Seasonal changes diver observations

The cold murky, winter waters around the British Isles are not an encouragement to divers. However, with the advent of effective thermal insulation in the form of modern dry suits, more divers (and biologists) are now diving all year round. They have been able to confirm, and document photographically, the dramatic seasonal changes that occur in the benthos and the plankton around the British Isles (and in other temperate seas).

Although macro-algae play only a small part in the productivity of the oceans as a whole, they form an essential part of the shallow rocky sublittoral ecosystem in temperate seas. Around the British Isles, the kelp Laminaria hyperborea forms forests in suitable rocky areas below the low-water mark. This kelp is, in effect, deciduous, and as the autumn storms rip through the beds, the large fronds are shed leaving behind the long-lived, tough stalks (stipes). The fronds are often cast ashore in great heaps and the nutrients in them are eventually returned to the sea via the food chain and DOM (see Section 4.3.2). As daylight starts to increase in March and April and the sea becomes less turbulent allowing greater light penetration, new fronds start to sprout from the stems. Kelp fronds make excellent surfaces for the attachment of epiphytic hydroids, brozoans, ascidians and algae and these reduce the light reaching other kelp fronds. Shedding the fronds gets rid of this epiphytic load.

Many red seaweeds are annuals and these die right back in winter leaving spores behind to start the growth cycle again in spring. Many of the smaller attached and encrusting animals, including hydroids, bryozoans, tunicates and sponges, are also annuals. They are either torn away or die back to an inconspicuous resting stage. Therefore, at very exposed sites, sublittoral rocks may appear very bare in winter with only a covering of pink encrusting, calcified algae. In early spring, attached animals such as the sea squirt Calvelina lepadiformis, and many others, start to re-grow as the planktonic food supply increases.

Changes in the abundance of the plankton are also evident to divers since they greatly affect the underwater visibility. In spring around March and April, visibility is often very poor. At this time increasing daylight length and intensity result in algal blooms in coastal waters, fed by an abundance of nutrients from the breakdown of tonnes of decaying marine life which died back over winter. During the summer, by about mid-June, visibility is usually good because the plankton have used up much of the nutrient excess and are being eaten more quickly than they can reproduce. The autumn often brings brief plankton blooms as storms churn up the water and release more nutrients. Poor visibility in winter is the result of storms stirring up the sea-bed and suspending fragments of plant and animal debris.

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