Dannielle S. Green

Impacts of Discarded Plastic Bags on Marine Assemblages and Ecosystem Functioning

The accumulation of plastic debris is a global environmental problem due to its durability, persistence, and abundance. Although effects of plastic debris on individual marine organisms, particularly mammals and birds, have been extensively documented (e.g., entanglement and choking), very little is known about effects on assemblages and consequences for ecosystem functioning. In Europe, around 40% of the plastic items produced are utilized as single-use packaging, which rapidly accumulate in waste management facilities and as litter in the environment. A range of biodegradable plastics have been developed with the aspiration of reducing the persistence of litter; however, their impacts on marine assemblages or ecosystem functioning have never been evaluated. A field experiment was conducted to assess the impact of conventional and biodegradable plastic carrier bags as litter on benthic macro- and meio-faunal assemblages and biogeochemical processes (primary productivity, redox condition, organic matter content, and pore-water nutrients) on an intertidal shore near Dublin, Ireland. After 9 weeks, the presence of either type of bag created anoxic conditions within the sediment along with reduced primary productivity and organic matter and significantly lower abundances of infaunal invertebrates. This indicates that both conventional and biodegradable bags can rapidly alter marine assemblages and the ecosystem services they provide.

Impacts of the invasive alga Sargassum muticum on ecosystem functioning and food web structure

Biological invasions have the potential to cause severe alterations to the biodiversity of natural ecosystems. At the same time, variation in the diversity and composition of native communities may have an important influence on the impact of invasions. Here, effects of the invasive Japanese wireweed, Sargassum muticum, were tested across a range of native marine algal assemblages using a combined additive and substitutive design. The invasive alga significantly reduced primary production, an important component of ecosystem functioning, and increased connectance, a key property of the food webs associated with the algal resources. These impacts were mediated by changes in the proportions of intermediate and top species, as well as apparent reductions in faunal species richness and diversity. Some key alterations to faunal species composition (including the arrival of generalist species associated with S. muticum) may have been important in determining these patterns. Overall results suggest that S. muticum not only directly impeded the native algal community, but that these effects extended indirectly to the native fauna and therefore caused major changes throughout the ecosystem.

Effects of Non-Indigenous Oysters on Microbial Diversity and Ecosystem Functioning

Invasive ecosystem engineers can physically and chemically alter the receiving environment, thereby affecting biodiversity and ecosystem functioning. The Pacific oyster, Crassostrea gigas, invasive throughout much of the world, can establish dense populations monopolising shorelines and possibly altering ecosystem processes including decomposition and nutrient cycling. The effects of increasing cover of invasive C. gigas on ecosystem processes and associated microbial assemblages in mud-flats were tested experimentally in the field. Pore-water nutrients (NH4 + and total oxidised nitrogen), sediment chlorophyll content, microbial activity, total carbon and nitrogen, and community respiration (CO2 and CH4) were measured to assess changes in ecosystem functioning. Assemblages of bacteria and functionally important microbes, including methanogens, methylotrophs and ammonia-oxidisers were assessed in the oxic and anoxic layers of sediment using terminal restriction length polymorphism of the bacterial 16S rRNA, mxaF, amoA and archaeal mcrA genes respectively. At higher covers (40 and 80%) of oysters there was significantly greater microbial activity, increased chlorophyll content, CO2 (13 fold greater) and CH4 (6 fold greater) emission from the sediment compared to mud-flats without C. gigas. At 10% cover, C. gigas increased the concentration of total oxidised nitrogen and altered the assemblage structure of ammonia-oxidisers and methanogens. Concentrations of pore-water NH4 + were increased by C. gigas regardless of cover. Invasive species can alter ecosystem functioning not only directly, but also indirectly, by affecting microbial communities vital for the maintenance of ecosystem processes, but the nature and magnitude of these effects can be non-linear, depending on invader abundance.

Microplastics Affect the Ecological Functioning of an Important Biogenic Habitat

Biological effects of microplastics on the health of bivalves have been demonstrated elsewhere, but ecological impacts on the biodiversity and ecosystem functioning of bivalve-dominated habitats are unknown. Thus, we exposed intact sediment cores containing European flat oysters (Ostrea edulis) or blue mussels (Mytilus edulis) in seawater to two different densities (2.5 or 25 μg L-1) of biodegradable or conventional microplastics in outdoor mesocosms. We hypothesized that filtration rates of the bivalves, inorganic nitrogen cycling, primary productivity of sediment dwelling microphytobenthos, and the structure of invertebrate benthic assemblages would be influenced by microplastics. After 50 days, filtration by M. edulis was significantly less when exposed to 25 μg L-1 of either type of microplastics, but there were no effects on ecosystem functioning or the associated invertebrate assemblages. Contrastingly, filtration by O. edulis significantly increased when exposed to 2.5 or 25 μg L-1 of microplastics, and porewater ammonium and biomass of benthic cyanobacteria decreased. Additionally the associated infaunal invertebrate assemblages differed, with significantly less polychaetes and more oligochaetes in treatments exposed to microplastics. These findings highlight the potential of microplastics to impact the functioning and structure of sedimentary habitats and show that such effects may depend on the dominant bivalve present.