Seasonal changes in plankton around the British Isles

Eggs, larvae and spores of benthic plants and animals are often a conspicuous part of the plankton of neritic water, and some of the most obvious seasonal changes are related to the reproductive seasons of the benthos. We can summarize certain seasonal features of the plankton of shallow water around the British Isles as follows (Boalch et al., 1978) (Figures 5.12-5.15).

Winter (November-March)

Surface cooling is continuous during the winter, reaching a minimum temperature throughout the water column, usually in early March, of about 7-8°C in the English Channel and 4-6°C in much of the North Sea. This is the season of minimum quantity of holoplankton. There is very little phytoplankton in the water, although certain diatoms are often present, mainly species of Coscinodiscus and Biddulphia. Dinoflagellates are very scarce except for Ceratium tripos. Copepods are few in number, overwintering mainly as the pre-adult copepodite stage V, and much of the holoplankton lies at a deeper level during winter than in summer. Many invertebrates and the majority of fishes in this area spawn during the latter part of the winter, setting free into the plankton a great number and variety of eggs and early larval stages. Usually the most abundant larvae in tow-net samples taken in late winter are nauplii of Semibalanus balanoides. There may also be nauplii of Verruca stroemia and Balanus crenatus, together with numerous plutei, trochophores, bivalve and gastropod veligers, zoeas and fish eggs of many species. The euphausid Nyctiphanes couchi often spreads southwards in the North Sea in winter, where it is not commonly found in spring and summer. In the western English Channel, water inflow in winter tends to be from the northwest, bringing in Aglantha digitale and associated species (see page 160).

Figure 5.12 Changes in the standing stock of phytoplankton in western part of English Channel as indicated by measurements of chlorophyll concentration.

(Data from Atkins, W.R.G. and Jenkins, P.G. (1953). J. Mar. Biol. Ass. UK, 31, 495-508 published by Cambridge University Press, and Jenkins, P.G. (1961). Deep Sea Research, suppl. to vol. 3, 58-67, published by Pergamon Press.)

Figure 5.12 Changes in the standing stock of phytoplankton in western part of English Channel as indicated by measurements of chlorophyll concentration.

(Data from Atkins, W.R.G. and Jenkins, P.G. (1953). J. Mar. Biol. Ass. UK, 31, 495-508 published by Cambridge University Press, and Jenkins, P.G. (1961). Deep Sea Research, suppl. to vol. 3, 58-67, published by Pergamon Press.)

Copepodites

Figure 5.13 Total numbers of zooplanktonts per 100 litres caught between the surface and 45 m off Plymouth through 1934. □ Copepod nauplii, ■ copepods and copepodites, § Appendicularia, S3 Cirripede nauplii, Eü Ciado cera, El Polychaete larvae, El Limacina, M Noctiluca, O Rotifer a, Ml etc.

(From Harvey, H.W. et al. (1935) in J. Mar. Biol. Ass. UK, 20, published by Cambridge University Press.)

Figure 5.13 Total numbers of zooplanktonts per 100 litres caught between the surface and 45 m off Plymouth through 1934. □ Copepod nauplii, ■ copepods and copepodites, § Appendicularia, S3 Cirripede nauplii, Eü Ciado cera, El Polychaete larvae, El Limacina, M Noctiluca, O Rotifer a, Ml etc.

(From Harvey, H.W. et al. (1935) in J. Mar. Biol. Ass. UK, 20, published by Cambridge University Press.)

Spring and early summer (March-June)

Diatoms rapidly increase in numbers to their annual maximum which usually occurs during late March or early April. The diatom population is now very mixed, Skeletonema, Chaetoceros, Lauderia, Thalassiosira, Coscinodiscus and Biddulphia often becoming very numerous. Different species tend to

Copepodites

Figure 5.14 Numbers of copepods per 100 litres caught between the surface and 45 m off Plymouth through 1934. ■ Temora longicornis, HE Pseudocalanus elongatuss § Oithona helgolandicas El Acartia clausiis ^ Euterpina acutifronss S3 Centropages typicuss IS1 Corycaeus anglicus, □ small copepodites, etc.

(From Harvey, H.W. et al. (1935) in J. Mar. Biol. Ass. UK, 20, published by Cambridge University Press.)

Figure 5.14 Numbers of copepods per 100 litres caught between the surface and 45 m off Plymouth through 1934. ■ Temora longicornis, HE Pseudocalanus elongatuss § Oithona helgolandicas El Acartia clausiis ^ Euterpina acutifronss S3 Centropages typicuss IS1 Corycaeus anglicus, □ small copepodites, etc.

