Light reductions drive macroinvertebrate changes in Amphibolis griffithii seagrass habitat

Abstract

Numerous anthropogenic activities can significantly reduce the amount of light reaching seagrass habitats. Typically these result in morphological and physiological changes to the plant and associated algal epiphytes. However, the flow-on effects to seagrass-dependent fauna induced by these disturbances has yet to be examined. This study investigated the effects of different light reduction intensity (high: ~92% reduction; moderate: ~84% reduction), duration (3, 6 and 9 mo) and timing (post-winter and post-summer) on the density and biomass of macroinvertebrate epifauna within an Amphibolis griffithii seagrass ecosystem (Western Australia). There were generally lower epifauna densities and biomass within shaded seagrass plots. When moderate intensity shading was imposed at the end of winter, total density in unshaded controls was 31% lower at 3 mo, and 78% lower at 9 mo. When high intensity shading was imposed, total density was 38% lower than in controls at 3 mo, and 89% lower by 9 mo. Although densities varied, similar magnitudes of decline occurred in post-summer shaded treatments. Taxa-specific responses were variable in terms of time, rapidity and magnitude of response. Amphipod, isopod and gastropod densities generally declined in response to shading. Bivalve densities declined with shading post-summer, but not post-winter. Ostracod densities had an inconsistent response to moderate shading. Changes in epifaunal density were largely associated with declines in algal biomass, leaf variables and stem biomass, indicating food and habitat limitations. It is likely that the significant declines in epifauna observed in this experiment would have flow-on consequences to higher trophic levels. KEY WORDS: Seagrass · Disturbance · Shading · Western AustraliaResale or republication not permitted without written consent of the publisher

Recovery from the impact of light reduction on the seagrass Amphibolis griffithii, insights for dredging management

“…Consequently, there is a higher biomass and diversity of algae and fauna living on A. griffithii compared to other seagrass species (Borowitzka et al, 1990;Edgar, 1990;Gartner et al, 2010), including a unique assemblage of fish characterised by different species and larger fish than those found in other large seagrass, such as Posidonia (Hyndes et al, 2003). Greater predation rates have also been observed in A. griffithii meadows compared to Posidonia meadows (Vanderklift et al, 2007).…”

Habitat preferences of macroinvertebrate fauna among seagrasses with varying structural forms

“…These changes can serve to modify the habitat such that while it still remains, there may be a reduction in the ecosystem functions it can provide (Hewitt, et al, 2008). In seagrass systems, these changes can have negative effects for the macroinvertebrate fauna that inhabit them (Gartner, et al, 2010) potentially removing an important trophic link to higher order consumers.…”

“…Such changes in the macroinvertebrate assemblage are largely associated with declines in algal epiphyte biomass, leaf canopy variables and stem biomass (Bostrom, Mattila, 1999;Edgar, 1990a;Gartner, et al, 2010), which all demonstrate sub-lethal responses to anthropogenic stressors (Lavery, et al, 2009). It is hypothesized that these associations are primarily driven by a reliance on these habitat resources by macroinvertebrate fauna for food, shelter and possibly protection from predation (Gartner, et al, 2010).…”

“…Such changes in the macroinvertebrate assemblage are largely associated with declines in algal epiphyte biomass, leaf canopy variables and stem biomass (Bostrom, Mattila, 1999;Edgar, 1990a;Gartner, et al, 2010), which all demonstrate sub-lethal responses to anthropogenic stressors (Lavery, et al, 2009). It is hypothesized that these associations are primarily driven by a reliance on these habitat resources by macroinvertebrate fauna for food, shelter and possibly protection from predation (Gartner, et al, 2010). However, while these outcomes have been observed in field investigations thus establishing correlative evidence (Gartner, et al, 2010), our general understanding remains limited and there are few studies which have confirmed the mechanisms which drive changes in macroinvertebrate assemblages via controlled experiments.…”

“…It is hypothesized that these associations are primarily driven by a reliance on these habitat resources by macroinvertebrate fauna for food, shelter and possibly protection from predation (Gartner, et al, 2010). However, while these outcomes have been observed in field investigations thus establishing correlative evidence (Gartner, et al, 2010), our general understanding remains limited and there are few studies which have confirmed the mechanisms which drive changes in macroinvertebrate assemblages via controlled experiments.…”