Laser Optical Plankton Counter Research Articles

The impact of advection on a Subarctic fjord food web dominated by the copepod Calanus finmarchicus

Fjord and shelf food webs are frequently supplemented by the advection of external biomass, which in high-latitude seas often comes in the form of lipid-rich copepods that can support a wide range of fish species, including Northeast Arctic cod (Gadus morhua). A seasonal match or mismatch at the lower trophic levels (phytoplankton and zooplankton) is central in determining how much energy and biomass is available for higher trophic levels (fish). Here, we quantify the inflow of the copepod Calanus finmarchicus into the Vestfjorden fjord system using high-resolution measurements of ocean currents and zooplankton (laser optical plankton counter). We evaluate a spatio-temporal match/mismatch between the phytoplankton bloom and Calanus and assess the input of advected copeods at the lower trophic level fjord and shelf food web based on an integrative approach employing stable isotope analyses (C, N), fatty acid trophic marker analyses, and biovolume spectrum analyses. Our results suggest two different sources of the Calanus population in the fjord/shelf system: one fraction overwintered locally and started ascending early to feed on the phytoplankton bloom that peaked around April 11. The other fraction had only recently (end of April) been and still was being advected from the oceanic overwintering habitats. Ca. 119 g C/s of Calanus were advected into the fjord, comparable to the biomass of Calanus advected into an Arctic fjord, and the mesozooplankton community was dominated by the copepod. The fjord food web was tightly coupled between the phytoplankton spring bloom, the local part of the Calanus population (trophic level 1.8–2.4) and cod larvae (high levels of wax esters). On the shelf, our results suggest that the impact of advected Calanus in the food web is at its starting point (low trophic level, large difference of δ13C of POM and Calanus). We highlight important factors that can contribute to the successful spawning of Northeast Arctic cod: an extended phytoplankton bloom that can support both locally and advected Calanus, which in turn can supply the essential nauplii prey for first-feeding cod larvae.

Effects of Mixing and Stratification on the Vertical Distribution and Size Spectrum of Zooplankton on the Shelf and Slope of the Northern South China Sea

The northern shelf and off-shelf regions of the South China Sea (SCS) present a stark contrast between being eutrophic from terrestrial runoffs of nutrients and biota and being oligotrophic with multiple nutrient limitations due to the distance to land sources and stable permanent stratification. The abundance, size, and trophic structures of plankton in the shelf and off-shelf regions were studied in a joint Sino-German cruise conducted between September 1 and 24, 2018. A laser optical plankton counter was mounted on a water sampler-CTD (conductivity–temperature–depth) system for studying the horizontal and vertical distributions of plankton structures in a size range between 0.1 and 35 mm and their relationships with hydrographic and biological features in the northern SCS and its shelf region. Results revealed the subsurface chlorophyll maximum layer (SCM) below the pycnocline and plankton aggregation near the SCM about a depth of 50–60 m. The distributions of small plankton between 0.1 and 0.5 mm were strongly correlated with stratification and SCM compared to those of large plankton. Analyzing the intercept and slope of a normalized biovolume spectrum (NBVS) as an indicator for abundance and size structure of a plankton community, results revealed that in the shelf region, the slopes exhibited no significant vertical variations in the water column regardless of stratification and SCM. In contrast, in the off-shelf stratified water column, the intercepts and slopes were lower and flatter in the surface layer, higher and steeper in the SCM layer, and the lowest and flattest in the deep layer. Stirring by the typhoon also altered both the abundances and size structures of the plankton communities, with significant regional differences. This study elucidates the variances of plankton abundances, distributions, NBVS slopes, and intercepts among different water column structures in both shelf and off-shelf regions of the northern SCS.

