Vertical Net Hauls Research Articles

Trophic relationships in the zooplankton – gelatinous zooplankton food chain in the shelf areas of the Crimean coast of the Black Sea

The seasonal and spatial dynamics of the key trophic characteristics were studied (food spectrum, feeding rate, and predatory impact on mesozooplankton) for populations of the jellyfish Aurelia aurita (Linnaeus, 1758) and ctenophore Mnemiopsis leidyi A. Agassiz, 1865. The investigation was carried out during four cruises of the RV “Professor Vodyanitsky” in the shelf areas of Crimean Peninsula in January to October 2016. The area was divided into inner (depth of < 50 m) and outer (51–150 m) shelves. To study the food spectrum and feeding rate of gelatinous predators, the composition of food items in the gastric cavity was analyzed under a binocular microscope. Daily ration (R, mg C·ind.−1·day−1) was calculated by the formula: R = Bz × DT−1 × 24, where Bz is zooplankton biomass in the predator gastric cavity (mg), and DT is zooplankton digestion time (h). Predatory impact of gelatinous zooplankton was estimated by the values of daily ration and mesozooplankton biomass. Zooplankton was sampled with a Juday plankton net with mouth diameter of 38 cm and mesh size of 140 µm. Vertical net hauls were performed: at the inner shelf stations, from the sea surface down to the bottom; at the outer shelf stations, down to the boundary of the hydrogen sulfide zone (δt = 16.2 conventional units according to a Sea-Bird probe). In the samples fixed with 4 % formalin solution, zooplankton abundance, its taxonomic composition, and size–age structure were quantified by standard method. In the food spectrum of the jellyfish, seasonal differences were revealed: predominance of Bivalvia veligers in winter and spring and wide species composition of Crustacea and other groups of prey in summer. The feeding rates of the studied species were similar: specific daily rations in winter, spring, and autumn did not exceed tenth of a percent of the carbon content in the body. Both species fed at a maximum rate in summer on the outer shelf: the specific rations reached 12.9 and 5.1 % C of the body for the jellyfish and ctenophore, respectively. A. aurita and M. leidyi populations consumed 0.2 to 5 % of the fodder zooplankton biomass per day; it did not result in a drastic reduction in zooplankton abundance and provided favorable feeding conditions for small planktivorous pelagic fish.

A method to choose water depths for zooplankton samples in lakes

AbstractAs methods in the literature to sample zooplankton in lakes mostly offered general guidance on the sample depths, a new one was developed. Using the principle of volume‐weighted sampling of the lake volume and an empirical function for the hypsometric curve, formulae for the volumes and areas of five equal sections of the lake were derived, which were then used to calculate section mean depths. Vertical net hauls taken at the mean depths are combined using a relation between their mean depths to produce one unbiased composite sample of the zooplankton. While generic formulae were derived, starting values for the depths that divide the lake volume into five equal sections are needed in order to apply the method, which then optimizes the depths; the method is implemented in a spreadsheet. The method was applied to four hypothetical lakes of maximum depth 12 m that cover a wide variation of lake form and how the sample depths vary with form was described; as lake form becomes more convex, the sample depths decrease, reflecting that more of the lake volume is at shallower depth. The method was used to estimate the whole‐lake abundance of zooplankton in 51 lakes and no practical difficulties were encountered. It can be used in lakes up to a few tens of km2 in area.

An 18S V4 rRNA metabarcoding dataset of protist diversity in the Atlantic inflow to the Arctic Ocean, through the year and down to 1000 m depth

