The influence of bivalve size on ecosystem functioning in intertidal estuaries with multiple stressors

Abstract

Estuarine ecosystems, as critical transition zones between land, freshwater and the sea, are hotspots for biogeochemical cycling and productivity. Most of the biogeochemical processes occur in estuarine sediments where macrofauna and microbes are key drivers of the ecosystem functions, nutrient regeneration and primary production. Large bivalves in particular are important drivers of these ecosystem functions; however, in many estuaries large bivalves have declined, altering ecosystem structure and functioning. Increased sedimentation and nutrient enrichment from land run off alter the macrofaunal community, including bivalves and the roles they play within the sediment. This thesis aimed to determine how changes in the biomass of large and small bivalves changed ecosystem functioning using an in situ manipulation experiment of Austrovenus stutchburyi, a key bivalve in New Zealand’s estuaries (Chapter 2). Further the thesis used a manipulative field experiment to investigate the effects of nutrient enrichment in contrasting muddy and sandy habitats on key ecosystem functions and the importance of large bivalves in ameliorating the effects of nutrient enrichment (Chapter 3 & 4). The ecosystem functions measured included nutrient regeneration, primary production and extracellular enzyme activities, critical functions that maintain healthy estuaries. Macrofaunal community and sedimentary parameters were also measured and the influence of these variables on ecosystem functions determined. Small bivalves (Austrovenus) were key drivers of benthic primary production and microbial activity associated with organic matter breakdown and nutrient cycling in sediments, while large bivalves drove nitrate fluxes (Chapter 2). Since high biomass shellfish beds could only be maintained with large individuals (due to predation of small Austrovenus), high biomass of the large individuals was important overall for nutrient regeneration and primary production. Thus, mixed size classes of large and small bivalves provide complementary ecosystem functions and are required for ecosystem wide benefits. The effect of nutrient enrichment was context dependent with experimentally elevated porewater nutrients interacting with mud content to affect ecosystem functions (Chapters 3 & 4). Extracellular enzyme activity, a rate-limiting step of organic matter breakdown and nutrient cycling, was greater in muddy than sandy sediment. Mud content also affected macrofaunal communities, reducing species diversity and abundance of large bivalves. Macrofaunal abundance increased extracellular enzyme activity, however bivalve density negatively influenced enzyme activity (Chapter 3). Interaction effects of nutrients and mud were also documented for nutrient regeneration and productivity, with small increases in mud content (over ~4 %) reducing the photosynthetic efficiency of benthic primary production (Chapter 4). Bivalve and macrofauna excretion, movement and bioadvection stimulated nutrient regeneration and primary production in sandy sediments similar to the results in chapter 2, however, increased mud content reduced the positive role of bivalves and macrofaunal communities on ecosystem functions. These results highlight the complex interaction (both cumulative and additive) between multiple stressors and the influence of key bivalves on ecosystem functions. Small changes in mud content due to increased sedimentation reduce ecosystem functions, showing the need for increased monitoring of mud content to improve estuarine management and health. The increased vulnerability of estuaries to nutrient enrichment with increasing mud content and the corresponding loss of large bivalves highlights the need for integrated management of multiple stressors rather than a single stressor approach focusing on the whole catchment for managing estuaries. Monitoring of size classes and biomass of key bivalves such as Austrovenus is essential to ensuring a mixed size class of these bivalves in order to maintain and increase ecosystem functions. Reducing nutrient and sediment run off and limiting the loss of large bivalves will improve and support key ecosystem functions and services that maintain and preserve healthy estuarine ecosystems into the future.