The effect of environmental factors on field and laboratory germination ina population of Zostera marina L. from southern England

The effects of temperature, oxygen availability and salinity on seed germin ation in a population of Zostera marina L. from the Fleet, a tidal lagoon i n southern England, were investigated in the laboratory. The fate of seeds under natural conditions was also monitored throughout the year from core s amples and from seed samples buried in the sediment layer in nylon sachets. Supported with temperature, salinity and oxygen level measurements at the study site, field data confirmed that following seed dispersal in September /October, the majority of seeds germinated in the Fleet during the winter a nd early spring. In the laboratory, seeds germinated more rapidly and to a higher final percentage under anaerobic conditions compared to aerobic cond itions. Under anaerobic conditions, rapid, near complete germination was po ssible at full salinity (30 g l(-1)) or when the salinity was reduced by 50 % (15 g l(-1)). Under aerobic conditions, seeds germinated more rapidly at lower salinity. The effects of temperature were strongly dependent on the p resence or absence of oxygen. Under anaerobic conditions, there was a prono unced optimum constant temperature for germination around 6 degrees C, corr esponding closely to the average water temperature recorded during the peri od mid-October to the end of March. On the other hand, under aerobic condit ions, very low levels of germination were recorded at low temperatures. The se findings support previously published evidence that seeds of Z. marina a re well adapted to germinate in anaerobic sediments and that water temperat ure is the key environmental variable that determines the timing of germina tion under natural conditions. Previous misuse of dormancy terms to describ e seed populations of Z. marina is discussed.