(From Harvey, H.W. et al. (1935) in J. Mar. Biol. Ass. UK, 20, published by Cambridge University Press.)

predominate in succession, although the order is not constant from year to year. Photoperiodism may play some part in setting off the rapid growth of particular species at different times. The effects of external metabolites may be partly accountable for the successive rise and decline of each species. The earliest to become dominant must thrive mainly on inorganic nutrients. Those which achieve dominance later may be utilizing DOM produced by preceding species (Butler, 1979). Following their spring peak the diatom population declines fairly rapidly. Often the colonial flagellate Phaeocystis pouchetii then becomes abundant.

During early spring the plankton contains many eggs and larvae, but by early summer many of these have metamorphosed and disappeared from the plankton. The nauplii of the barnacle, S. balanoides, most abundant in March, develop to cypris stages in April and have mostly settled by mid-May. Over the same period, many crab zoeas advance to megalopas, and fish larvae to young fish stages. Overwintering copepodites mostly become adult in February and produce a brood of eggs in March, hatching to nauplii by April. The individuals from this brood usually grow to larger size than any others during the year. The

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Figure 5.15 Numbers of smaller copepods per cubic metre through the year 1947 off Plymouth, all to same scale. Solid black: copepodites and adults. Continuous line: all stages including nauplii. Dotted line: total plus eggs. (a) Pseudocalanus elongatus; (b) Paracalanus parvus; (c) Centropages typicus; (d) Temora longicornis; (e) Acartia clausii; (f) Oithona helgolandica; (g) Oithona nana; (h) Oncaea venusta; (i) Corycaeus anglicus; (j) Unidentified nauplii; (k) Euterpina acutifrons.

(From Digby, P.S.B. (1950). J. Mar. Biol. Ass. UK, 29, 398-438, published by Cambridge University Press.)

Figure 5.15 Numbers of smaller copepods per cubic metre through the year 1947 off Plymouth, all to same scale. Solid black: copepodites and adults. Continuous line: all stages including nauplii. Dotted line: total plus eggs. (a) Pseudocalanus elongatus; (b) Paracalanus parvus; (c) Centropages typicus; (d) Temora longicornis; (e) Acartia clausii; (f) Oithona helgolandica; (g) Oithona nana; (h) Oncaea venusta; (i) Corycaeus anglicus; (j) Unidentified nauplii; (k) Euterpina acutifrons.

(From Digby, P.S.B. (1950). J. Mar. Biol. Ass. UK, 29, 398-438, published by Cambridge University Press.)

cladocerans Podon and Evadne often reach their greatest numbers between April and May. The euphausid Thysanoessa inermis sometimes enters the northern part of the North Sea.

Mid-summer (June-August)

Diatoms continue to decrease in number and are often scarce between July and August, species of Rhizosolenia often being the commonest at this period. Dinoflagellates are now more numerous, especially species of Peridinium, and usually reach their maximum abundance between June and July. The majority of larvae produced in the spring have now completed their metamorphosis and disappeared from the plankton, but other species require warm water for breeding and shed their eggs and larvae at this time. For example, the nauplii of Chthamalus montagui, B. perforatus and Elminius modestus appear in plankton samples taken off our south-west coastline. However, it is the holoplanktonic species, copepods, chaetognaths, ctenophores and larvaceans, which become very numerous and form the greater part of the zooplankton during the summer months. The copepods Acartia clausi and Paracalanus parvus, mainly summer to autumn forms, are often specially abundant. In the western English Channel, inflow now tends to be from the southwest, with species of Muggiaea, Liriope, etc. (see page 161) appearing. Pilchard eggs sometimes become very numerous in the English Channel in summer to autumn, though less so in recent years.

During the summer the plankton often comprises two distinct communities, one above and the other below the thermocline. Sometimes Sagitta setosa occurs in the upper warm layer, and S. elegans in the colder water below. Surface water temperature usually reaches its warmest in late August to early September, 16-18°C in the English Channel and 13-17°C in the North Sea.

Autumn (September-October)

After the thermocline breaks, diatoms show a brief but definite increase in numbers and again produce a very mixed population in which species of Rhizosolenia, Coscinodiscus, Biddulphia and Chaetoceros are often found together. Autumn plankton samples are sometimes rich in the larvae of benthic invertebrates, mainly plutei and bivalve veligers. An autumn brood of copepods appears which in some cases does not complete its development to the adult stage but survives the winter as stage V copepodites. In the English Channel the copepods Oncaea venusta, Oithona nana, Corycaeus anglicus and Euterpina acutifrons, which are not numerous earlier in the year, have their main abundance from August to the end of the year (Figure 5.14).

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Responses

  • yohannes
    How does phytoplankton changes with season in the english channel?
    8 years ago
  • claire cameron
    What cause very little plankton in the water?
    6 years ago

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