Abstract. Arctic marine protist communities have been understudied due to challenging sampling conditions, in particular during winter and in deep waters. The aim of this study was to improve our knowledge on Arctic protist diversity through the year, in both the epipelagic (< 200 m depth) and mesopelagic zones (200–1000 m depth). Sampling campaigns were performed in 2014, during five different months, to capture the various phases of the Arctic primary production: January (winter), March (pre-bloom), May (spring bloom), August (post-bloom), and November (early winter). The cruises were undertaken west and north of the Svalbard archipelago, where warmer Atlantic waters from the West Spitsbergen Current meet cold Arctic waters from the Arctic Ocean. From each cruise, station, and depth, 50 L of seawater was collected, and the plankton was size-fractionated by serial filtration into four size fractions between 0.45–200 µm, representing picoplankton (0.45–3 µm), small and large nanoplankton (3–10 and 10–50 µm, respectively), and microplankton (50–200 µm). In addition, vertical net hauls were taken from 50 m depth to the surface at selected stations. The net hauls were fractionated into the large nanoplankton (10–50 µm) and microplankton (50–200 µm) fractions. From the plankton samples DNA was extracted, the V4 region of the 18S rRNA-gene was amplified by polymerase chain reaction (PCR) with universal eukaryote primers, and the amplicons were sequenced by Illumina high-throughput sequencing. Sequences were clustered into amplicon sequence variants (ASVs), representing protist genotypes, with the dada2 pipeline. Taxonomic classification was made against the curated Protist Ribosomal Reference database (PR2). Altogether, 6536 protist ASVs were obtained (including 54 fungal ASVs). Both ASV richness and taxonomic composition varied between size fractions, seasons, and depths. ASV richness was generally higher in the smaller fractions and higher in winter and the mesopelagic samples than in samples from the well-lit epipelagic zone during summer. During spring and summer, the phytoplankton groups diatoms, chlorophytes, and haptophytes dominated in terms of relative read abundance in the epipelagic zone. Parasitic and heterotrophic groups such as Syndiniales and certain dinoflagellates dominated in the mesopelagic zone all year, as well as in the epipelagic zone during the winter. The dataset is available at https://doi.org/10.17882/79823 (Egge et al., 2014).

Желетелый макропланктон открытой пелагиали и шельфа Чёрного моря: распределение осенью 2016 г. и межгодовые изменения биомассы и численности

Сообщество желетелого макропланктона Чёрного моря обновляется ежегодно и функционирует по-разному в зависимости от сложной системы трофических отношений и межгодовых колебаний биотических и абиотических факторов. Мониторинг этого сообщества необходим для оценки современного состояния и векторов возможной эволюции всей пелагической экосистемы. С этой целью в ходе планктонных работ по программе 89-го рейса НИС «Профессор Водяницкий» (сентябрь ― октябрь 2016 г.) в прибрежных и открытых районах моря к югу и юго-западу от полуострова Крым на 62 станциях собраны данные по составу, численности, биомассе и распределению массовых видов желетелых. Пробы отбирали сетью Богорова ― Расса (площадь входного отверстия ― 0,5 м², ячея ― 300 мкм) методом тотальных вертикальных ловов от дна до поверхности моря в области мелководного шельфа и от нижней границы кислородной зоны [по данным зонда CTD SBE plus (Sea Bird)] до поверхности моря ― в его глубоководной части. Сцифомедуза Aurelia aurita (Linnaeus, 1758) и три вида гребневиков (Mnemiopsis leidyi A. Agassiz, 1865; Pleurobrachia pileus (O. F. Müller, 1776); Beroe ovata Bruguière, 1789) присутствовали совместно на 56 станциях. В глубоководных районах моря отмечено увеличение средней биомассы каждого из видов (A. aurita ― до 260 г·м−2). Достоверных различий между показателями 2016 и 2010 г. не обнаружено (p > 0,05), при этом численность медузы возросла почти на порядок (p < 0,001), P. pileus ― в 2–5 раз (p < 0,001), B. ovata ― в 3–15 раз (p < 0,01). Более высокая численность M. leidyi наблюдалась лишь на внешнем шельфе (p < 0,01); в других районах моря её увеличению могло помешать хищничество B. ovata. Скорость соматического роста медузы в 2016 г. была одной из самых низких за последние 15 лет. Опережающий рост численности желетелых по отношению к их биомассе свидетельствует о тотальном уменьшении размеров особей как в популяциях гребневиков, так и в популяции медузы. Низкая скорость соматического роста медузы указывает на неблагоприятные условия существования этого вида в 2016 г. Тем не менее из-за высокой численности особей весеннего поколения биомасса медузы была выше, чем в прежние годы. Вследствие ослабления конкуренции со стороны гребневиков-планктофагов, в перспективе на ближайшие годы следует ожидать ещё большего увеличения экологической валентности A. aurita среди желетелых хищников.

