Interannual–interdecadal variations in zooplankton biomass, chlorophyll concentration and physical environment in the subarctic Pacific and Bering Sea

Interannual, decadal and interdecadal variations in summer plankton biomass during 1954–1994 in the whole subarctic Pacific and Bering Sea were compared among regions as well as with climatic and oceanographic conditions. The zooplankton biomass and chlorophyll concentration during the mid 1960s to early 1970s in the central and western subarctic Pacific were a few times higher than those in the preceding and following decades. The values in the eastern Bering Sea and eastern subarctic Pacific also increased in the mid 1960s, but remained at an elevated level until the end of the 1980s. These decades of higher and mid plankton biomass levels during the mid 1960s to early 1970s and mid 1970s to late 1980s correspond to the period of positive and negative values of the Northern Hemisphere zonal index (NHZI), respectively. In the decadal scale, one can see a significant positive correlation between the summer plankton biomass and the wind speed during winters in the eastern Bering Sea. The effect of grazing by biennially fluctuating Asian pink salmon on zooplankton biomass and its effect on chlorophyll concentration in the central subarctic Pacific is also significant.     

Interdecadal variations of plankton biomass and physical environment in the North Pacific

In the central and western subarctic Pacific, zooplankton biomass and chlorophyll concentrations during the mid 1960s to mid 1970s were a few times higher than in the preceding and following decades, corresponding to higher values of the atmospheric Northern Hemisphere Zonal Index (NHZI). In the Alaskan Gyre, however, it was reported that biomass of zooplankton and nekton doubled after the atmospheric regime shift in the mid 1970s. In the subtropical North Pacific, chlorophyll a concentration decreased drastically after 1980, although a decrease of zooplankton biomass was clearly seen only in the northern part of the subtropical gyre. Chlorophyll concentration in the central subarctic Pacific and zooplankton biomass in the Oyashio have been decreasing since the early 1980s. Additionally, chlorophyll concentration in the western subarctic Pacific and eastern Bering Sea, and zooplankton biomass in the central subarctic Pacific and eastern Bering Sea have also been decreasing since the late 1980s. In these regime-shift situations, there is a general tendency for intensification of wind speed or de-stratification to cause plankton biomass to decrease in regions where the upper mixed layer is deep, such as the western subarctic and north-western subtropical water, whereas in relatively stratified areas, such as in the eastern subarctic and south-western subtropical water, the effect is an increase of plankton biomass.     

Evidence for a substantial increase in gelatinous zooplankton in the Bering Sea, with possible links to climate change

We examined quantitative catches of large medusae from summer bottom trawl surveys that sampled virtually the same grid station on the eastern Bering Sea shelf using the same methodology every year from 1979 to 1997. This series shows a gradual increase in biomass of medusae from 1979 to 1989, followed by a dramatic increase in the 1990s. The median biomass increased tenfold between the 1982–1989 and 1990–1997 periods. Most of this biomass was found within the Middle Shelf Domain (50 <  z  < 100 m). The greatest rate of increase occurred in the north-west portion of this domain. Whether this dramatic increase in biomass of gelatinous zooplankton has resulted from some anthropogenic perturbation of the Bering Sea environment or is a manifestation of natural ecosystem variability is unclear. However, several large-scale winter/spring atmospheric and oceanographic variables in the Bering Sea exhibited concomitant changes beginning around 1990, indicating that a possible regime change occurred at this time.     

Planktivory and diet-overlap of densely rakered whitefish (Coregonus lavaretus (L.)) in a subarctic lake

Abstract –  Three sympatric whitefish ( Coregonus lavaretus (L.)) forms, one being pelagic and two benthic, segregate available habitat and food resources in subarctic Lake Muddusjärvi, northern Finland. Zooplankton availability in the lake, food composition, diet-overlap and growth of densely rakered (DR) whitefish were examined during June to September to explore the reasons for the small individual size of the pelagic form. DR whitefish used zooplankton as main food item and prey selection followed zooplankton species density proportions in the lake. Zooplankton density and water temperature was highest in July–August. The average lengths of Bosmina spp., Daphnia spp., Calanoida and Cyclopoida in DR whitefish stomach were higher than in zooplankton sample during June–September, except Calanoida in June. Diet-overlap between DR whitefish age groups was high at all months indicating intercohort resource competition. DR whitefish reached sexual maturity at 3 years of age and at the length of 12 cm, after which somatic growth almost ceased. Reason for the small average size and slow growth of DR whitefish were connected to high diet-overlap between age groups and early sexual maturation.     

