Shallow Soft Bottom Habitats
II. Community Zonation
In Monterey Bay, wave disturbance is correlated with the zonation of animal communities living in sandy sediments (Oliver et al. 1980). There are two broad zones: a shallow region dominated by highly motile peracarid crustaceans and a deeper area dominated by more sedentary polychaete worms.
These crustacean and polychaete zones are observed from San Diego to Washington and extend into much deeper water along the more wave-exposed open coast (Lie and Kisker 1970, Van Blaricom 1978, 1982, Oliver et al. 1980). The zones are more distinct in Monterey Bay, probably because of the relatively wave-protected environment. The zonation of benthic communities is best developed along the inner continental shelf, where environmental gradients such as wave disturbance are also steepest. However, there are also broad faunal zones in much deeper water, especially at the break between the continental shelf and slope. The nearshore zonation pattern in Monterey Bay is interrupted at the wave-protected ends of the Bay, in the submarine canyon, and around the extensive rock reefs that border much of the open coast (see III.)
The crustacean zone continues up into the surf zone and intertidal beach zone; two highly mobile sedimentary environments. The main peracarid crustacean groups include sand-burrowing amphipods (haustoriids and phoxocephalids) and surface-active cumaceans and ostracods. All burrow into superficial sediments, and flourish in wave-disturbed sand bottoms (Oakden 1981, Slattery 1980, Slattery 1985). In Monterey Bay, these groups and the crustacean zone are best developed in water depths of 10 meters. Here, very few animals live in relatively persistent burrows or tubes - most live close to the sediment surface and do not burrow deeply. There is also little biogenic structure on the sand surface such as burrow or fecal mounds, feeding or movement tracks, egg cases, emergent tubes or burrow casings. These are swept away by the first winter waves and benthic sand storms. Benthic fishes are also less abundant in the crustacean zone than further offshore (Kukowski 1973, Hulberg and Oliver 1978).
Sand dollars (Dendraster excentricus) form unique narrow communities along the subtidal beach at the shoreward edge of the crustacean zone (and see Sandy Beach Community section). Just seaward of the wave break, sand dollars are sparsely dispersed near the shore but form a dense bed with more than 1000/m² at the seaward border (Oliver et al. 1980). Although sand dollars can burrow into the sediment, they often extend more than half-disc up into the water, with many individuals forming a dense layer of upright shingles. The seaward border of the bed can end very abruptly and probably acts as a barrier to predacious sea stars, which can move over the bed only at significant risk of tumbling into the surf zone with a passing wave (Morin et al. 1985, Kastendiek 1982). A number of animals are most abundant in and around the sand dollar bed. Cancer crabs (Cancer gracilis) nestle into the bed but still become prey to wolf-eels (Anarrhichthys ocellatus) and other fishes (Hulberg and Graber 1980). Dense beds of small olive snails (Olivella pycna) form mucus sheets one to several meters across. The entire sand dollar bed migrates shoreward in the summer and seaward in the winter in response to seasonal wave action (Oliver et al. 1980, Morin et al. 1985, Kastendik 1982).
Sand dollar beds are often dominated by single age groups suggesting that recruitment is episodic and only occasionally successful (Oliver et al. 1980, Morin et al. 1985, Kastendiek 1982, Cameron and Rumrill 1982). One explanation for the single age class is that ecological filters usually prevent recruitment, especially among the younger and smaller sand dollars (less than 1 cm), until the filter is disrupted by extreme storm waves. Only then does a short period of successful recruitment result in the dominance of one age class (Dayton and Oliver 1980). For example, larvae of sand dollars commonly settle more abundantly in deeper water but rarely survive to several millimeters here (Oliver et al. 1980). The first biological filter for the youngest sand dollars may be the predacious phoxocephalid amphipods in the crustacean zone (Oliver et al. 1982). The next size filter may be young crabs that largely forage at night, followed by a filter which is likely to be young fishes. A winter of extremely strong wave action could reduce the numbers of animals in these size filters, permitting recruitment of sand dollars from larvae to sizes of several centimeters. Since wave disturbance is most severe near the surf zone where adult sand dollars live, predators on larger sand dollars are also likely to be less abundant after the extreme storm event. Note that a voracious ecological filter is responsible for age and size class patterns in other sedimentary environments as well (Oliver and Slattery 1985), and may account for many population and community patterns beyond the sand dollar bed.
As wave disturbance decreases, the sedimentary environment gradually shifts to a more stable fine sand with a significant muddy fraction. In Monterey Bay, the polychaete zone is well developed in water depths of 20 meters or greater. Animals build more permanent tubes and burrows, large suspension feeding clams live deep in the sediment, many sessile and suspension feeding animals increase dramatically, and the bottom is alive with biogenic structures that persist for many years. While the deeper zone is dominated by polychaete worms, many other relatively sessile and suspension feeding groups are common (Oliver et al. 1980). For example, just beyond scuba diving depths there is a dense community of suspension feeding brittle stars (Hodgson and Nybakken 1973) which are incapable of survival in shifting, shallow sands.
Section I. Physical Features of Shallow Soft Bottom Communities