Seasonal growth rate and population dynamics of a freshwater sponge

Five sites of various water depths on four transects were sampled on a seasonal basis to determine Ephydatia fluviatilis population dynamics. The temporal occurrence of life cycle events was influenced by factors (e.g. water temperature) that varied with water depth. The shallow-water (< 1.0 m deep) portion of the population was characterized by seasonal standing crop fluctuations and fall gemmulation. Gemmulation was rare and standing crop relatively constant in the perennial deep water (>1.5 m deep) portion of the population. Sexual reproduction occurred in the spring. Seasonal growth rates were determined for 45 E. fluviatilis colonies on 6 rocks. Growth was maximal during late spring and summer at a mean rate of 26.75% increase in area per week. Growth rates during the winter were slow (2.0% increase/ week). Growth rates were not significantly different between small (<4.0 mm²) and large (>4.0 mm²) colonies. Seasonal changes were positively correlated with colony growth rates, whereas, micro-habitat parameters that may vary from rock to rock within a small area were not correlated.