Effects of fish stocking on the zooplankton community structure in a shallow lake in China

Abstract  Changes in the zooplankton community structure in relation to fishery practices in Lake Donghu, Wuhan, China were examined. The number of Protozoa species increased slightly, whereas the number of rotifers and crustaceans decreased from the 1960s to the 1990s. The total annual average densities of zooplankton increased 15–20 times in the 1990s compared with the 1960s. This increase was largely attributed to Protozoa, which contributed 93.4% by number of the total zooplankton density in 1991. Cladoceran densities decreased markedly from 1987. Changes in densities of rotifers and copepods were not evident. Trends in zooplankton biomass were similar to density. Large changes in zooplankton community structure coincided with markedly changes in concentration of chlorophyll a and transparency in Lake Donghu in 1987. The year 1987 seems to be the threshold year when the zooplankton community structure changed considerably. These changes were related to continuously increasing fish stock biomass in the lake. It was suggested that fish stocking and fish biomass should be a better managed for improvement of the quality of the lake's environment.     

Assessing zooplankton advection in the Barents Sea using underway measurements and modelling

The coastal shelf of northern Norway and the Barents Sea is highly advective and mainly affected by the North Atlantic Current and the Norwegian Coastal Current. The oceanographic conditions are an important factor for the spatial and temporal distribution of zooplankton in the region. To quantify zooplankton advection over the western border of the Barents Sea, a Scanfish/Optical Plankton Counter (OPC), Acoustic Doppler Current Profiler (ADCP), Multiple Opening and Closing Nets and Environmental Sampling System (MOCNESS), and hydrodynamic model were used. This study provides data from two time windows (June and July 1998) by continuous measurements between northern Norway and Bear Island. The zooplankton community structure was obtained by net tows and zooplankton abundance fields were mapped by an OPC counting zooplankton in the size range 0.7–14 mm equivalent spherical diameter. A simple zooplankton community structure was found with the copepod Calanus finmarchicus (CIII–CV) as the dominant species in this size fraction. Ocean currents were measured by a ship‐mounted ADCP and the residual currents were calculated by subtracting the tidal component obtained from a hydrodynamic model. Two measurements conducted in June and July 1998 shows a net eastward transport of water of 3.5 and 1.3 Sverdrup over the section. For the same two periods, zooplankton biomass transport is also positive towards east but varies by two orders of magnitude between the two measurements.     

Strategies for Development of Rotifers as Larval Fish Food in Ponds

Strategies to sustain rotifer peak biomass, distribution of rotifer resting eggs in the sediment, and relationship between rotifers and larval fish growth were studied in a series of pond experiments. After the ponds were filled with water, herbivorous rotifers (e.g., Brachionus calyciflorus ) developed first, but were gradually replaced by predatory rotifers (e.g., Asplanchna ). Subsequently, herbivorous cladocerans (e.g., Moina sp) eventually replaced rotifers and dominated the zooplankton community. The occurrence of Asplanchna and Moina indicated the decline of B. calyciflorus . Peak rotifer biomass developed 8–10 d after the ponds were filled with water at 20–25 C, 10–15 d at 17–20 C, 15–20 d at 15–17 C, 20–30 d at 10–15 C, and >30 d at < 10 C. The abundance of resting eggs in the top 5-cm sediment varied from 6 to 83/cm². About 25% of resting eggs were buried in the top 5-cm sediment but the number of resting eggs decreased with increased sediment depth. Optimum rotifer biomass for silver carp Hypophthalmichthys molitrix larvae stocked at 1,500,000/ha was 20–40 m/gL. High rotifer biomass (>20 mg/L) usually lasted 3–5 d, but could be prolonged by pond fertilization or cladoceran controls. A weekly application of dipterex at 0.05 mg/L reduced cladoceran biomass but enhanced rotifer biomass. Our results indicate with a careful management plan it is possible to synchronize the rotifer development with larval fish stocking.     

