Environmental Control of Green Sea Urchin, Strongylocentrotus droebachiensis, Spawning in the St. Lawrence Estuary

Environmental factors and spawning of the green sea urchin, Strongylocentrotus droebachiensis, were examined during 1983 and 1984 in the St. Lawrence Estuary. In both years, spawning occurred in June, which contrasts sharply with the February to early May spawnings reported for other locations. This difference does not appear to be related to temperature, but to the much delayed spring increase of phytoplankton in the Estuary. In both 1983 and 1984, sea urchin spawning coincided with the first marked and sustained increase in phytoplankton abundance (chlorophyll a levels of 1–2 mg∙m−3 for > 3 d) which took place about 2 wk before the main spring bloom; this first increase coincided with a sharp increase in salinity resulting from a decrease in freshwater runoff. Sea urchin spawned later in 1983 than in 1984. This difference did not appear to be related to either a critical temperature level or short-term temperature fluctuations; however, it did coincide with a greater freshwater runoff and a consequently delayed seasonal increase in salinity and phytoplankton. We hypothesize that onset of the phytoplankton bloom initiates spawning; freshwater runoff may indirectly determine the time of spawning by controlling when the bloom will occur.