Spatio-temporal patterns in the copepod community in Malangen, Northern Norway

Abstract

The objective of this study was to identify the key copepod species and their life cycles, and provide evidence for any seasonal and spatial changes in the copepod community in Malangen, a fjord located 30 km to the south of Tromso in Northern Norway (69°30'N, 18°21 'E). As a result of high levels of freshwater run-off in May, the fjord became highly stratified with a sharp pycnocline at 10-30 m depth from May to August. The generation patterns of six copepod species are described. Calanus finmarchicus produced one generation during the spring that year, whereas two generations appeared to be produced by both Pseudocalanus acuspes and P. minutus: one in spring (March-June) and the other in autumn (August-December). However, it is uncertain to what extent P. minutus regularly produces a second generation. Two peaks of CI-CIII Metridia spp. were found; there were differences along the length of the fjord in the timing of these, but the relative contributions of M. longa and M. lucens are uncertain. Chiridius armatus CI-CIII peaked in abundance in the spring, which indicates that one main generation was produced at the outer station of the fjord. The copepod community in Malangen could be grouped into three entities according to their numerical abundance during the year: one group of highly abundant forms, generally with maxima >50 000 individuals m-3 (C. finmarchicus, Microcalanus sp., Oithona similis, Oithona spinirostris, Acartia sp. and Pseudocalanus spp.), a second group of less abundant species with a clear seasonality in abundance, varying from 500 to 50 000 individuals m-3 (M. longa, M. lucens, Calanus hyperboreus, C. armatus, Temora longicornis, Oncaea sp., Euchaeta norvegica and Scolecithricella minor), and a third group of 14 holoplanktonic species, sporadically occurring in the fjord. The study demonstrates clear gradients in the abundance of five species along the length of the fjord: the recruiting generation of C. finmarchicus occurred in higher abundances at the outer station in May and June compared to the other inner sites. Later in the season, the reverse situation appeared, in which the population was more abundant in the inner part of the fjord. Calanus hyperboreus increased abruptly in abundance from low winter levels to a maximum in April-May, and declined steadily during the season (except at the innermost station). Metridia lucens, M. longa and C. armatus demonstrated different distribution patterns in Malangen that matched their preferred areas of distribution. Both M. lucens and C. armatus are known as oceanic and deep-water species, respectively, and these were prevalent at the two outer sites in Malangen. Metridia longs is a more neritic species and was found in highest numbers at the two innermost sites. The mechanisms for the differences in abundance among these species within the fjord are discussed.