limitation. Eutrophication also enhances the proliferation of faster growing phytoplankton, epiphytic algae, and macroalgae that compete with seagrass for light and space (Kemp et al., 1983; Phillips et al., 1978; Short et al., 1995; Twilley et al., 1985). Light limitation of seagrasses leads to diminished growth and stature, increased shoot mortality and declines in shoot density (Duarte, 1995; Moore et al., 1996; Short et al., 1995), resulting in declines in seagrass habitat area. Initial habitat fragmentation by trawling and dredging can make seagrass habitats more susceptible to the negative effects of eutrophication.
The maintenance of the ecosystem in an alternative state will depend on interactions with adjacent ecosystems and the intensity of the new biologic links. Additional disturbance generated by natural events or by new trawling and dredging can help maintain the assemblage in this state of equilibrium or transfer it to a new state. In the benthos, disturbances can be physical (hurricanes, suspension of sediment by surf, lateral transport by bottom currents, seasonal hypoxia generated by the input of nutrients, limited export of biogenic carbon) or biological (predation, flux and export of biogenic carbon, deposition of debris, bioturbation, competitive exclusion). Their common action is to remove organisms and to open spaces for colonization by other organisms. If disturbances are frequent, gaps will constantly reset to one of the multiple stable stages. If disturbances are rare, most of the community will remain in a stable state for most of the time. The loss of complexity and biodiversity can threaten important ecologic functions (the cycling of important elements or the control of populations of particular species) or the resilience of ecosystems to change or disturbance.
For the most part, existing information about the direct responses of benthic communities to trawling and dredging is consistent with the general principles that govern how ecologists expect communities and ecosystems to respond to acute and chronic physical disturbance. Trawling and dredging change the physical habitat and biologic structure of ecosystems and therefore can have potentially wide-ranging consequences. Mobile gear reduces benthic habitat complexity by removing or damaging the actual physical structure of the seafloor, and it causes changes in species composition. The reduction of physical structure in repeatedly trawled areas results in lower overall biodiversity. Of direct concern to commercial and recreational fisheries is the possibility that losses of benthic structural complexity and shifts in community composition will compromise the survival of economically important demersal fishes. Mobile gear also can change surficial sediments and sediment organic matter, thereby affecting the availability of organic matter for microbial food webs.
It is clear that the links between habitat alteration and loss of fisheries production can be subtle and diverse and that they operate on many spatial scales, from site-specific to regional. Most studies have been done in shallow water in small areas. Researchers have examined acute disturbances, rather than chronic, and they have studied short-term response focused in animal communities, as opposed to ecosystem processes such as nutrient regeneration. Although there have been many acute studies, few have examined the effects of short-term multiple passes, and future research should address this type of disturbance.
Perhaps the biggest research gap is on chronic effects and recovery dynamics. More studies on chronic disturbance by fishing gear are needed to determine the dose–response relationship as a function of gear, return time, and habitat type. Research also should address recovery dynamics, with consideration given to estimating the large-scale effects at current fishing intensities (e.g., Collie et al., 1997). This research should include quantitative studies undertaken in deeper water (>100 m) and studies in stable and structurally complex habitats, for which the recovery trajectory will be measured in years to decades. The statistical power to detect fishing effects will be greatest when biologic sampling can be combined with high-resolution spatial data on fishing effort.