Spatial variation in recruitment of native and invasive sessile species onto oyster reefs in a temperate soft-bottom lagoon

Abstract

Although spatial variability in recruitment is a strong force structuring many marine communities, relatively few data exist on recruitment variability in sessile oyster reef communities. In a soft-bottom lagoon in Virginia, we tested if recruitment differed among three reefs situated across a mainland-lagoon-barrier-island transect and among elevations (>90–80, >80–70, >70–55 and >55–20 cm below MSL) on the lagoon reef. The most abundant taxa (the invasive algae Gracilaria vermiculophylla and Codium fragile and the indigenous oyster Crassostrea virginica and algae Ulva curvata) had highest recruitment at the lagoon reef, where propagule supply was likely highest. The mainland reef had lowest algal richness (1.4–3.1 panel −1) and abundances (<4% cover) compared to lagoon and island reefs (3.3–6.5 panel −1, 14–20%), but there were no differences between sites for animals. Overall, animals and algae were equally dominant at the mainland reef, whereas algae dominated at lagoon and island reefs. High water turbidity and suspended solids are typical algal stressors at mainland reefs, and these may account for the low algal abundance in that region. For many species (at least 9 out of 14) differences in recruitment success were observed over elevation differences as small as 10–30 cm, e.g. G. vermiculophylla and C. fragile mainly recruited up to >70–55 and >80–70 cm respectively (probably limited upward by desiccation), U. curvata down to >70–55 cm (probably limited downward by grazing or competition), whereas C. virginica recruited at all elevations. Animal richness was highest at the two lowest elevations (2.0–2.5 vs. 1.1–1.8 panel −1), but there was no effect of elevation on algae (3–6 panel −1) because of species substitutions between elevation levels. Thus, as in rocky intertidal systems, spatial variability in recruitment is important for community structure on oyster reefs, and if biodiversity is considered an important reef conservation goal, managers should focus conservation and restoration on locations and elevations that support successful recruitment and survival of many different species.