Large scale patches of Calanus finmarchicus and associated hydrographic conditions off the Lofoten archipelago

Large swarms of individuals at different spatiotemporal scales characterise the distributions of many animal species. In the ocean several mesozooplankton taxa aggregate in large patches or swarms driven by active behavioural responses to hydrographic structures, although intrinsic biotic characteristics of species´ life cycles not related with the environment can also affect spatial distributions. To understand the mechanisms of aggregation and disentangle environmental and pure spatial effects contributing to patch formation, we extensively sampled a large Calanus finmarchicus patch off the Lofoten islands, northern Norway, in spring 2017 by means of Laser Optical Plankton Counter (LOPC) cross shelf transects and biophysical samplings at fixed stations. We observed a clear association between the buoyant layer of the fresh, cold Norwegian Coastal Current (NCC) and high surface copepod abundances. Off shelf lateral displacements of the NCC along its path may retain copepods from the offshore basins. At deeper layers, copepod abundances increased markedly at regions with low sea level anomalies and reduced vertical flows. Our results also suggest that copepods performed short range diel vertical migration within the patch. Potential future changes in the seasonality and structure of the NCC and their impact on swarm occurrence and formation are discussed.

Pelagic forage fish distribution in a dynamic shelf ecosystem – Thermal demands and zooplankton prey distribution

The fine-scale distribution of pelagic forage fish is shaped by competing factors as fish optimise foraging while avoiding predation. We investigated the distribution of forage fish in surface waters of a dynamic coastal environment during two spring seasons to examine their distribution in relation to environmental variables. Using a multi-frequency echosounder and a towed Laser Optical Plankton Counter (LOPC), we investigated the effects of bathymetry, temperature, chlorophyll a concentration and zooplankton biomass on forage fish density. Relationships between fish density and these variables were consistent between surveys, despite large differences in total acoustic energy attributed to fish. Fish density showed a strong positive relationship with bathymetry and water temperature, and no relationship with surface zooplankton biomass density or chlorophyll a. This mismatch between fish and zooplankton may be caused by differences in the way fish perceive the distribution of prey versus temperature and predators in shallow coastal waters. Seeking out warmer temperatures along the shelf break may also improve fish physiological performance when cooler spring temperatures are below their thermal optimum. Understanding the distribution of coastal forage fish may contribute to interpreting nearshore movements of their predators.

Heterogeneity in zooplankton distributions and vertical migrations: Application of a laser optical plankton counter in offshore Lake Michigan

Zooplankton distributions are patchy due to multiple physical, chemical, and biological processes, including diel vertical migration (DVM) behavior. Heterogeneity in the offshore environment is difficult to study with net tows, but newer technologies measure finer-scale distributions. Here, we use laser optical plankton counter (LOPC) data, informed by net tows, to study distributions and DVM of zooplankton in offshore Lake Michigan during July and September 2015. Water column (5–60 m) zooplankton biomass varied by an order of magnitude among transects and a factor of two within individual transects (6–19 km distances); transect coefficients of variation (SD/mean) ranged from 7 to 22% (~0.5 km scale). Horizontal patterns in zooplankton biomass varied among size groups but were consistent from day to night, suggesting that processes driving heterogeneity persist for hours to days. Fine-scale LOPC data show that zooplankton often aggregate in thin layers (1–3 m) within the metalimnion, a feature undetectable by coarser net sampling. Although DVM was not consistently observed, some patterns emerged. Small zooplankton including copepodites, diaptomids (Leptodiaptomus ashlandi, L. minutus), and Diacyclops thomasi often migrated to surface waters at night, and large zooplankton (Limnocalanus macrurus) migrated upward at night in most cases. Beam attenuation coefficient (proxy for phytoplankton biomass) was a significant predictor for zooplankton mean depth (p < 0.001) although it explained more of the variation for night data (R2 = 0.72) than day data (R2 = 0.53). The heterogeneity observed in zooplankton distributions has implications for planktivorous fish feeding in the offshore zone, as prey density varies greatly with depth.

