Environmental factors affectingHirschmannia viridis(O. F. Müller) (Ostracoda) in shallow brackish water

Abstract

The ecology of ostracods, particularly Hirschmannia viridis, living among green and brown algae in shallow water in Salviken, Øresund, was studied in 1966–69. The green algal zone in the area occurs on open shores and in rock pools. The latter offer extreme, at times fatal, conditions for the fauna. Temperature ranges between +32 and -1° C. In calm weather, temperature among algae closely follows air temperature, while it is considerably lower among brown algae. In a normal winter the ice cover has no adverse effect on the fauna. Salinity in open water ranges between 4 and 22.5%, with an annual mean of 9–10%. In small, temporarily isolated pools in the green algal belt, salinities may fluctuate between 0.5 and 40%. Diel oxygen variation among green algae embraces 8–180% saturation. The maximum value noted was 260%. The high tide slows down the fall in oxygen content at night. Periodical anaerobic conditions, due to accumulation of decomposing algae, are common in the green algal belt. When lasting long these periods exterminate the ostracod fauna. In connection with accumulation of decomposing algae, the pH in the green algal belt may decrease to 4–6. In the center of a green-algal patch on a stone the pH is normally 6.5 to 7. Hirschmannia viridis, one of the most abundant species, has a life span of one year. Larvae hatch in May–June, spend the winter in stage 7–8, mature in March–April, and die after copulation and egg-laying. The abundance of H. viridis among green algae varies with the periodical disasters which hit the biotope. The abundance on brown algae depends on the extent of epiphytic coverage and the life history of the brown algae. On green algae it is dependent on immigration from areas outside. An empty area is inhabited by a “normal” ostracod population after one week, and H. viridis is the first species which attain a “normal” density.