Abundance and spatial distribution of Mysis diluviana in Lake Ontario in 2008 estimated with 120 kHz hydroacoustic surveys and net tows

Mysids are an important component of Great Lakes foodwebs, both as a prey for fish and as a predator on zooplankton. We monitored mysid abundance in Lake Ontario using lake-wide hydroacoustics data and vertical net hauls collected 1–5 August 2008 during the Ontario Ministry of Natural Resources and New York State Department of Conservation forage fish survey. Acoustic volume backscattering strength was highly correlated with both density and biomass of mysids although the correlation with biomass was stronger. The slopes of these relationships were not significantly different from theory (0.10) indicating a linear relationship between abundance and backscattering strength. Size structure significantly affected the relationship between backscattering strength and density but not between backscattering strength and biomass. Average target strength for areas with small mysids was −91.9 dB per mysid, and in areas with both small and large mysids it was −88.2 dB per animal. Acoustic estimates for Lake Ontario calculated with these regressions provided a lake-wide estimate weighted by lake area within depth intervals, of 228 mysids m−2 (CV 17.6%) and 0.93 g dw m−2 (CV 12.9%). Density and biomass in 14 net hauls averaged 316 mysids m−2 (range 0 to 1113) and 1.35 g dw m−2 (range 0 to 3.89). Mysid biomass was 33% of the biomass of other crustacean zooplankton in the summer of 2008. As in 2005, areas of low mysid abundance were detected in the middle of the transects and along the north shore. Lakewide mysid abundance in the summer of 2008 was higher than in a similar acoustic survey in the summer of 2005 and in whole-lake autumn 2000–2007 net surveys, suggesting that mysids are persisting in Lake Ontario and abundant relative to other Great Lakes.

Oxygen minimum zone of the open Arabian Sea: variability of oxygen and nitrite from daily to decadal timescales

Abstract. The oxygen minimum zone (OMZ) of the Arabian Sea is the thickest of the three oceanic OMZ. It is of global biogeochemical significance because of denitrification in the upper part leading to N2 and N2O production. The residence time of OMZ water is believed to be less than a decade. The upper few hundred meters of this zone are nearly anoxic but non-sulfidic and still support animal (metazoan) pelagic life, possibly as a result of episodic injections of O2 by physical processes. We report on discrete measurements of dissolved O2 and NO2–, temperature and salinity made between 1959 and 2004 well below the tops of the sharp pycnocline and oxycline near 150, 200, 300, 400, and 500 m depth. We assemble nearly all O2 determinations (originally there were 849 values, 695 of which came from the OMZ) by the visual endpoint detection of the iodometric Winkler procedure, which in our data base yields about 0.04 mL L−1 (~ 2 μM) O2 above the endpoint from modern automated titration methods. We acknowledge that much lower (nanomolar) O2 values have been measured recently with the STOX (Switchable Trace amount OXygen) sensor in the eastern tropical South Pacific, and that similar conditions may also prevail in the Arabian Sea OMZ. In spite of the error in O2 measurements at vanishingly low levels, we argue that the temporal trends of the historic data should still hold. We find 632 values acceptable (480 from 150 stations in the OMZ). The data are grouped in zonally paired boxes of 1° lat. and 2° long. centered at 8, 10, 12, 15, 18, 20, and 21° N along 65 and 67° E. The latitudes of 8–12° N, outside the OMZ, are treated in passing. The principal results are as follows: (1) an O2 climatology for the upper OMZ reveals a marked seasonality at 200 to 500 m depth with O2 levels during the northeast monsoon and spring intermonsoon seasons elevated over those during the southwest monsoon season (median difference, 0.08 mL L−1 [~ 3.5 μM]). The medians of the slopes of the seasonal regressions of O2 on year for each of the NE and SW monsoon seasons are −0.0043 and −0.0019 mL L−1 a−1, respectively (−0.19 and −0.08 μM a−1; n = 10 and 12, differing at p = 0.01); (2) four decades of statistically significant decreases of O2 between 15 and 20° N but an opposing trend toward an increase near 21° N are observed. The mechanisms of the balance that more or less annually maintain the O2 levels are still uncertain. At least between 300 and 500 m, the replenishment is inferred to be due to isopycnal re-supply of O2, while at 200 (or 250?) m it is diapycnal, most likely by eddies. Similarly, recent models show large vertical advection of O2 well below the pycnoclines and oxyclines. The NO2– distribution, taken as an indicator of active NO3– reduction, does not show a trend in the redox environment for a quarter of a century at a GEOSECS station near 20° N. In the entire OMZ, the regression slopes on year within seasons for the rather variable NO2– do not present a clear pattern but by other measures tended to an increase of NO2–. Vertical net hauls collect resident animal (metazoan) pelagic life in the NO2– maximum of the OMZ at O2 levels well below the lower limit of the Winkler titration; the extremely low O2 content is inferred from the presence of NO2– believed to be produced through microbial NO3– reduction. Instead of the difficult measurement by the STOX sensor, the relation between the very low O2 inferred from presence of NO2– and mesozooplankton should be studied with 100 to 150 L bottles rather than nets. The spatial (within drift stations) and temporal (daily) variability in hydrography and chemistry is large also below the principal pycnocline. The seasonal change of hydrography is considerable even at 500 m depth. Future O2 or nutrient budgets for the OMZ must not be based on single cruises or sections obtained during one season only. Steady state cannot be assumed any longer for the intermediate layers of the central Arabian Sea.