Dynamics of Calanus Copepodite Structure during Little Auks’ Breeding Seasons in Two Different Svalbard Locations

Populations dynamics of key zooplankton species in the European Arctic, Calanus finmarchicus and Calanus glacialis (hereafter defined as Calanus) may be sensitive to climate changes, which in turn is of great importance for higher trophic levels. The aim of this study was to investigate the complete copepodite structure and dynamics of Calanus populations in terms of body size, phenology and their relative role in the zooplankton community over time in different hydrographic conditions (two fjords on the West Spitsbergen Shelf, cold Hornsund vs. warm Kongsfjorden), from the perspective of their planktivorous predator, the little auk. High-resolution zooplankton measurements (taken by nets and a laser optical plankton counter) were adapted to the timing of bird’s breeding in the 2015 and 2016 summer seasons, and to their maximal diving depth (≤50 m). In Hornsund, the share of the Calanus in zooplankton community was greater and the copepodite structure was progressively older over time, matching the little auks timing. The importance of Calanus was much lower in Kongsfjorden, as represented mainly by younger copepodites, presumably due to the Atlantic water advections, thus making this area a less favourable feeding ground. Our results highlight the need for further studies on the match/mismatch between Calanus and little auks, because the observed trend of altered age structure towards a domination of young copepodites and the body size reduction of Calanus associated with higher seawater temperatures may result in insufficient food availability for these seabirds in the future.

Vertical Motions and Their Effects on a Biogeochemical Tracer in a Cyclonic Structure Finely Observed in the Ligurian Sea

AbstractVertical velocities can be estimated indirectly from in situ observations by theoretical frameworks like the ω‐equation. Direct measures of vertical exchanges are challenging due to their typically ephemeral spatiotemporal scales. In this study we address this problem with an adaptive sampling strategy coupling various biophysical instruments. We analyze the 3‐D organization of a cyclonic mesoscale structure finely sampled during the Observing Submesoscale Coupling At High Resolution cruise in the Ligurian Sea during fall 2015. The observations, acquired with a moving vessel profiler, highlight a subsurface low‐salinity layer (≃50 m), as well as rising isopycnals, generated by geostrophic cyclonic circulation, in the structure's center. Reconstructed 3‐D fields of density and horizontal velocities are used to estimate the vertical velocity field down to 250 m by applying the adiabatic QG ω‐equation, for the first time in this region. The vertical motions are characterized by multipolar patterns of downward and upward velocities on the edges of the structure and significantly smaller vertical velocities in its center. Both the 3‐D distribution of particles (size ≥100 μm), measured with a laser optical plankton counter, and the Synechococcus and Prochlorococcus abundances (cell per cubic meter) measured by flow cytometry are consistent with the 3‐D velocity field. In particular, a secondary vertical recirculation is identified that upwells particles (from 250 to 100 m) along isohalines to the structure's center. Besides demonstrating the effect of vertical patterns on biogeochemical distributions, this case study suggests to use particle matter as a tracer to assess physical dynamics.

Spatial variability and size structure of particles and plankton in the Fram Strait

Changes within marine pelagic fields in the Arctic affect global biogeochemical cycles. The size structure and distribution of particles and plankton (P&P) were studied in the main northward passage of Atlantic water to the Arctic region, which is affected in various ways by climate change. The wide size spectrum of P&P (3 µm–5 mm) was assessed by combining measurements from two complementary optical counters, a Laser In-Situ Scattering and Transmissometry (LISST) instrument and a Laser Optical Plankton Counter (LOPC). The measurements were performed in the upper 100 m layer at 55 stations in summer in 2013. The P&P were attributed to various functional groups – particles, phytoplankton, marine snow, and zooplankton – to test their spatial coupling with hydrodynamic forcing. Although our study confirms that the Fram Strait is a region of strong biophysical contrasts, the expected coastal-offshore gradient has not been distinguished clearly. Three mechanisms could explain P&P patches: particle flocculation due to contact with fresh waters from ice and fjords, enhanced production due to coastal upwelling as well as widening the range of warm surface water advection northwards. Particularly rich biological hot spots were recognised in the northern, shallow parts of the west Spitsbergen shelf, and near the marginal ice edges, confirming that these areas are highly vulnerable to marine predators. Moreover, combining optical measurements from two size-specific optical counters contributes greatly to reducing the substantial knowledge gap regarding wide size structure and distribution of thus far neglected seawater components, such as non-living particles and marine snow aggregates, occurring in this vulnerable Arctic region.

