Herbivory in a subtropical seagrass ecosystem: separating the functional role of different grazers

Abstract

Seagrass meadows provide many important ecosystem services, but they are threatened by human activities and are in decline globally. In particular, eutrophication arising from human activities promotes algal growth, which negatively affects seagrass. Herbivores consume algae and can, therefore, reduce eutrophication effects, but they may also consume seagrass. Little is known, however, about grazer−epiphyte−seagrass interactions in subtropical seagrass in the Indo-Pacific. We used a 5 wk exclusion experiment to quantify the influence of different grazers in seagrass (dominated by Zostera muelleri) in Moreton Bay, eastern Australia. Our results show that herbivory does indeed affect seagrass−epiphyte dynamics in this region and that different grazers can exert different effects in seagrass ecosystems. In particular, exclusion of small mesograzers (i.e. amphipods and juvenile shrimp) caused epiphyte biomass to increase by up to 233%. Exclusion of medium mesograzers (i.e. small fish and prawns) resulted in increases of up to 10% in seagrass cover, 53% in shoot height and 29% in shoot density. Large mesograzers (i.e. adult fish) and macrograzers (i.e. turtles and dugong) did not appear to play a role in the study system. These results demonstrate that mesograzers can be important in controlling epiphytic algae in subtropical Indo-Pacific seagrass, and show that different mesograzers can affect seagrass− epiphyte dynamics in different ways. It is critical that the functional effects of different herbivores be considered when implementing programs for seagrass conservation and restoration.

Unravelling complexity in seagrass systems for management: Australia as a microcosm

“…grazing pressures), and may be inferred from the physical classification described above. Heavy grazing by macrograzers can remove substantial biomass and cause meadows to fragment , moderate grazing can stimulate growth and productivity (Christianen et al, 2012), and even small grazers extensively influence meadow structure (Ebrahim et al, 2014).…”

Identifying knowledge gaps in seagrass research and management: An Australian perspective

“…How does herbivory influence the structure and function of seagrass? Studies in Australia are limited but suggest biogeographic differences in the relative influence of meso-, macro-and megaherbivory on seagrass (Aragones et al, 2006;Cook et al, 2011;Ebrahim et al, 2014;Verhoeven et al, 2012). A better understanding is required of how herbivory affects seagrass biomass and species composition, and how those effects interact with water and plant nutrient concentrations.…”

Quantifying the influence of small omnivorous fishes on seagrass epiphyte load

“…Moreton Bay in subtropical eastern Australia is a large, shallow, subtropical embayment containing extensive beds of seagrass, especially of eelgrass Zostera muelleri (Stevens & Connolly, ). Previous studies on Moreton Bay have shown that the removal of invertebrate grazers via caging and insecticides increases epiphyte biomass by 233% after 5 weeks, and that there is potentially a limited influence of small fishes (Ebrahim et al ., ). No studies, however, have experimentally and explicitly quantified the influence of omnivorous fish species whose diet includes seagrass epiphytes.…”

“…Epiphyte load on Z. muelleri blades in Moreton Bay averages c . 0·015–0·025 g dry mass (DM) leaf −1 and is dominated by algal epiphytes (Ebrahim et al ., ). Previous studies have found a significant effect of bottom‐up drivers (especially water column nutrient availability and water clarity) on algal biomass on reefs in Moreton Bay (Gilby et al ., ), but the influence of nutrient and light availability and sedimentation on seagrass algal loads is hitherto untested.…”