Effect of different estuarine habitats on biodiversity
Science Centres: Aquatic Biodiversity and Biosecurity
This sampling quadrat highlights the rich biodiversity of mudflat communities.
Tairua estuary in the Coromandel.
NIWA scientists are studying the links between habitat types as part of a FRST-funded programme to better understand how estuaries recover from catastrophic sedimentation events and longer-term chronic degradation by sedimentation.
In an estuarine ecosystem there are often multiple habitat types, including salt-marshes, mangroves, sea-grass beds, unvegetated intertidal flats, tidal channels, and subtidal habitats. These environments promote diversity by enhancing the recruitment and maintenance of species that require multiple resources, and may also modify the outcome of biological interactions such as competition, predation, foraging behaviour, and predator avoidance. However, these habitats are often examined separately, so we do not know much about how intertidal and subtidal vegetated habitats and unvegetated habitats interact and function together.
Research on estuaries often focuses on the habitat type of interest without considering the potential effects on surrounding habitat types. Yet these systems are a mosaic of habitats connected by water, and many of the animals using estuaries can potentially move between habitat types, giving them the opportunity to use resources (e.g., food and shelter) from different parts of the estuary. Studies that focus on a single habitat or simply compare one habitat with another do not adequately explain why there are variations in the abundance of animals at different locations and times, or how modification (either natural or anthropogenic) of one habitat type affects the overall production or biodiversity in an estuary. Therefore, understanding the links between habitats in diverse landscapes and their effect on the abundance, movement, and growth of estuarine organisms is essential in the management of ecosystems.
New Zealand estuaries are increasingly affected by changes in catchment land-use, mainly because of increased sediment loading which can affect the biodiversity and functioning of an estuary. For example, an increase in sediments can smother animals living in the sediment, affect the heath of suspension feeders, and change fringing vegetated habitats such as salt-marshes and mangroves.
Changes in habitat can also influence the recovery of estuaries. For example, disturbed areas rely on adjacent or nearby habitats for sources of colonists for the recovery of sediment disturbance. To predict the potential for an estuary to recover after being disturbed we need to understand the links between different habitats, particularly the movement of organisms between habitats.
We use hydrodynamic modelling techniques and field experiments to identify the key features of the recolonisation and recovery of estuarine ecosystems after sediment disturbance. The location of a disturbance within an estuary (e.g., tidal channel, sandflat) is one of the most important factors affecting recovery of the estuary. For example, higher intertidal locations like mudflats and tidal creeks have lower rates of recolonisation than estuarine sandflats. An organism’s life-history also has a major influence on the recovery of estuarine ecosystems. For example, some tube-dwelling polychaetes (worms) with crawling larvae have a low chance of moving long distances and colonising disturbed areas compared with many bivalves (clams). Other minor influences on the movement of organisms include the tides, release time and height, and flood and wind events.
To test these model predictions we used sediment traps for colonising fauna at seven sites in Whitford estuary in Auckland, which included mangroves, mudflats, a channel, and upper and lower sandflats. Our preliminary field results confirmed model predictions, showing strong differences between mudflat and mangrove sites and all other estuarine sandflat locations. For example, there were low rates of sediment-trap colonisation in the mudflats and mangroves compared with nearby sandflats.
Many species also seemed to have very different colonisation, transport, and recovery rates. For example, worms were a major part of the benthic community at the mud site, but were poorly represented in the sediment-trap samples because of their methods of dispersal. However, bivalves were found in high numbers at most sites because of their ability to drift as juveniles .
This information gives us an insight into how benthic communities in different parts of estuaries recover, based on the total amount of the estuary that is disturbed and its location. Estuarine habitats are subject to many forms of human disturbance, so the potential for chronic and broad-scale degradation is high. Our research allows us to examine the impacts of human disturbance at an ecosystem scale, including the links between different habitat types. Our preliminary results confirm model predictions that soft-sediment benthic assemblages are subject to local dispersal processes, that transport within and between habitats can differ, and that recovery times are dependent on the dispersal stage of each organism. Varying rates of colonisation by different organisms suggest that high levels of disturbance can decrease biodiversity within estuarine habitats if recovery times are not sufficient for some species to colonise a disturbed habitat. Future work will focus on the likely recovery times and effects of estuarine disturbance on the biodiversity of benthic soft-sediment assemblages.