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Abstract

Population size of Ephemeroptera, Plecoptera and Trichoptera (EPT) emergence in a given year and the assemblage of the community are predictable based mainly on the discharge pattern of the previous year. We analysed the relationships between precipitation, discharge. water temperature and the emergence of 41 EPT species over 25 years from the Breitenbach, Central Germany. We assumed that the number of emerging individuals does not depend on a single event but was determined particularly by the annual pattern of key variables during larval development in the previous period. Precipitation, discharge and water temperature were tested for variation among years (predictability), predictable persistence (constancy), and predictable fluctuation (contingency). Water temperature had a clear seasonal pattern, precipitation and discharge were rather constant. Precipitation was mediated through the climatic conditions in the catchment into a seasonal discharge pattern. Discharge strongly affected the community. We classified the yearly discharge patterns (1969-1993) into four different groups: (i) permanent 'good flow', (ii) winter and spring spates, (iii) permanent low flow, and (iv) floods during 'unusual' periods (non-seasonal); patterns were discriminated by mean discharge, the yearly sum of the standard deviation of monthly discharge maximum, and the yearly sum of the monthly coefficient of discharge stability (an inverse of the coefficient of variation) within the respective group. The long-term constancy of the EPT populations was determined using the inverse of the coefficient of variation as well. Caddis flies with heavy mineral cases, large stone-flies, and abundant may-flies had stable populations. Variable populations included fine particle feeders and caddis flies with net spinning larvae or light cases. Site scores (based on species abundance) in indirect gradient analysis were best associated with groups defined by the discharge pattern in the year before emergence. Years with permanent good flow, winter and spring spates, or low flow were ordered along the horizontal axis; years with unpredictable (non-seasonal) flow were shifted along the vertical axis. Direct gradient analysis found water temperature, precipitation and discharge in the winter before emergence, and discharge in the summer of the respective year, and of the year before to influence the EPT emergence. High or low abundance of many species was associated with one of the discharge patterns. Community diversity was highest in years with winter and spring floods, and lowest in years with random floods. Differing discharge patterns forced the community into different community "states".