Conditions for assessing zooplankton abundance with LOPC in coastal waters

Recent technical advances in laser-based systems to measure zooplankton distribution have opened new perspectives in ecological and behavioral studies by significantly improving the horizontal and vertical sampling resolution, providing information on zooplankton patchiness and on the influence of small scale physical processes. The application of laser-based systems also led to new challenges on the identification of organisms vs. particulate matter. In areas with high detritus abundances, zooplankton abundances might be overestimated by counting plankton and detritus together. We investigated the contribution of detritus in Laser Optical Plankton Counter (LOPC) data collected during two cruises on the continental shelf of the Gulf of Lion (NW Mediterranean Sea). The study area was characterized by several types of ecoregions owing to the influence of winds, freshwater runoff and intrusion of oligotrophic waters from offshore. We identified the main mechanisms leading to the formation of detritus as a function of environmental conditions and developed a method to assess the contribution of detritus in LOPC counts based on the proportion of large particles (multi-element plankton, MEPs). Highest percentages of detritus (up to 90% of the counts, mainly particulate organic matter from various sources) were found in stratified conditions associated with relatively high chlorophyll a concentration (chl-a; ca 2 mg m−3). Discontinuities in density profiles alone also resulted in peaks of particles concentrations. We suggested a threshold of 2% of MEPs in LOPC counts above which the LOPC is most likely counting more detritus than organisms. This easy check of the detritus contribution to total LOPC counts was applied to datasets from different marine ecological situations (glacial input, clear water, productive shelf) and gave successful results in different biogeographical regions (e.g. high latitude and tropical habitats).

Fine-scale zooplankton diel vertical migration revealed by traditional net sampling and a Laser Optical Plankton Counter (LOPC) in Lake Ontario

Abstract Traditional net hauls used to monitor zooplankton population change in the Great Lakes do not provide information on vertical distribution – an important aspect of the availability of zooplankton to predators. During 2013, we combined net data with data from a Laser Optical Plankton Counter (LOPC) to track the vertical distribution of zooplankton in Lake Ontario in May, July, and September. Closing nets (64 μm mesh) sampled three strata at stations of north-south transects. For a 10 km segment of each transect in May and September, a LOPC was towed from the surface to 60 m depth. In May, biomass was highest in the epilimnion both day and night. Diacyclops thomasi dominated in the epilimnion while Limnocalanus macrurus dominated deeper. During strongly stratified conditions in July and September, closing net data confirmed that biomass was highest in the metalimnion during the day and in the epilimnion during the night. D. thomasi, Daphnia retrocurva, and L. macrurus were major components and vertical migrators. In May, LOPC data showed that biomass was highest in the upper 10 m of the epilimnion for day and night. In September, LOPC data showed two layers during the day, a narrow 5 m layer in the epilimnion at about 20 °C and a 15 m layer below the thermocline at 10 °C. The lower layer followed the deepening thermocline from west to east. At night, the LOPC detected most biomass throughout the epilimnion. The LOPC data confirmed diel vertical migration and detailed the fine-scale structure of zooplankton layers.

