A 100-year record of changing Pseudo-nitzschia species in a sill-fjord in Denmark related to nitrogen loading and temperature

We present a detailed record of shifts in species composition of the toxic diatom genus Pseudo-nitzschia from 1905 to 2001 in Mariager Fjord, Denmark, most probably driven by changes in nitrogen loading to the coastal environment and increasing temperatures in the sea. The fertile Mariager Fjord is a long narrow sill-fjord with a permanently anoxic basin. The fjord has experienced increased nutrient loading during the last 100 years and is presently highly eutrophic. 210Pb-dated sediment cores from the anoxic basin in Mariager Fjord were used in a palaeoecological study to explore changes in species composition. Within the genus Pseudo-nitzschia we observed a change in species composition where Pseudo-nitzschia multiseries was dominant before 1947, when a shift occurred towards dominance by Pseudo-nitzschia pungens. At the same time an increasing relative abundance of Pseudo-nitzschia seriata and Pseudo-nitzschia americana was observed. The increase in relative abundance of P. pungens was positively correlated to nitrogen loading and suggests that increased nitrogen loading during the last 100 years contributes to the observed shift. We hypothesise that increased water temperature in the area could also have contributed to the observed shift in species composition. Three of the species identified (P. heimii, P. americana, P. pseudodelicatissima) are new records for Denmark and Scandinavia. One of these species, P. americana, has been linked to increased ammonium concentrations. The preservation of the frustules was excellent and valves of Pseudo-nitzschia were identified to species level in sediment dating as far back as 7900 years before present.