Effects of accumulations of sediments and drift algae on recruitment of sessile organisms associated with oyster reefs

Increases in sediment and drift algae accumulations have caused degradation of coastal lagoons worldwide. It is well known that these factors are stressors of seagrass beds, sediment fauna and coral reefs. However, little is known about the impacts on temperate hard-bottom assemblages within soft-bottom lagoons. To test if accumulations of sediment and drift algae (stress) affected recruitment of sessile oyster reef organisms, we constructed cages in Hog Island Bay, Virginia that trapped drifting macroalgae (≈ 2.7 kg WW m²) and facilitated sedimentation (≈ 7 mm per 2-3 month). The stress treatments and unstressed controls were placed in front, between, and behind reefs (position) to represent wave exposed (≈ 0.3 m, windy conditions), current exposed (≈ 0.2 m s^− 1, peak tide) and protected (≈ 0 m, 0.0 m s^− 1) habitats. The percentage cover of recruited taxa onto bricks was mapped 4 times during a 1-year period. There were strong significant effects of stress on the total assemblage, plant (but not animal) richness, total plant and animal cover, and cover of the most common taxa. Unstressed bricks had high plant richness, high animal and plant cover, and high cover of the oyster Crassostrea virginica, the alien algae Gracilaria vermiculophylla and Codium fragile, the alga Agardhiella subulata, and high to medium cover of the opportunistic algae Ulva curvata and Enteromorpha spp. In comparison, sediment-stressed bricks had low plant richness, low animal and plant cover, and low cover of C. virginica, G. vermiculophylla, C. fragile, A. subulata, U. curvata and Enteromorpha spp. Similarly, algae-stressed bricks also had low cover of animals, C. virginica, G. vermiculophylla, C. fragile, and A. subulata, but intermediate plant richness and plant cover and high cover of U. curvata and Enteromorpha spp. Although reef position caused significant multivariate results, this factor was clearly less important than the stress factor. Our study shows that accumulations of sediments and drift algae have an adverse impact on sessile temperate reef organisms, reducing richness and abundance, but favoring a few small opportunistic taxa. As the reef-generating oysters themselves performed poorly under these stressors, the long-term impact of the causes of these stressors, eutrophication and urbanization, is likely to be diminished reefs with cascading adverse effects on sessile reef organisms.