The Relationship between Green Sea Urchin Spawning, Spring Phytoplankton Blooms, and the Winter-Spring Hydrography at Selected Sites in Maine

The relationship between green sea urchin spawning, spring phytoplankton blooms, and hydrography were examined at multiple spatial scales during the winter-spring of 2000 at selected sites along the coast of Maine. To determine factors contributing to the variation observed in the timing of green sea urchin spawning, sea urchins, phytoplankton, and oceanographic variables were sampled biweekly at four sites in central Maine and three sites in eastern Maine. Water column properties and phytoplankton was intensively examined at sites in central Maine, while sites in eastern Maine were less well characterized. Analysis of gonad indices showed that spawning was protracted in central Maine (occurring from late February to May and encompassing a period of 60 + days), while spawning was relatively discrete in eastern Maine (occurring from April to May and encompassing a period of 34 - 50 days). Despite significant variations in gonad indices between sites, changes in gonad indices were synchronous between males and females within each site. Female gonad indices were significantly greater than males during the peak of the spawning period, although this difference diminished over time and varied between sites. Spawning was significantly correlated with both the first, sustained increase in phytoplankton chlorophyll a and with increasing water temperatures at most sites. The strength of this relationship, however, varied between males and females and between sites. Further, sea urchin spawning times were similar between sites despite significant differences in temperature regimes (5-6 C in central Maine versus 4-5 C in eastern Maine) and water column properties. The coastal waters surrounding the sites in central Maine Islands during the winter-spring 2000 were characterized by high concentrations of inorganic nutrients (Si04 \u3e 8 pM; NO3 + NO2 \u3e 5 pM) and low phytoplankton standing biomass (chl a \u3c 2 pg/L) and cell abundance (\u3c 5 x lo3 cells L -\u27) within a wellmixed water column. Phytoplankton abundance during 2000 exhibited trends inconsistent with a typical, pronounced spring phytoplankton bloom, which suggests that blooming phytoplankton may not be a reliable proximate spawning cue. Despite the relatively consistent pattern, there is considerable variability in the timing, duration, and environmental correlates, especially water temperature and chlorophyll a, of spawning. The timing of spawning in the green sea urchin may influence the recruitment of this species, which furthermore may have important ecological and economic implications. Futhermore, micro- and meso-scale processes affect both phytoplankton bloom dynamics and sea urchin spawning, and the interaction between these factors may result in locally disparate or atypical patterns