Examination of a high resolution laser optical plankton counter and FlowCAM for measuring plankton concentration and size

Many commercial ships will soon begin to use treatment systems to manage their ballast water and reduce the global transfer of harmful aquatic organisms and pathogens in accordance with upcoming International Maritime Organization regulations. As a result, rapid and accurate automated methods will be needed to monitoring compliance of ships' ballast water. We examined two automated particle counters for monitoring organisms ≥50μm in minimum dimension: a High Resolution Laser Optical Plankton Counter (HR-LOPC), and a Flow Cytometer with digital imaging Microscope (FlowCAM), in comparison to traditional (manual) microscopy considering plankton concentration, size frequency distributions and particle size measurements. The automated tools tended to underestimate particle concentration compared to standard microscopy, but gave similar results in terms of relative abundance of individual taxa. For most taxa, particle size measurements generated by FlowCAM ABD (Area Based Diameter) were more similar to microscope measurements than were those by FlowCAM ESD (Equivalent Spherical Diameter), though there was a mismatch in size estimates for some organisms between the FlowCAM ABD and microscope due to orientation and complex morphology. When a single problematic taxon is very abundant, the resulting size frequency distribution curves can become skewed, as was observed with Asterionella in this study. In particular, special consideration is needed when utilizing automated tools to analyse samples containing colonial species. Re-analysis of the size frequency distributions with the removal of Asterionella from FlowCAM and microscope data resulted in more similar curves across methods with FlowCAM ABD having the best fit compared to the microscope, although microscope concentration estimates were still significantly higher than estimates from the other methods. The results of our study indicate that both automated tools can generate frequency distributions of particles that might be particularly useful if correction factors can be developed for known differences in well-studied aquatic ecosystems.

Plankton patchiness in the Polar Front region of the West Spitsbergen Shelf

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 560:1-18 (2016) - DOI: https://doi.org/10.3354/meps11925 FEATURE ARTICLEPlankton patchiness in the Polar Front region of the West Spitsbergen Shelf Emilia Trudnowska*, Marta Gluchowska, Agnieszka Beszczynska-Möller, Katarzyna Blachowiak-Samolyk, Slawomir Kwasniewski Institute of Oceanology Polish Academy of Sciences, 81-712 Sopot, Poland *Corresponding author: emilia@iopan.gda.pl ABSTRACT: Plankton patchiness, a widely known but poorly studied phenomenon, was investigated concurrently with measurements of environmental conditions in the Polar Front region west of Spitsbergen in summer (June-July) 2012. Continuous high-resolution observations were made using a laser optical plankton counter, a fluorometer and CTD along several sections in the surface 50 m of the sea with additional data from zooplankton net sampling and full-depth range hydrographical measurements at stations. The observed patches differed in terms of size, number and distribution patterns. They occupied only a minor portion of the studied ocean area (2-17%), but contained plankton concentrations that were 3-17 times higher than the concentrations in the background waters. The generation and existence of patches was associated with particular conditions created by increased primary production, optimal hydrography, discontinuities in the density field and/or the presence of meanders or eddies. The distribution of small size fraction patches, consisting of the phytoplankton, inorganic particles and detritus, was closely related to specific patterns in hydrography. The medium-size zooplankton and marine snow were concentrated mainly in conjunction with discontinuities in density field. The distribution of the large copepods was independent of any oceanographic feature detected in this study, which points towards biological cues as factors regulating zooplankton aggregation processes. Diverse mechanisms of patch formation as well as their ecological significance are discussed in the context of planktonic responses to the structured environment in the region of the Polar Front. KEY WORDS: Zooplankton · Patchiness · Zooplankton distribution · Laser optical plankton counter · Eddy · Polar Front · West Spitsbergen Current Full text in pdf format Information about this Feature Article NextCite this article as: Trudnowska E, Gluchowska M, Beszczynska-Möller A, Blachowiak-Samolyk K, Kwasniewski S (2016) Plankton patchiness in the Polar Front region of the West Spitsbergen Shelf. Mar Ecol Prog Ser 560:1-18. https://doi.org/10.3354/meps11925 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 560. Online publication date: November 24, 2016 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2016 Inter-Research.