Mesozooplankton community development at elevated CO<sub>2</sub> concentrations: results from a mesocosm experiment in an Arctic fjord

Abstract. The increasing CO2 concentration in the atmosphere caused by burning fossil fuels leads to increasing pCO2 and decreasing pH in the world ocean. These changes may have severe consequences for marine biota, especially in cold-water ecosystems due to higher solubility of CO2. However, studies on the response of mesozooplankton communities to elevated CO2 are still lacking. In order to test whether abundance and taxonomic composition change with pCO2, we have sampled nine mesocosms, which were deployed in Kongsfjorden, an Arctic fjord at Svalbard, and were adjusted to eight CO2 concentrations, initially ranging from 185 μatm to 1420 μatm. Vertical net hauls were taken weekly over about one month with an Apstein net (55 μm mesh size) in all mesocosms and the surrounding fjord. In addition, sediment trap samples, taken every second day in the mesocosms, were analysed to account for losses due to vertical migration and mortality. The taxonomic analysis revealed that meroplanktonic larvae (Cirripedia, Polychaeta, Bivalvia, Gastropoda, and Decapoda) dominated in the mesocosms while copepods (Calanus spp., Oithona similis, Acartia longiremis and Microsetella norvegica) were found in lower abundances. In the fjord copepods prevailed for most of our study. With time, abundance and taxonomic composition developed similarly in all mesocosms and the pCO2 had no significant effect on the overall community structure. Also, we did not find significant relationships between the pCO2 level and the abundance of single taxa. Changes in heterogeneous communities are, however, difficult to detect, and the exposure to elevated pCO2 was relatively short. We therefore suggest that future mesocosm experiments should be run for longer periods.

Distribution of life-history stages of the salp Thalia democratica in shelf waters during a spring bloom

Swarms of the salp Thalia democratica periodically occur off southeast Australia follow- ing the austral spring bloom of phytoplankton. The present study aimed to determine the abundance and size/stage distribution of T. democratica and their relationship with copepods in 3 water types of the western Tasman Sea. Samples were taken from vertical net hauls along 4 cross-shelf transects spaced along 200 km of the New South Wales coast, from the East Australian Current (EAC) sepa - ration zone, around 32.5° S, to off Sydney (34° S). Temperature-salinity signatures grouped stations into 3 distinct water types: inner shelf water, EAC and upwelled water. Although common across all stations, T. democratica was significantly more abundant in inner shelf waters compared to both EAC and upwelled water. Analysis of population structure (aggregate buds, aggregate females, aggregate males, immature solitaries and mature solitaries) also identified higher proportions of reproductive aggregates and their offspring in inner shelf water. This salp population structure was significantly different in the EAC regions, characterised by a paucity of the solitary stages, higher temperatures and lower chlorophyll a concentrations. A weak negative correlation was identified between T. demo- cratica and copepod abundance. In the present study, the maximum abundance of T. democratica was twice the highest globally recorded abundance and 10-fold greater than maximum abundances sampled from the continental shelf and slope waters off southeast Australia during the period from 1938 to 1942.

