Dynamics and ecological significance of daily internal load of phosphorus in shallow Lake Balaton, Hungary VERA ISTVA ´ NOVICS,* ANDRA ´ S OSZTOICS † AND MA ´ RK HONTI* *Water Resources Management Group of the Hungarian Academy of Sciences, Department of Sanitary and Environmental Engineering, Budapest University of Technology and Economics, Budapest, Hungary † Department of Civil and Environmental Engineering, Budapest University of Technology and Economy, Budapest, Hungary SUMMARY 1. As supported by field data, turbidity recorded by light scattering sensors could reliably be converted into concentration of suspended particulate matter (SPM) and coefficient of vertical light attenuation (K d ) in Lake Balaton. 2. Autocorrelation analysis revealed that proper determination of SPM concentration and K d required daily sampling. To approximate daily rate of resuspension, 15 min or more frequent measurements were needed. Thus, routine monitoring provides very little insight into environmental variability of shallow lakes as habitats for phytoplankton. 3. The internal P load was estimated from daily rate of resuspension and P desorption capacity of sediments. The latter was assumed to be proportionate to the potentially mobile inorganic P content of SPM. A comparison with net primary production and nutrient status of phytoplankton showed that the proposed method of estimating time series of internal P load captured seasonal trends. 4. The daily rate of resuspension was high whereas that of internal P load was low in Lake Balaton relative to other shallow lakes. The latter reflects favourable behaviour of the calcite-rich sediments. As a consequence, carrying capacity of Basin 1 of Lake Balaton was P-determined. 5. The timing of external and internal loads was radically different. While the former showed mostly seasonal changes, large pulses characterised the latter. As a consequence, internal load may supply more P to phytoplankton growth during the critical summer months than external load. However, the relative importance of these sources may show substantial interannual variability. 6. Large resuspension events often followed each other during periods of 10–15 days. It has been shown that disturbances in this frequency range are of key importance in maintaining the diversity of phytoplankton. We propose that resuspension can be perceived not only as a disturbance factor but also as a factor that periodically relaxes nutrient stress. The former feature may dominate the instantaneous effect, whereas the latter may determine the persistent effect of resuspension on succession of phytoplankton. Keywords: light attenuation, nutrient status of algae, P desorption, pulses in P load, resuspension, turbidity Introduction In most shallow lakes, internal P load is the principal component of P supply for phytoplankton growth during summer. When the carrying capacity of a lake is P-determined (Reynolds, 1992), internal load is a key factor in regulating year-to-year variability of Correspondence: Vera Istva ´novics, Department of Sanitary and Environmental Engineering, Budapest University of Technology and Economics, H-1111 Budapest, Mu ˆ egyetem rkp. 3., Hungary. E-mail: istvera@goliat.eik.bme.hu Freshwater Biology (2004) 49, 232–252 232 Ó 2004 Blackwell Publishing Ltd