Shallow Seas by Peter Hayward

Author:Peter Hayward
Language: eng
Format: epub
Publisher: HarperCollins Publishers
Published: 2016-02-22T16:00:00+00:00

FIG 118. Biomass and shoot density of Zostera marina at five eutrophic sites on the Swedish coast. (After Jephson et al., 2008)

FIG 119. Biomass of epiphytes and mesograzers on Zostera marina at five sites on the Swedish coast. AFDW, ash-free dry weight; DWT, dry weight. (After Jephson et al., 2008)

Interactions between the successive trophic levels of an eelgrass community vary in intensity along environmental gradients, and in relation to the biodiversity of each level. Where nutrient levels are minimal epiphyte growth is immediately consumed by modest mesograzer populations, and the eelgrass increases the proportion of below-surface biomass through additional root growth; predation has no effect on algal biomass, and the eelgrass may be grazed by a different suite of mesograzers, especially small gastropods. Taxonomic diversity within a trophic level will weaken top-down effects because highly diverse assemblages will include a proportion of predator-resistant species, and will also promote greater niche diversity, and fewer strong interactions; eelgrass habitats are structurally complex, and complexity is enhanced by epiphyte growth, providing maximum refuge from predation for mesograzers. However, these mitigating effects may still be negated by especially effective keystone predators which may strongly affect the structure and function of the assemblage. For example, pipefish are visual predators adapted for foraging in complex, three-dimensional habitats, and it has been demonstrated in Mexican Z. marina beds that two or three adult pipefish per square metre of eelgrass habitat provide effective control of mesograzer populations (Jorgensen et al., 2007). Hydrodynamic factors may also drive trophic cascades within seagrass beds. In brackish habitats the small mud snail Peringia ulvae is an ecologically significant mesograzer, and Nanozostera noltii beds in the leeward shelter of Scolt Head, on the north coast of Norfolk, supported a mean of 32 invertebrate species, and 6,568 individuals per 0.1 m2; total faunal density scaled up to 51,300 per square metre, of which 43,000 were P. ulvae (Barnes & Farnon Ellwood, 2011). At such densities, in low-energy environments, P. ulvae will effectively control epiphyte biomass, but as current strength increases, in high-energy habitats the snails are swept away, epiphyte growth is unchecked (Fig. 120), and seagrass growth is inhibited (Schanz et al., 2002). The primary producers benefiting from trophic cascades within eelgrass beds are, of course, the epiphytic macroalgae, while the eelgrass is actually disadvantaged through competitive pressure. There is an interesting contrast here with kelp habitats: kelps are negatively affected by herbivory, and herbivore pressure is contained through predation of grazing echinoderms by lobsters and fish. Eelgrass has a positive mutualistic association with mesoherbivores at one end of a nutrient-enrichment gradient, where epiphytes are checked by grazing, and a negative relationship at the other, where the mesograzer community consumes eelgrass tissue in mesotrophic systems.

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