Response of Chrysaora quinquecirrha medusae to low temperature

Because of their high abundance in Chesapeake Bay, Chrysaora quinquecirrha medusae may be an important reservoir of organic matter. The timing and location of the decomposition of biomass from medusae may have implications for carbon cycling in the bay. Our objective was to identify the cause of C. quinquecirrha medusa disappearance to better understand when and where decomposition occurs. A time series of visual surface counts and vertical net hauls in the Choptank River, a tributary of Chesapeake Bay, showed that as temperatures approached 15°C, C. quinquecirrha medusae disappeared from the surface, but persisted in net hauls until temperatures reached 10°C. In order to test whether medusae sink upon cooling, we exposed C. quinquecirrha medusae to low temperatures in large static tanks and measured their depth and pulsation rates twice daily for at least 6 days. This procedure was repeated three times through the 2008 jellyfish season. On average, individuals exposed to temperatures below 15°C were found deeper and pulsed slower than those in the warmer control tank. This suggests that low temperatures cause the medusae to sink before cooling to the limit of their physiological tolerance and may have implications for the deposition of organic matter associated with the seasonal disappearance of medusae from Chesapeake Bay.

Distribution, abundance and life history of Mysis relicta ( Lovén) in the Feldberg Lake District, Germany

Distribution, abundance and life history characteristics of Mysis relicta were studied in the Feldberg Lake District (Lake Breiter Luzin, Lake Schmaler Luzin, Lake Zansen) located in northeastern Germany. Between July 2001 and November 2002 mysids were collected by vertical net hauls. In order to determine the impact of the current trophic conditions on the distribution of mysids in these lakes, oxygen concentration, total phosphorus, chlorophyll a and water transparency were also measured. All investigated lakes are mesotrophic at present. Lake Breiter Luzin exhibited great seasonal and spatial variations in mysid abundance. Density of adults and juveniles had a mean of 44.9 ± 57.1 and 68.7 ± 99.6 m −2, respectively. Highest abundance of adults was 110.4 ± 76.5 m −2 in summer, lowest abundances of 2.0 ± 4.0 m −2 occurred in spring. For juveniles, highest density of 218.4 ± 174.6 m −2 was detected in summer and lowest of 0.8 ± 1.8 m −2 in winter. No mysids were caught in any of the daytime hauls, but they were widely distributed throughout the water column at night. Size frequency distribution of mysids suggested that reproduction occurred year-round, the most consistent influx of juveniles occurred in early summer and a smaller second cohort in autumn. Highest mysid abundance was 189.2 ± 318.6 adults and 127.0 ± 66.3 juveniles m −2 in Lake Schmaler Luzin, and 59.6 ± 5.6 adults and 79.4 ± 11.2 juveniles m −2 in Lake Zansen. There were great spatial differences in abundance in both lakes.

Comparison of biomass and size spectra derived from optical plankton counter data and net samples: application to the assessment of mesoplankton distribution along the Northwest and North Iberian Shelf

Abstract Data from vertical net hauls and from a submersible optical plankton counter (OPC) were compared in terms of biomass and slope of the normalized biomass–size spectra (NB–SS), a proxy for the size structure of the community. The relationship between the estimates of biomass in the overlapping range sampled by both methods (0.2–2 mm equivalent spherical diameter (ESD)) was linear and not significantly different to 1 (BOPC:BNET=1.1±0.1, r2=0.84). However, the relationship varied depending on the size fraction considered; the ratio BOPC:BNET was 0.10±0.04 (r2=0.30), 0.8±0.2 (r2=0.66), and 2.4±0.5 (r2=0.64) for the size fractions 0.2–0.5, 0.5–1, and 1–2 mm ESD, respectively. The discrepancies between methods were presumably due to the combined effect of the limitations of the instrument in the lower detection limit and the errors in the volume of water sampled by the net for the smallest size fraction, and to net avoidance enhanced by clogging for the largest size fraction. The agreement between methods improved when the NB–SS of the different data sets were compared. The slope (b=−1.1) and the intercept (a=14.6) of the NB–SS integrated across stations were not significantly different (Student's t-test) for the linear model fitted to net samples, OPC data, or pooled data from both methods. Station by station, the slopes of the NB–SS from the net (bNET) and the OPC (bOPC) were not significantly different in 61% of the stations. This percentage increased to 78% when the comparison was limited to the mesoplankton size range. As an example of the applicability of the OPC, we showed the distribution of mesoplankton biomass and size structure along the NW and N Iberian Shelf during the winter–spring transition of 2002 and its relationship with the hydrographic scenario.

