−2 yr −1) 1000 100 10 Freshwater wetlands, lakes Forests Agroecosystems Estuaries, coastal marine 0.1 100 1000 10 000 100 000 N Inputs (mmol m−2 yr−1) Figure Phosphorus and nitrogen inputs to various types of ecosystems, showing the highest nutrient enrichment for estuaries and coastal waters Reprinted with permission from Wassmann (2005) Eutrophication, primary production and vertical export, Figure 9.1 In: Wassmann P and Olli K (eds.) Drainage Basin Inputs and Eutrophication: An Integrated Approach, pp 126–138 Norway: University of Tromso Available at: http://lepo.it.da.ut.ee/~olli/eutr/ Eutrophication.pdf (Figure 3) Total phosphorus (TP) is typically considered in evaluating freshwaters for P limitation, rather than inorganic P (Pi – phosphate, PO4 À3 ) which is the form most often used by algae TP is used because algae commonly take up and store more P than they need in a process called luxury consumption In natural systems or surface waters that are affected only by low anthropogenic inputs, PO4 À3 ions are rapidly taken up by algae and bacteria as fast as they are released, sometimes within seconds, so PO4À3 concentrations typically appear to be low (o5 mg lÀ1) yet offer only a ‘‘glimpse’’ of the P actually available for algal growth However, in areas that are strongly influenced by anthropogenic nutrient enrichment – such as sewage discharges from treatment plants with only primary or secondary treatment, or discharges from large phosphate mines – PO4À3 concentrations can be 0.5–2 mg lÀ1 or more Phosphorus also has been the most important nutrient-limiting plant growth in many tropical coastal marine waters and certain pelagic ocean waters (Downing, 1997; Burkholder et al., 2007, and references therein) In temperate and polar coastal marine environments, nitrogen historically has been the most important nutrient that first limits primary production Inorganic N (Ni), especially nitrate (NO3À) and ammonia (NH3) and its ionized form, ammonium (NH4ỵ ), is directly taken up by phototrophs, although certain organic N forms such as urea can also be important N sources (Hecky and Kilham, 1988; Smith, 2006) Similarly as for Pi, in some cases Ni concentrations may appear to be low in the water column, but the total availability can be much higher due to rapid regeneration and biological recycling At the estuarine interface between marine and freshwater habitats, both N and P can colimit plant production, especially in late winterspring seasons of high precipitation and accompanying high inorganic N inputs Other nutrients, notably silica and iron, 13 13 16 16 27 20 16 10 17 40 12 27 100 20 16 17 13 43 22 16 10 24 (a) 23 Chlorophyll a (µg l−1) Chlorophyll a (µg l−1) 23 100 349 43 23 12 40 24 13 10 50 TP (µg l−1) 100 200 50 (b) 100 500 1000 TN (µg l−1) 4000 Figure Relationship between summer maximum chlorophyll a content and (a) TP or (b) TN in surface and near-surface waters of some lakes in Japan Numbers next to points represent the N : P ratio (by mass) Note that lakes with exceptionally low and exceptionally high N : P ratios are outliers in the TP-chlorophyll a and TN-chlorophyll a relationships, respectively Similar log–log relationships between TP and chlorophyll a, and between TN and chlorophyll a, are characteristic of many natural lakes in the world that have low abiotic turbidity Modified by Sakamoto (1966) in Kalff J (2002) Limnology, Prentice Hall, Upper Saddle River, NJ, with permission from Jacob Kalff and Pearson Education