Comparison of acoustical and optical zooplankton measurements using an acoustic scattering model: A case study from the Arctic frontal zone

Abstract High-frequency acoustic measurements supplemented by a modern optical method, Laser Optical Plankton Counter (LOPC), allowed us to perform a comparative analysis through the application of a mathematical model. We have studied the correspondence between measured and modelled echoes from zooplankton aggregations consisted mainly of two Calanus species. Data were collected from the upper 50 m water layer within the hydrographical frontal zone on the West Spitsbergen Shelf. The application of a “high-pass” model of sound scattering by fluid-like particles to the distribution of zooplankton sizes measured by LOPC resulted mostly in very good agreement between the measured (420 kHz BioSonics) and modelled values, except for cases with very low zooplankton abundance or with occurrence of stronger scatterers (e.g. macrozooplankton, fish). An acoustic model validated for the elastic parameters of zooplankton confirmed that particles smaller than 1mmin diameter, although highly abundant, did not contribute significantly to the sound scattering process at a frequency of 420 kHz. The implementation of diverse complementary methods has great potential to obtain high spatial and temporal resolution in zooplankton distribution studies; however, their compatibility has to be tested first.

Zooplankton abundance and biomass size spectra in the East Antarctic sea-ice zone during the winter–spring transition

Sea ice is an influential feature in Southern Ocean-Antarctic marine environments creating a 2-phase vertical ecosystem. The lack of information on how this system influences community structure during the winter-spring transition, however, is largely lacking. Zooplankton form the link that bridges these environments, with the meiofaunal and algal communities within sea ice directly influencing the epipelagic zooplankton community at the ice-water interface. A combination of methods including sea-ice coring, umbrella net sampling and Laser Optical Plankton Counter were used to describe the vertical structure of zooplankton and meiofaunal communities. The distribution of meiofauna and chlorophyll a both played important roles in structuring the zooplankton community within this dynamic region. Many dominant taxa, including Calanus propinquus and Oithona similis, directly responded to the high availability of algae present within the bottom strata of sea ice. The sea-ice associated species Stephos longipes represented a strong link between this 2-phase ecosystem. Observations of the vertical distribution of biomass obtained from the LOPC suggests that the responses of these species to the sea ice directly influences the vertical structure of zooplankton during the winter-spring transition.

Springtime zooplankton size structure over the continental shelf of the Bay of Biscay

Abstract. Linking lower and higher trophic levels requires special focus on the essential role played by mid-trophic levels, i.e., the zooplankton. One of the most relevant pieces of information regarding zooplankton in terms of flux of energy lies in its size structure. In this study, an extensive data set of size measurements is presented, covering parts of the western European continental shelf and slope, from the Galician coast to the Ushant front, during the springs from 2005 to 2012. Zooplankton size spectra were estimated using measurements carried out in situ with the Laser Optical Plankton Counter (LOPC) and with an image analysis of WP2 net samples (200 μm mesh size) performed following the ZooScan methodology. The LOPC counts and sizes particles within 100–2000 μm of spherical equivalent diameter (ESD), whereas the WP2/ZooScan allows for counting, sizing and identification of zooplankton from ~ 400 μm ESD. The difference between the LOPC (all particles) and the WP2/ZooScan (zooplankton only) was assumed to provide the size distribution of non-living particles, whose descriptors were related to a set of explanatory variables (including physical, biological and geographic descriptors). A statistical correction based on these explanatory variables was further applied to the LOPC size distribution in order to remove the non-living particles part, and therefore estimate the size distribution of zooplankton. This extensive data set provides relevant information about the zooplankton size distribution variability, productivity and trophic transfer efficiency in the pelagic ecosystem of the Bay of Biscay at a regional and interannual scale.

