930  2002 Estuarine Research Federation Estuaries Vol. 25, No. 5, p. 930–937 October 2002 Phytoplankton, Nutrients, and Transparency in Danish Coastal Waters SØREN LAURENTIUS NIELSEN 1, *, KAJ SAND-JENSEN 2 ,JENS BORUM 2 , and OLE GEERTZ-HANSEN 2, † 1 Department of Life Sciences and Chemistry, Roskilde University, P. O. Box 260, DK-4000 Roskilde, Denmark 2 Freshwater Biological Laboratory, University of Copenhagen, 51 Helsingørsgade, DK-3400 Hillerød, Denmark ABSTRACT: We present a comparative analysis of 1400 data series of water chemistry (particularly nitrogen and phos- phorus concentrations), phytoplankton biomass as chlorophyll a (chl a) concentrations, concentrations of suspended matter and Secchi depth transparency collected from the mid-1980s to the mid-1990s from 162 stations in 27 Danish fjords and coastal waters. The results demonstrate that Danish coastal waters were heavily eutrophied and had high particle concentrations and turbid waters. Median values were 5.1 g chl a l -1 , 10.0 mg DW l -1 of suspended particles, and Secchi depth of 3.6 m. Chlorophyll concentration was strongly linked to the total-nitrogen concentration. The strength of this relationship increased from spring to summer as the concentration of total nitrogen declined. During summer, total nitrogen concentrations accounted for about 60% of the variability in chlorophyll concentrations among the different coastal systems. The relationship between chlorophyll and total phosphorus was more consistant over the year and correlations were much weaker than encountered for total nitrogen. Secchi depth could be predicted with good precision from measurements of chlorophyll and suspended matter. In a multiple stepwise regression model with ln-transformed values the two variables accounted for most of the variability in water transparency for the different seasons and the period March–October as a whole (c. 80%). We were able to demonstrate a significant relationship between total nitrogen and Secchi depth, with important implications for management purposes. Introduction Nitrogen has often been regarded as the main limiting nutrient of phytoplankton production in temperate coastal marine areas (Ryther and Dun- stan 1971; Grane ´li 1978; Perry and Eppley 1981; Boynton et al. 1982, 1996; Kemp et al. 1990; Baird et al. 1995; Galloway et al. 1996; Nixon 1997), but arguments for phosphorus limitation (Redfield 1958; Smith 1984; Conley et al. 1995; Glibert et al. 1995), co-limitation by nitrogen and phosphorus (Eppley et al. 1973), or seasonal shifts between phosphorus and nitrogen limitation (McComb et al. 1981; D’Elia et al. 1986; Malone et al. 1996; Conley 1999; Fisher et al. 1999) have also been presented. The case for nitrogen as the main lim- iting nutrient is supported by the fact that the ratio of dissolved inorganic nitrogen to dissolved inor- ganic phosphorus (DIN:DIP) in coastal marine wa- ters is usually well below the Redfield ratio (Red- field 1958; Ryther and Dunstan 1971; Grane ´li 1984). In some cases, several biogeochemical pro- cesses drive coastal marine areas toward nitrogen limitation as a result of a preferential loss of nitro- * Corresponding author; tele: +45-4674 2277; fax: +45-4674 3011; e-mail: nielsen@ruc.dk. † Current address: 3 COWI A/S, 2 Parallelvej, DK-2800 Kon- gens Lyngby, Denmark. gen relative to phosphorus (Nixon et al. 1980; Car- aco et al. 1990; Galloway et al. 1996). In other in- stances, terrestrial runoff to coastal marine areas is enriched in phosphorus, compared to the Redfield ratio ( Jaworski 1981; Nixon 1997), but this could be changing in some coastal areas due to increased tertiary treatment of sewage over the last decade. Indications of an increasing importance of phos- phorus limitation have been shown for some Dan- ish coastal areas (Conley et al. 2000). In temperate lakes, phytoplankton biomass and production is often phosphorus limited (Dillon and Rigler 1974; Prepas and Trew 1983; Masson et al. 2000), and several simple but powerful empiri- cal lake models have been established relating phosphorus loading or phosphorus concentrations to phytoplankton biomass and transparency (McCauley et al. 1989; Jeppesen et al. 1990; Mas- son et al. 2000). Models based on phosphorus loading are generally not a good predictor of net primary production in estuaries, apart from a few instances (e.g., Jaworski 1981; Lee and Jones 1981; Boynton et al. 1982). Models based on nitrogen loading are more suitable and have been devel- oped during the last 10–15 yr (Howarth 1988; Boynton et al. 1996; Nixon 1997). Many coastal marine areas are shallow and high- ly influenced by resuspension of sediment parti-