Phytoplankton,pelagic community and nutrients in a deep oligotrophic alpine lake: ratios as sensitive indicators of the use of P-resources (DRP:DOP:PP and TN:TP:SRSi)

The different use of P-resources between two sites in the deep oligotrophic Traunsee was studied by seasonal and vertical patterns of phytoplankton and nutrients from 12/1997 to 10/1998. The P-resources were evaluated from the proportion between the P-fractions, the dissolved reactive P (DRP), dissolved non-reactive P (DOP) and particulate organic P (PP) and from the stoichiometry between nutrients, the total N (TN), the total P (TP) and soluble reactive Si (SRSi). Significant differences between an inshore site impacted by industrial tailings (Ebensee Bay, EB) and an open water reference site (Viechtau, VI) were evident from vertical profiles of both the P-accumulation (%PP of TP) evaluated by DRP:DOP:PP and the distribution of phytoplankton assessed by Si-exhaustion (TN:TP:SRSi), but not from the seasonal patterns of phytoplankton composition, S:V ratios of the algal community or surface layer nutrient dynamics. Low TP and the stable stratification from May to September triggered the relative accumulation of epilimnetic P at VI as it was evident from both the higher portion of particulate P within TP (%PP of TP) and from the shift towards P-enrichment in nutrient stoichiometry of TN:TP:SRSi. The predominance of around 55-52% algal carbon over bacteria at the surface layer to 20m coincided spatially with the lowest Si content relative to N and P. The disturbances at the impacted site was summarised by: up to 11% less P accumulation by organisms at the surface, no stoichiometric shift towards TP in the epilimnion when compared with deeper layers and a reduction of the trophogenic zone to the top 10m. Reasons for this disturbance are seen in the unstable stratification, turbidity, higher TP and the metazoan dominated food chain. Both triple ratios, DRP:DOP:PP and TN:TP:SRSi, were sensitive indicators of the use of P-resources by plankton communities, while inorganic dissolved fractions (DIN:DRP:SRSi, DIN = dissolved inorganic N) provided only insufficient information on nutrient resources in Traunsee.