The Continuous Plankton Recorder in the Southern Ocean: a comparative analysis of zooplankton communities sampled by the CPR and vertical net hauls along 140 E

A repeat transect was run south of Tasmania, along ∼140°E, during November and December 2001. NORPAC nets were deployed during a CTD transect on the southern leg, sampling four depth zones at each of 19 stations: 0–20, 20–50, 50–100 and 100–150 m. A Continuous Plankton Recorder (CPR) was deployed on the northern leg (average sampling depth = 10.5 m). Both net systems were harnessed with 270 μm mesh and all sampling was conducted between 47°S and the Southern Polar Front (S-PF) at ∼61°S. Zooplankton in the top 150 m of the water column demonstrated strong, small-scale, vertical distribution patterns. Species richness and diversity increased with depth, and were lowest for CPR samples. Conversely, dominance decreased with depth and was highest for CPR samples. Evenness was similar for all sample groups, indicating that all communities had a similar distribution of abundance amongst species. There was little variation in abundance between NORPAC depth zones (average = 82 ± 47 individuals m−3), while abundance was substantially higher in the CPR samples (average = 144 ± 103 individuals m−3), despite it under-sampling fast-moving and delicate components of the plankton community. The higher CPR abundance was due to significantly higher abundance levels of Appendicularia, Oithona similis and Rhincalanus gigas nauplii. The NORPAC samples showed that these three taxa were most abundant in the surface waters. The significant increase in abundance in the CPR samples was attributed to the growth in size during the period between the NORPAC and CPR surveys (minimum 15 days) increasing their catchability. Both the NORPAC nets and CPR surveys identified distinct communities to the north and south of the Southern Sub-Antarctic Front. Owing to its shallow towing depth, the CPR focuses on species with surface distributions. Despite under-sampling some components of the zooplankton, the CPR provided sufficient taxonomic resolution to identify biogeographic zones in the Southern Ocean. The utility of the CPR as a long-term monitoring tool in the Southern Ocean is discussed.

Cladocera Chydoridae of high altitude water bodies (Serra da Mantiqueira), in Brazil.

The diversity of species of Cladocera, Chydoridae in the water bodies belonging to the water management unity of Serra da Mantiqueira, was studied inside the BIOTA/FAPESP Program. A total of 21 water bodies were analyzed and the sampling was carried out between 9th and 11th September, 1999. Horizontal and vertical net hauls were taken at littoral and limnetic region in each lake. Most water bodies in Serra da Mantiqueira unity are small, shallow, and densely colonized by macrophytes. Most species of Cladocera found belong to the Chydoridae family, being typical inhabitants of the littoral zone, living associated with macrophytes. A total of 12 species were found, 9 of the subfamily Aloninae and 3 Chydorinae. In more than half of the water bodies the relative abundance of chydorids reached 100%, and they were dominant in most of the others, with few exceptions. Alona rustica was the most frequent species, occurring in 62% of the water bodies. The results evidenced the importance of the littoral zone as propitious habitats for the species of Chydoridae and contributing significantly to the diversity of Cladocera as a whole.

Rotifer resting egg densities in lakes of different trophic state, and their assessment using emergence and egg counts

The methods for estimating rotifer resting egg densities from lake sediment, and the use of emergence for determining community composition from lakes, were investigated in Lakes Tikitapu and Okaro, New Zealand. Significantly lower (P<0.01) estimates of resting egg densities were obtained by monitoring emergence from incubated sediments than from egg counts. However, incubation of sediments did not require the ability to recognise species on the basis of egg morphology as did egg counts. Significantly higher (P<0.01) densities of resting eggs were found in sediments from eutrophic Lake Okaro than from oligo-mesotrophic Lake Tikitapu. Incubation of sediments did not result in the emergence of all species found in the plankton during an earlier thirteen-month seasonal survey, including some common species. This currently limits the use of this method as a tool for rapid assessment of community composition. However, the method is likely to provide advantages over some traditional methods, such as vertical net hauls, by providing more complete species lists if only a single visit to a site is possible.

