Interannual phenological variability in two North-East Atlantic populations of Calanus finmarchicus

Phenological variations of the marine copepod Calanus finmarchicus were studied in Svalbard and northern Iceland, where samples were collected in summer and spring, respectively, over two decades. Four phenological indices, developed for copepodite stage-structured data, were used: the proportion of CV to total abundance (CVT), the population development index (PDI), the average weighted stage (AWS), and the average age in days (AAD). The variation of these indices was compared within and between locations to evaluate their suitability for the analysis of phenological effects. For both populations, phenology was related to local temperature and spring bloom dynamics, influenced by Atlantic water inflow. Large-scale climate was related to phenological variation only in the Svalbard population. C. finmarchicus phenology advanced under warmer conditions in both locations. We conclude that vertical phenological indices, i.e. based on interannual changes in copepodite stage structure, are useful to investigate zooplankton phenology, especially when data series covering the whole life cycle are unavailable. We suggest that AWS and AAD can be applied irrespective of sampling time, while PDI and CVT should be applied for early and late sampling seasons, respectively. When multiple phenological indices are needed, AAD in combination with either CVT or PDI should be preferred.     

Seasonal abundance and distribution of Caridea larvae in Isafjord-deep, north-west Iceland

The abundance and distribution of Candea larvae was studiedin Ísafjord-deep, north-west Iceland, at approximatelymonthly intervals from February 1987 to February 1988 Zooplanktonsampling was made at nine stations along the length of the fjord,while temperature and chlorophyll a measurements from one ofthe stations are also presented Larvae of six species occurredin the samples, Eualus pusiolus and Pandalus borealis were mostnumerous, constituting 62 8 and 25 9% of the larvae respectively.The other species were, in declining order of abundance, Pandalusmontagui, Spirontocaris spp. (S spinus and s lilljeborgii) andSabinea septemcarinata. Eualus pusiolus was of highest abundancein the outer and middle parts of the fjord, while P.borealiswas most common in the middle and inner parts The onset of hatchingof all species in April–May appeared closely linked tothe phytoplankton spring bloom, while the temperature in thefjord was by then near the annual low (2–3°C). Exceptfor E pusiolus, of which a small part of the population produceda second brood during the summer, most of the larvae had disappearedfrom the plankton by the middle of August The monthly carapacegrowth of P.borealis larvae during the summer months was estimatedto be 1.0 mm.     

Life-cycle strategies and seasonal migrations of oceanic copepods in the Irminger Sea

Abundance and seasonal vertical distribution of dominant zooplankters in the Irminger Sea was studied from data collected during four cruises between November 1996 and June 1997. In addition, egg production of Calanus finmarchicus was measured during winter, spring and summer 1996–2001. Five taxa constituted >95% of the copepod biomass, C. finmarchicus, Pareuchaeta norvegica, C. hyperboreus, Oithona spp. and Oncaea spp. A seasonal migration pattern was evident for C. finmarchicus, P. norvegica and Oithona spp.: from December to February, they inhabited the deeper layers, whereas, from April to June, they were most abundant in the upper layers. Oncaea spp. also stayed deep during winter and only a very limited part of the population rose to the surface during summer. C. hyperboreus remained deep from December to April, but had virtually disappeared in June. Reproduction of C. finmarchicus took place in May in the surface layers and was linked to the phytoplankton spring bloom. In contrast, reproduction of P. norvegica occurred at depth in February and was uncoupled with the spring bloom. C. hyperboreus did not reproduce in the Irminger Sea. Data on Oithona spp. and Oncaea spp. indicated that the former reproduced between April and June in the upper layers, whereas the latter reproduced year-round at depth. Thus, data on vertical distribution and seasonal stage composition suggested that the dominant copepods are separated, at least partly, at spatial and temporal scales with regard to overwintering and development.     

Seasonal cycle of zooplankton southwest of Iceland

Seasonal variations in biomass, abundance and species compositionof zooplankton in relation to environmental parameters and chlorophylla were studied in both the Coastal water [salinity] (S) <35.0     

Seasonal variations in abundance, development and vertical distribution of Calanus finmarchicus, C. hyperboreus and C. glacialis in the East Icelandic Current

The seasonal variation in abundance and development of Calanusfinmarchicus, Calanus hyperboreus and Calanus glacialis in relationto hydrography and chlorophyll (Chl) a was studied in the Arcticwaters of the East Icelandic Current to the north-east of Icelandfrom March 1995 to February 1996. The sampling was carried outat approximately monthly intervals on a transect of five stationsextending from 67°00'N, 13°55'W to 68°00'N, 12°40'W.In April, May and June, vertical distribution was also investigated.Spring warming of the surface waters began in May, with maximumtemperatures recorded in August (~5°C, mean for uppermost50 m). Below 75 m, temperature remained at <0°C throughoutthe year. The spring bloom of phytoplankton started in earlyMay and the highest Chl a concentrations were measured duringlate May to early June (~1 mg Chl a m^-3). Calanus finmarchicusdominated in terms of numbers (~75%), while C. hyperboreus dominatedbiomass (~76%). Calanus glacialis occurred only in low numbers(~1%) and was only a small portion of biomass (~0.7%). The abundanceof all species was low during the winter and peaked once duringsummer: C. finmarchicus in July (~16 000 ind. m^-2) and C. glacialisand C. hyperboreus in June (~370 and ~7700 ind. m^-2, respectively).The biomass of C. finmarchicus had two maxima, in April (~1.9g m^-2) and July (~1.5 g m^-2), while C. hyperboreus peaked inJune (~12.3 g m^-2). Calanus finmarchicus was estimated to spawnin early May at about the start of the spring bloom, while C.hyperboreus spawned prior to the spring bloom, in late Februaryto early March. On the basis of copepod stage distribution,C. finmarchicus was considered to have a 1-year life cycle andC. hyperboreus at least a 2-year life cycle.     

Abundance and distribution of early life stages of krill around Iceland during spring

Abundance, distribution and development of early life stages of krill (eggs, nauplii, calyptopes and furciliae) around Iceland were studied during the latter half of May 2013. Multivariate analyses were used to examine the relationships between water mass characteristics and phytoplankton spring bloom dynamics and distribution of krill. The results show that krill eggs, nauplii and calyptopes were most abundant over the shelf edges off the southwest and east coasts, while furciliae were most abundant on the shelf off the southwest coast. Meganyctiphanes norvegica and Thysanoessa longicaudata larvae were found mainly in the southwest, while T. inermis larvae were found in highest numbers on the east coast. Redundancy analysis showed that phytoplankton biomass, temperature and bottom depth explained 41% of the distribution pattern of early ontogenetic krill stages. In areas where krill eggs and larvae were most abundant (off the southwest coast), the phytoplankton spring bloom was in an advanced state, and the phytoplankton biomass and temperature were particularly high.