What makes the estuarine system of Montrose Basin so special?
The salinity, exposure to wave action, particle sizes and tidal height all contribute to the physical nature of the Basin, without which the biological diversity would be diminished.
The interaction between the freshwater of the River South Esk and the tidal seawater has a marked effect on the biology of the estuary. leading to the exclusion of freshwater species unable to tolerate saltiness and those marine forms sensitive to reduced salinity.
The Montrose Basin estuary, like a further nine of the 50 or so Scottish estuaries, is of a type known as bar-built. It is relatively young geologically and owes its existence in the past to the post-Holocene sea- level rise around 10,000 years ago.
The lifespan of the Basin will be typically only a few thousand or a few tens of thousands of years, depending on rates of sea-level change (although this may be much reduced by sea-level rises due to global warming), and offshore sedimentation. It is a micro-tidal estuary, having a range of around 0-2 metres. The wind and storm-generated waves dominate water movement and the sediment is often sandy, accumulating in banks and spits.
The "mud" of the Basin is composed of shelly gravel and coarse sand down to fine silts, and there are good scientific reasons for their dispersal to different areas of the Basin. These different sediments hold a diverse range of species, from lug and ragworm to Corophium, a tiny shrimp-like creature, and the equally small Hydrobia snail.
Because these species inhabit different layers in the sediments, a wide range of birds can be found feeding, from ducks sifting through the surface layers, to the curlew, whose bill can be up to six inches in length, allowing it to probe deeply in the search for food.
The food source of the Basin is mainly organic detritus, of plant origin, including the remains of eel grasses such as Zostera, an important food source for Wigeon. Faunal sources of detritus include the faeces of the mudflat invertebrates. The high invertebrate populations of estuaries support large predators such as shrimps and crabs, as well as fish and birds.
Direct grazing of the surrounding saltmarsh vegetation occurs particularly by the overwintering flocks of pink-footed geese (when they are not favouring the farmer's winter wheat).
There are many threats to estuaries including landfill (which occurred for a time in the north-east corner of the Basin at Tayock), poor water quality and contamination. Additionally, inorganic nutrients in the form of agricultural run-off of chemicals such as nitrogen and phosphorus can have severe consequences.
The growth of green macro-algae such as Enteromorpha can cause mats, which deoxygenate the underlying sediments with disastrous effects on the invertebrates within. We are fortunate at Montrose Basin that none of these pose an immediate threat.
The conservation value of estuaries can be appreciated at all levels ranging from the entire system to individual species. The food source of estuaries, particularly at different times of the year to migrating birds, and those that stay with us all year round, is a good indicator of the general health of the estuary.
When you next see a bird feeding at the Basin, give some thought to the complex cycles of events which lead to the availability of its food source.