Mid-summer mesozooplankton biomass, its size distribution, and estimated production within a glacial Arctic fjord (Hornsund, Svalbard)

The estimation of secondary production constitutes an integrating proxy of pelagic ecosystem status, its functions as well as its responses to environmental stressors. The combination of high-resolution automatic measurements with a Laser Optical Plankton Counter (LOPC) and size spectrum analyses was utilized to estimate the secondary production of a high Arctic fjord during a summer post bloom situation in 2012. The dataset comprised 28 vertical and extensive horizontal hauls of a LOPC–CTD–fluorometer platform plus four zooplankton net sampling stations for taxonomic composition designation. A clear gradient in temperature, salinity, chlorophyll a concentrations as well as mesozooplankton abundance, biomass and production was demonstrated along Hornsund fjord axis. The outer fjord part was under the influence of water advection and had the highest chlorophyll a concentrations, numerous opaque mesozooplankton individuals and flat slopes of size spectra, pointing to long food chains in which biomass is recycled several times. The opposite state was found in the glacial bays, where the glacier meltwater discharge led to low chlorophyll a concentrations but high abundance of small and amorphous particles. It resulted in steep size spectra slopes and high intercepts implying higher potential productivity there. The model of mesozooplankton production demonstrated that Hornsund fjord is a highly productive ecosystem, particularly its upper water layer and its central parts. However, we would like to emphasize that a careful approach is needed before going deeper into ecological interpretations based on size spectra analysis, especially in reservoirs, where non-zooplankton particles contribute to the size spectra.

Secondary production at the Polar Front, Barents Sea, August 2007

To investigate spatial patterns of secondary production we sampled four core hydrographical regions of the Polar Front in the Barents Sea (Arctic Water, ArW; Polar Front Water, PFW; Atlantic Water, AtW; and Melt Water, MW) by towing an undulating instrument platform along a transect crossing the front from August 8–9, 2007. Sensors mounted on the platform provided data on the hydrography (CTD), fluorescence (Fluorometer, F) and zooplankton abundance in the size range between 0.1 and 30mm (Laser Optical Plankton Counter, LOPC). These continuous, biophysical data with high-spatial resolution were supplemented by discrete water and zooplankton net samples at stations for sensor calibrations. After in depth quality assessments of the biophysical data, estimates were made of the vital rates based on biovolume spectrum theory. Five size groups were distinguished from the LOPC data: small (S), mainly Oithona spp. and the appendicularian Fritillaria sp.; medium (M), mainly Pseudocalanus spp. and Calanus spp. CI–CIII; large (L), mainly Calanus spp. CIV–CV; and extra large (XL and 2XL), juvenile and adult euphausids. Size groups were further divided based on transparency of organisms. Vital rates based on the biophysical in situ data in combination with biovolume spectrum theories agreed generally well with data from empirical and numerical models in the literature. ArW was characterised by subsurface maxima of chlorophyll a (chl a), and an estimated population growth of ca. 13mgCm−3d−1 for CI–CIII Calanus spp. and some older Pseudocalanus within the chl a maxima. Frontal waters were characterised by low chl a concentrations, but high abundances and production (around 1gCm−3d−1) of small copepods (Oithona spp.) and appendicularians (Fritillaria sp.). The estimated production of small-size zooplankton was an order of magnitude higher than the production of all other size groups combined, including large copepods. The high loss rates (−166 to −271mgCm−3d−1) of small zooplankton may contribute a substantial amount of carbon to the benthos and to pelagic predators such as young capelin. AtW was the most productive water mass, with surface chl a maxima and an estimated population growth of 134mgCm−3d−1 for small zooplankton, 3.6mgCm−3d−1 for medium-sized copepods and 0.9mgCm−3d−1 for CIV–CVI Calanus. For those Calanus spp. in the surface layer, the estimated specific mortality rates were up to −0.35d−1, partly due to high predation pressure by hydrozoans and chaetognaths.

