Estuary mudflats seem an unlikely setting for some of the most productive habitats in the world, but here at the interface of land and sea life abounds – for the select few. Certain plants, seaweeds and photosynthetic micro-organisms (phytoplankton) absorb nutrients at a fast rate, grow rapidly and produce lots of food.
Estuaries are termed ‘open’ ecosystems because they are vitally linked to the wider environment. Nutrients are carried in from the land via rivers, and from the sea by the tides. Some of these nutrients are then taken out again when animals such as fish and birds leave the estuary. Some are also flushed out to sea on outgoing tides.
Living organisms within an estuary co-exist in a network of interdependent feeding relationships, known as a food web. An estuarine food web contains the following elements:
- Phytoplankton. These microscopic organisms manufacture food by photosynthesis and absorb nutrients such as phosphorus and nitrogen from the water.
- Detritus (dead organic matter).
- Microscopic animals known as zooplankton eat some of the phytoplankton. The remainder of the phytoplankton becomes detritus, when it dies.
- Larger estuary animals such as filter-feeding worms, shellfish and hungry young fish feed on the zooplankton and detritus.
Mangroves, seagrass and rushes, the dominant plants of the estuary, produce tonnes of rotting leaf material, broken down into detritus. At Pāuatahanui Inlet near Wellington, 38 hectares of seagrass contribute 34 tonnes of plant material to the food web each year, and 30 hectares of sea rushes contribute 189 tonnes.
Natural water filters
Animals that get their food from matter suspended in the water are filter feeders (or suspension feeders). In the case of shellfish such as cockles, their gills and fine hairs (cilia) filter or strain out any food particles as water passes over them. On marine worms, their tentacles act as a filter.
Mud snails, crabs and marine worms abound in estuarine muds. They eat quantities of mud, digesting the detritus and discarding the sand and silt. By eating this decaying organic matter, these mud-dwellers become good pickings for the larger animals of the estuary – fish, birds and humans. And by excreting nutrient-rich wastes, mud-dwellers play a critical role in recycling nutrients within an estuary.
Mangrove and seagrass are also home to rich communities of filter-feeding animals around their roots. At least 30 types of fish also feed in the waters around mangroves. In 2002 at Whangapoua Harbour, juvenile snapper, trevally, garfish, kahawai, gurnard, mullet, flounder and sole were all found around seagrass, but were not so common in water above bare ground. Mangroves and rushes trap sediments entering the estuary and provide an important habitat for native wetland birds such as the bittern, pūkeko, banded rail and spotless crake.