A study of the effects of migratory zooplankton on microbial populations in surface waters of the N.W. Mediterranean

We examined the hypothesis that metazoan zooplankton may directly influence on microbial populations in an oligotrophic marine system. Diel variations of microbial populations, relative to changes in the biomass and feeding activity of metazoan zooplankton, were studied at an offshore station in the North Western Mediterranean Sea. Two different parts of the water column, representing relatively low, 0.05 μg l −1, and high, 0.12 μg l −1, chlorophyll zones were repeatedly sampled. At 3 h intervals, samples were obtained from 5 m and 40 m for estimates of microbial population abundances, size-fractions of chlorophyll and bacterial production; metazoan zooplankton were sampled concurrently with vertical net hauls between 0 and 50 m. Overall, there were no large day/night differences or clear diel rhythms in parameters of microbial populations. Bacterial concentrations peaked in the early evening at 40 m and later at 03:00 h at 5 m depth. Cell specific thymidine incorporation rates showed a distinct peak in the early morning hours at 5 m and were nearly constant at 40 m. Photosynthetic and heterotrophic nanoflagellates showed dissimilar temporal trends both relative to one another and between the two sampling depths. Chlorophyll, in the <10 μm or <3 μm size-fractions, increased at both 5 m and 40 m over the study period. Chlorophyll >10 μm was a negligible part of the chlorophyll crop at 5 m but represented about 50% of the chlorophyll at 40 m and showed considerable variability. In contrast to parameters of microbial populations, we found clear and large day/night differences in both abundances and feeding activities of metazoan zooplankton. Abundances of total zooplankton were higher by about a factor of 4 during nightime hours. Metazoan zooplankton peak abundances occurred from midnight to 03:00 h corresponding with peak activities of the digestive enzymes amylase and trypsin in total zooplankton as well as gut fluorescence in the most abundant copepod, Centropages typicus. There were few significant relationships detectable between parameters of microbial populations and metazoan zooplankton. The link between metazoan zooplankton and microbial populations, even in low-chlorophyll surface waters was apparently weak.

The Invasive Asian Copepod Pseudodiaptomus inopinus in Oregon, Washington, and British Columbia Estuaries

At least six species of Asian copepods have been recently introduced to the west coast of North America. Among them isPseudodiaptomus inopinus, which became established and abundant in the Columbia River estuary between 1980 and 1990. Subsequently, the tidal regions of 18 smaller rivers in the Pacific Northwest were sampled using near-bottom pumps and vertical plankton net hauls. These samples indicate thatP. inopinus has not only invaded at least seven other estuaries in this region, but that it is sometimes the dominant zooplankter in these systems.P. inopinus occurred in estuaries of both large and small rivers and in rivers with and without international shipping. Two factors may be important in the establishment of this copepod: temperature and extent of salinity intrusion. In our samples, average autumn temperature of rivers with establishedP. inopinus populations was 19.3°C, whereas those withoutP. inopinus had an average temperature of 12.4°C. Rivers withP. inopinus had salinity intrusion zones of more than 1 km length and those withoutP. inopinus usually had intrusion zones of less than 1 km.

Thalassiosiraspecies (Bacillariophyceae) from a Scottish sea-loch

During the spring diatom blooms of 1992 and 1993 in Loch Creran, Scotland, regular water column and sediment samples were collected, examined, and species composition of the phytoplankton determined. Thalassiosira species were found to be a major bloom constituent. Species were identified from specimens cultured from sediments and isolated from vertical net hauls, using light and scanning electron microscopy. Of the 17 species positively determined, only Thalassiosira angulata, T. anguste-lineata, T. eccentrica, T. gravida, T. minima, T. nordenskioeldii, T. rotula and T. tenera have been previously reported in Scottish waters. Other species, for example Thalassiosira pseudonana and T. punctigera, have been noted elsewhere in the British Isles but not along the Scottish west coast. Several species, namely Thalassiosira concaviuscula, T. constricta, T. delicatula, T. kuschirensis, T. oceanica, T. pacifica and T. tealata, are new records for the British Isles. One unidentified taxon is illustrated. Descriptions, illustrations and a key are provided for the 18 taxa.

