Subtidal Coexistence: Storms, Grazing, Mutualism, and the Zonation of Kelps and Mussels

Abstract

At exposed sites in the Gulf of Maine, USA, subtidal mussels (Modiolus modiolus) dominated space on upper rock surfaces at intermediate depths (11—18 m), but not at shallow depths (4—8 m), where dominants were the kelps Laminaria digitata and L. saccharina. Observations and experiments were conducted to test the following hypotheses about factors limiting the vertical (depth) zonation of mussels and kelp: (1) that the upward distribution of Modiolus is limited by interference effects of kelp, or (2) that it is limited by the failure of larval recruitment to shallow depths, and (3) that sea urchin grazing controls the downward distribution of kelp. The recruitment failure hypothesis was rejected. Results indicated that storm—generated dislodgement of mussels overgrown by kelp was the mechanism reducing the ability of Modiolus to maintain and hold space in the shallow kelp zone. Dislodgement following kelp overgrowth was the most significant mortality source of large mussels. Removal of sea urchins, Strongylocentrotus droebachiensis, from the lower edge of the kelp zone resulted in the downward shift of kelp to a 12.5 m depth, demonstrating that the lower depth limit of kelp is set by urchin grazing. Storm disturbances varied in frequency and severity of damage to mussel populations, and were an important agent of patch creation in the subtidal zone. Small storms occurred on a monthly basis. Severe storms struck three times in 1982 and twice in 1983; an average of 11 658 mussels with attached kelp were dislodged and cast ashore on monitored beaches during such storms. Storm disturbances were synchronous between coastal and offshore sites. The mean density of patches cleared in mussel beds (2.5 patches/0.25 m2) was highest after fall storms. Mean patch size ranged from 91.0 cm2 (fall) to 122.0 cm2 (winter). With the exception of urchin fronts at 10.5—12.0 m, sea urchin densities were significantly higher inside Modiolus beds than outside. The hypothesis that urchins increase Modiolus survivorship by grazing kelp off mussels and decreasing the risk of mussel dislodgement was tested by an urchin—removal experiment. Removal of urchins from mussel beds led to rapid kelp recruitment, resulting in a 30—fold increase of mussel mortality (via kelp—induced dislodgement) compared to control beds with resident urchins. Significantly more force was required to pull large urchins out of mussel beds than to remove them from rock surfaces outside the beds, suggesting that urchins are less susceptible to predation and dislodgement—caused mortality while in Modiolus beds. The Modiolus—Strongylocentrotus interaction is identified as a facultative mutualism that appears to facilitate the coexistence of kelp and mussels at shallow depths. The ability of kelp and mussels to recover from dislodgement disturbance was examined by clearing patches in mussel beds and algal turf. Kelps recolonized and dominated all algal—turf patches and 47% of mussel patches within 7 mo. In contrast, Modiolus did not close patches by recruitment, or by a leaning response of mussels surrounding the patch, over a 3—yr period. This suggests that the ability of kelps to bounce back from dislodgement disturbance may enhance their competitive superiority. For Modiolus, the rate of storm—generated disturbance exceeded the rate of recovery.