Translation of Issue 53 pages 30-33 Tro’r tymhorau ar hyd ein glannau
Seasonal changes along our coasts
NIA HAF JONES is our guide through the seasonal changes seen at the seaside and beneath the waves, and shares a few gems of knowledge with us about some of our coastal species.
Britain lies in an area of significant biogeographical transformation. To the north-east we have coldwater (boreal arctic) species and, to the south-west, warm water, marine (Lusitanian) species, due to the warming effect of the North Atlantic Drift. Because of this, and also given the wide variety of habitats that exists on our shores, from mud flats to rugged cliffs, Britain has an extremely rich marine biodiversity. The boundary line referred to as the “General Limit of Southern Types” by Edward Forbes in 1858 – which is interesting from the point of view of the distribution of certain species – runs directly through the coastal area of the County of Conwy. Although over 150 years have passed since Forbes’s map was printed for the first time, much of it remains correct to this day. Species’ ability to survive and reproduce within a given marine temperature range is the main factor which determines their classification.
Marine species live in a far more limited temperature range than freshwater species, since sea temperature is, by and large, stable and changes very slowly. It’s a different story altogether in the intertidal zone, where species are challenged by environmental extremes as they come into contact with the open air. As the sea temperature changes because of climate change we are seeing variations in this biogeographical classification, with some northern species already disappearing and other more southern ones expanding.
Because of its temperate, latitudinal location there are changes in temperature and weather from season to season here in Wales, and similarly we see seasonal changes in the classification and presence of marine species. On land it is easy to see and experience significant changes which occur with the passing of the seasons but, although there may be some obvious signs along the coasts such as the migration of seabirds and the blooming of coastal vegetation, it is much more difficult to notice what happens beneath the waves.
Let us glance, therefore, at the year, along the coastline and underneath the waves…
In the sea waters
With its name originating from the Greek word planktos (meaning ‘wanderer’) the classification of plankton, especially phytoplankton, has a seasonal pattern. Since plankton is the base of the marine food chain, it has a substantial effect on the distribution of other sea life. We can split plankton, broadly, into two types – diatoms and dynoflagellates. At the beginning of the year, when there is not much light and after the winter storms have troubled the waters, there are almost no phytoplankton in the water. In early spring, as the days lengthen, there is an increase in the production of diatoms and a thermocline starts to develop. This is the plankton’s first yearly blossoming. As spring progresses the silica which is necessary for the growth of diatoms becomes scarcer, and this enables the dynoflagellates to increase. By summer the thermocline has established itself and the dynoflagellates prevail, since they are able to cope with low levels of nutrients. When the powerful autumn winds trouble the waters the thermocline disintegrates and, if conditions allow, nutrients can be extracted from the water, creating a second bloom of dynoflagellates.
In addition to the increase in phytoplankton in spring there is also an increase in zooplankton. Several marine species reproduce and release their eggs into the sea waters, coinciding with the bloom of phytoplankton, and therefore the meroplankton (zooplankton which spend part of their lives as plankton) has a wide distribution at this time of year. Before long, the holoplancton (zooplankton which spend their entire life cycle in the plankton) also flourishes. This means that food is plentiful in the sea in spring and summer and, as a result, we see several migrating species, such as basking sharks and other fish, reaching our waters. This is also why the birds we are so familiar with come to nest on the cliffs.
On the shore and in deep waters
Changes also occur on the seashore where the environmental conditions are much more volatile. Exposed to freezing winds in winter and scorching sun in summer, life in the intertidal zone is tough.
In a normal season several coastal species can cope successfully with these arduous conditions. This is especially true as regards sessile animal species. However, for these an unusually extreme winter can be very destructive. Species which are able to move migrate down the coastline as the weather cools, and to deeper waters where the water’s temperature is much more stable. There is an annual shift of rock-pool fish, such as blennies and gobies, and this is also true for several species of crustacea such as prawns and crabs. Some species, for example periwinkles Littorina littorea, may become lethargic and inactive during winter, rolling down the coast with the tide to warmer waters.
In autumn, in the shallow sea waters, species such as Dead man’s fingers Alcyonium digitatum retreat for the winter. This species is a soft coral with hundreds of polyps sharing a soft skeleton (the ‘fingers’). When it feeds during the spring and summer months these polyps appear on the surface giving the coral a furry appearance. In autumn the polyps retire for months. By spring, they will have discarded their outer skin and will have resumed feeding.
In Spring signs of new life are visible in the rock-pools. Annual seaweeds begins to appear while the perennial species begin their seasonal growth. Other, stranger looking creatures are also seen in the rock-pools, such as the sea hare Aplysia punctata, whose name derives from its ear-like tentacles. These slugs come to the intertidal waters to form mating chains, and since this is a hermaphroditic species individual slugs can act as male and female within these chains. Their eggs form long pink strands which can be seen in rock-pools at low tide.
On the shore
The signs of the changing seasons are also seen on the shore. Although several of these are visible throughout the year, winter is the best time to search for foreign species cast ashore by strong winds.
It is worthwhile looking at the barnacle Lepas anatifera, which is frequently found on the shores of Wales after storms and strong north-westerly winds. The barnacle’s natural home is driftwood floating in open sea, which provides it with anchorage for growth. In the past barnacle shells were found on natural driftwood, such as trees and timber, but these days they also grow on plastic objects. Their translucent shells, the capitulum, have been formed from several plates, supported by a long stalk. Due to their similarity to the barnacle goose Branta leucopsis, and the fact that no one had ever seen the geese’s eggs, it was believed at one time that the barnacles were young geese (at that time people did not know that birds migrated to nest). In his description in 1188 of the Irish landscape and people, Gerald of Wales noted that the geese developed “hanged by their beaks” on driftwood and flew away after feeding from “juices of the wood and sea-water”. Because of this belief, these geese weren’t considered to be born of flesh and therefore it was possible to eat them during religious fasts.
To alleviate extreme temperatures of cold and heat, boring species dig even deeper into sand and mud, where they maintain a more stable temperature. After terrible storms, like those we saw last winter, large numbers of these creatures are cast onto the shore. Despite their ability to bore, the perilous waters can churn the mud to such an extent that species such as the sea potato Echinocardium cordatum and the sea cucumber Thyone fusus are washed up in their thousands.
As the strong winter winds recede fewer things are seen on the sea shore but an increase in the number of empty outer shells of the European spider crab Maja squinado is a definite sign of the approach of spring. During the summer months they congregate in large numbers to shed their outer shells, forming piles to protect themselves while their carapaces are soft, and it is only at this time that females accept the attention of the males.
So there is plenty to see as the seasons change on the land, but the next time you notice the swallows returning or the leaves falling, think about the sea shore and the changes which happen beneath the waves.
Nia Haf Jones is a Marine Officer with the North Wales Wildlife Trust. Her aim is to encourage and enable children (of all ages!) to join her in discovering the wonders of Welsh coastal waters 01248 351541 / 07855454132 [email protected]