The Fraser River plume: some preliminary observations on the distribution of juvenile salmon, herring, and their prey

ABSTRACTZooplankton and fish densities in the southern Strait of Georgia were observed to coincide with variations in surface salinities resulting from the outflow of the Fraser River. Vertical net hauls in the euphotic zone revealed that copepods, amphipods, and euphausiids were significantly more abundant per m3 in the brackish estuarine plume (surface salinities ‐ 10–15 ppt) when compared to the area covered by the freshwater of the Fraser River plume (0–10 ppt) and the region of the Strait of Georgia (25–30 ppt) unaffected by the outflow of the Fraser River.The estuarine and riverine plumes had significantly higher fish densities (adult and juvenile herring, and juvenile salmonids [excluding chinook]) than the Strait of Georgia region, with no significant differences in densities of juvenile chinook salmon observed between regions. The highest catches of juvenile salmonids were at the boundary between the estuarine plume and the Strait of Georgia. Zooplankton found in the stomach contents of both adult and juvenile herring suggested that the herring were filter‐feeding on the zooplankton in the estuarine plume. Juvenile salmonids fed primarily on small unidentifiable juvenile fish. The existence of increased densities of prey items in the estuarine plume is proposed to be the primary mechanism resulting in increased residence time in this region by outmigrating juvenile salmonids. Utilization of aggregated zooplankton could lead to increased salmonid growth rates and therefore to enhanced survival of individuals utilizing the Fraser River plume environment.

Life History and Productivity of Mysis Relicta in Lake Ontario

Mysis relicta is a common food of many fish species in Lake Ontario, yet little was known about its life history or production. We determined growth rates, cohort development patterns, reproductive patterns, and production of the population at a deep water station over a 4-year period. Four replicate vertical net hauls were collected monthly from April until October, each year. All animals were counted and 100 from each replicate were measured. The original mysid data of N.H.F. Watson, collected in 1971–72, were reanalyzed and the results compared with the present study. Most reproduction occurred in late autumn; the eggs hatched late the following winter; and the offspring reached maturity approximately 18 month later. In 1971 and 1972, abundances were similar to that in 1984 but cohort development was less synchronous. Average growth rate (1984–1987) in the first summer was 0.035 mm.d −1, through the winter it fell to 0.012 mm.d −1, and during the second summer it was 0.029 mm.d −1. Production was 1.90g.m −2 in 1984 and 2.62 g.m −2 in 1986. Examination of sources of error suggest these values might underestimate production by 11% to 27%. Immigration prevented the estimation of production in 1985 and 1987

Spatial and seasonal changes in the species composition of armored dinoflagellates in the Southwestern Atlantic Ocean

To compare the spatial and temporal (seasonal) distribution of dinoflagellates, vertical net hauls were taken along similar cruise tracks in the Scotia Sea, Weddell Sea and across the Polar Front Zone in the austral spring and the austral fall. Sixty-three species of armored dinoflagellates were identified and enumerated. Chisquare and hierarchical cluster analyses were performed to define spatial and seasonal patterns in genera and species assemblages. The dominant genera were Protoperidinium, Dinophysis and Ceratium. The Polar Front Zone was an important biogeographical barrier with Blepharocysta, Gonyaulax, Heteroschisma, Oxytoxum and Podolampas occurring mainly north of the Front. Species found primarily in the austral spring were Ceratium fusus, Ceratium lineatum, Dinophysis antarctica, Dinophysis simplex, Gonyaulax digitale, Protoperidinium pyriforme and Protoperidinium variegatum. Austral fall species included Dinophysis tuberculata and Protoperidinium elegantissum. Distribution of armored dinoflagellates in the Southwestern Atlantic Ocean is influenced at the generic level by spatial considerations, particular with relation to the Polar Front Zone, whereas species composition can be effected by both region and season.