Polar light regime and piscivory govern diel vertical migrations of planktivorous fish and zooplankton in a subarctic lake

Abstract –  Seasonal variation in light intensity has strong impacts on invertebrate and vertebrate habitat selection creating trade-offs between foraging gain and risk of death. Diel vertical migration (DVM) has received a particularly interest, but multitrophic level studies in lakes under polar light regime have not been conducted. Here, we examined habitat selection of pelagic zooplankton, planktivores and piscivores in subarctic Lake Muddusjärvi with polymorphic whitefish ( Coregonus lavaretus (L.)). Seasonal change in light was hypothesized to be the most important abiotic factor inducing DVM, whereas predation was considered as an ultimate biotic factor. During period of mid-night sun in June, no DVM was observed at any trophic level, whereas during normal day and night light in September planktivores and zooplankton migrated. DVM was top-down controlled, where piscivorous brown trout ( Salmo trutta L.) used pelagic habitat continuously inducing DVM of its main prey, pelagic whitefish morph, which cascaded to reverse DVM of zooplankton. Top-down control of lower trophic level DVMs by piscivores might be more general pattern in lakes than previously considered.     

Covariation in climate, zooplankton biomass and mackerel recruitment in the southern Gulf of St Lawrence

Between 1982 and 1991, an annual survey of stage I egg production of Atlantic mackerel ( Scomber scombrus ) was conducted in June/early July in the southern Gulf of St Lawrence. We investigated the relationship between interannual variability in biomass of zooplankton, determined from the archived survey plankton samples, and mackerel recruitment, estimated from the proportion of three-year-olds in the catch of the commercial fishery. Zooplankton biomass varied by a factor of 2.5, primarily owing to fluctuations in the >1000 μm size fraction. The index of mackerel recruitment fluctuated by a factor of ≈20 and was positively related (linear regression: P  < 0.05; n  = 10) to variations in the zooplankton biomass. Both mackerel recruitment and zooplankton biomass were negatively related (linear regression: P  < 0.05) to RIVSUM, a measure of freshwater discharge from the St Lawrence River system and an index of variability in the region's climate. Three hypotheses are put forward to explain these observations: (1) there is a strong link between interannual variation in abundance of copepod females, which produce prey for mackerel larvae, and larval survival; the exceptional recruitment and subsequent year class resulted from an exceptional production of Calanus finmarchicus nauplii; (2) years of high zooplankton biomass provide better feeding conditions and consequently higher survival of mackerel juveniles; and (3) mackerel recruitment and zooplankton biomass are independently under the control of an underlying physical process, without strong trophic linkage. The first hypothesis is supported by a study of copepod species composition and female abundance conducted for four of the survey years. At the present time, none of these hypotheses can be ruled out.     

North–south contrasts in decadal scale variations in lower trophic-level ecosystems in the Japan Sea

Regional comparisons of interannual variations in springtime lower trophic-level ecosystems were made for northern subarctic regions, and for southern Tsushima Current regions of the Japan Sea, based on archival hydrographic and biological data sets collected from the mid-1960s to the early 1990s. Variations related to the Pacific Decadal Oscillation were detected for plankton biomass in both northern and southern regions, although there were regional differences with respect to mechanisms and timing. Springtime stratification increased after the late 1970s in the north, roughly coinciding with the northern Pacific regime shift in 1976/77. Stratification also increased due to warming in the south in the early 1980s, several years after the 1976/77 regime shift. Responding to the increase in stratification, springtime biomass of phytoplankton and zooplankton increased in the north and decreased in the south. Principal component analysis revealed that hydrographic conditions during spring, rather than winter, determined springtime phytoplankton biomass. In northern regions, spring phytoplankton production may be enhanced by increased light availability, due to mixed layer stabilization. In the south, where background nutrient concentration within the water column was low, increases in stratification were likely to limit nutrient supply to the surface layer, resulting in decreases in phytoplankton production. A positive relationship between phytoplankton and zooplankton biomass suggested bottom-up control of secondary production in northern regions. The nature of the links between phytoplankton and zooplankton production was not clear in southern regions, where hydrographic conditions during winter seemed to be responsible for variations in springtime secondary production.