Evaluation of chitobiase-based estimates of biomass and production rates for developing freshwater crustacean zooplankton communities

Increasing attention has been devoted to the development of alternative (often biochemical) methods for measuring crustacean zooplankton productivity because conventional methods are not globally applicable and rarely practical when community-level rates are required. Here we evaluate the chitobiase method as a rapid, routine and instantaneous method for measuring the productivity of freshwater crustacean zooplankton communities. Chitobiase, a moulting enzyme, is liberated into water following moulting and production rates are calculated by measuring its turnover rate in the water column. First, using literature-based instarand stage-specific individual body mass values, we found a common relationship between post-moult body size (and individual chitobiase activity) and the biomass produced between successive moults for common freshwater groups. Secondly, using a time-series of weekly measurements in a North-Temperate lake, we found a good correspondence between the standing activity of chitobiase in the water column (CBANAT) and the biomass sampled by a plankton net and laser optical plankton counter (LOPC). Overall, however, CBANAT-based biomass more closely corresponded to LOPC-based biomass estimates. Lastly, depth-specific biomass production rates and daily production to biomass estimates varied positively with temperature. Daily production to biomass ratios also varied closely with

Examining shifts in zooplankton community variability following biological invasion

We examined 20 yr of zooplankton samples from Harp Lake for shifts in zooplankton variability following invasion by zooplankton predator Bythotrephes longimanus, using organism body size—as measured at high resolution by Laser Optical Plankton Counter (LOPC)—as the primary metric of investigation. A period of transitory high variability in the 2 yr post‐invasion was observed for both body size compositional variability and aggregate variability metrics, with both measures of variability shifting from low or intermediate to high variability immediately following invasion, before shifting again to intermediate variability ∼ 2 yr post‐invasion. Aggregate and compositional variability dynamics were also considered in combination over the study period, revealing that the period of transitory high variability coincided with a shift from a community‐wide stasis variability pattern to one of asynchrony, before a shift back to stasis 2 yr post‐invasion. These dynamics were related to changes in the significant zooplankton species within the Harp Lake community over the pre‐ and post‐invasion periods, and are likely to be indicative of changes in the stability in the zooplankton community following invasion by Bythotrephes. The dual consideration of aggregate and compositional variability as measured by LOPC was found to provide a valuable means to assess the ecological effects of biological invasion on zooplankton communities as a whole, extending our knowledge of the effects of invasion beyond that already revealed through more traditional taxonomic investigation.

Capturing quantitative zooplankton information in the sea: Performance test of laser optical plankton counter and video plankton recorder in a Calanus finmarchicus dominated summer situation

We compared two optical plankton counters, the Laser Optical Plankton Counter (LOPC) and the Video Plankton Recorder (VPR) for their abundance estimates of Calanus finmarchicus during an early summer situation (June 2008) in two North Norwegian fjords. The LOPC was mounted on the VPR frame in order to sample the same body of water. The combined system of LOPC and VPR was operated by vertical profiling from the surface to 100m of depth in several locations of the fjords representing different blooming conditions and zooplankton community structures. Data from the two instruments, as well as from CTD-F, were logged concurrently and retrieved on deck after about 15 depth profiles. Primary data were analysed according to standard routines, and choices made during sampling and analyses (sampling volume, selection of size range, transparency of particles, statistics) are discussed. Data were averaged for every 5, 10 and 15m depth bins. The vertical profiles of C. finmarchicus CIV–CVI abundance that were obtained by LOPC and VPR, respectively, showed a striking similarity. No significant differences between profiles sampled by these two instruments were observed when data were binned into 15m bins. At low abundances (<100 Calanus sp. L−1) profiles were significantly different when data were binned into 5- or 10-m bins. This is attributed to the small sampling volumes of the LOPC and the VPR, and to very patchy distributions of copepods, resulting in a high standard deviation between consecutive profiles. Based on the results we conclude that the time is mature for a more extensive use of optical instruments to estimate zooplankton abundances and distributions in the sea.