Offshore Plankton Research Articles

Long-term trends of heat waves and ecosystem responses in Jiaozhou Bay, the Yellow Sea

With global warming, extreme weather frequently occurs, yet the consequences remain unexplored. A total of 156 heat waves and their characteristics were detected on the basis of a temperature dataset from 1954 to 2022 in Jiaozhou Bay (JZB). The increment is 0.62 times decade−1 for the number of heat waves, 6.65 days decade−1 for the sum of participating heat wave days and 0.66 days for the duration of each heat wave. The intensity of heat waves showed regular fluctuations with progressively shorter periods. Based on the dataset of 12 stations in JZB from 2003 to 2022, the zooplankton abundance was significantly greater during heat waves, which was strongly attributed to the greater abundance of copepods and gelatinous zooplankton during heat waves. However, the responses of plankton to heat waves were seasonally heterogeneous. Our study provides new insight into and a scientific basis for understanding the effects of heat waves on offshore plankton ecosystems.

Offshore Plankton Blooms via Mesoscale and Sub‐Mesoscale Interactions With a Western Boundary Current

AbstractWe investigate mesoscale circulations in an oligotrophic western boundary current, the East Australian Current, during a sustained offshore plankton bloom. Using a series of high resolution hydrographic sections taken a few days apart, supplemented with in situ samples of nutrients, satellite and long‐term mooring measurements, we describe a dynamic situation by which the East Australian Current's velocity core and associated front, interacts with a mesoscale eddy, migrating zonally by approximately 100 km over the course of 10 days. This interaction between the boundary current and mesoscale eddy field occurred concurrently with a sustained offshore plankton bloom. Sub‐mesoscale upwelling motions on the inshore flank of the boundary current core coincides with increased nutrient and plankton concentrations in the near surface. Calculations based on the quasi‐geostrophic omega equation and finite size lyapunov exponents suggest that these vertical motion arise from the interaction of the mesoscale with the East Australian Current. Frontolysis (the destruction of horizontal buoyancy gradients) leads to a thermally indirect ageostrophic secondary circulation that has the potential to supply nutrients to the near surface ocean. Using satellite data, we investigate the mesoscale conditions associated with all offshore phytoplankton blooms identified by an automated method, finding similar mesoscale patterns to those observed during the field campaign. We conclude that the interaction between the East Australian Current and mesoscale eddies is a recurrent catalyst for the complex sub‐mesoscale dynamics we observed, and is likely a fundamental processes in driving offshore biological productivity in the region.

At what scale should we assess the health of pelagic habitats? Trade-offs between small-scale manageable pressures and the need for regional upscaling

Major planktonic lifeforms such as diatoms, dinoflagellates, meroplankton and holoplankton have recently shown significant and alarming changes in abundance - mainly downwards trends - around the northwest European shelf. This has major implications for food web connections and for ecosystem services including seafood provision and carbon storage. We have quantified these changes in abundance for 2006–2019/20 using a Plankton Index (PI) and show that the scale of spatial aggregation is critical to the ability of the PI to detect change, understand causal mechanisms, and provide advice to policymakers.We derived PI statistics in the Celtic and North Seas from data from the Continuous Plankton Recorder survey offshore and England’s Environment Agency inshore using three sets of spatial units: (i) Ecohydrodynamic (EHD) units based on hydro-biogeochemical modelling, (ii) ‘COMP4′ areas based on cluster analysis of satellite data for chlorophyll a and primary productivity, and (iii) English coastal and estuarine Water Framework Directive (WFD) waterbodies. For the largest scale areas, the EHD units (median size 87,000 km2), we find greater change in plankton communities than previously reported, suggesting that these shifts have continued and possibly intensified in recent years. The smaller-scale COMP4 areas (median size 6,700 km2) appear to encompass more spatially coherent changes in plankton community structure than EHD units; at this scale PI values indicate community shifts of greater magnitude. These COMP4 areas provide a reasonable compromise scale for linking offshore plankton communities to large-scale drivers of change such as climate warming. For inshore plankton communities, larger changes are detected at the smaller WFD waterbody scale (median size 11 km2). This scale allows direct links to coastal management measures and is more suitable for linking to land-sourced pressures.Recent integration of the UK’s OSPAR and WFD plankton monitoring data management enables the exploration of changes across spatial scales to develop a holistic understanding of ecosystem health. Regional-sea scale derivation of the PI for coastal waters provides a clear indication that changes are occurring, at least in phytoplankton communities, while localised PI statistics offer an additional layer of information which can be an important tool for linking to localised drivers of change including coastal anthropogenic pressures. Broadscale inshore zooplankton monitoring is needed to evaluate the coastal plankton community holistically; zooplankton communities offshore are also changing but these changes cannot currently be linked to coastal processes. Layering information across spatial scales provides a breadth of system-level understanding beyond what any one typology can provide.

Phytoplankton community structure changes during autumn and spring in response to environmental variables in Methana, Saronikos Gulf, Greece

Phytoplankton community was investigated during two contrasting periods using offshore plankton samples in the volcanic area of Methana peninsula (Saronikos Gulf): the first at early autumn (warm period, September 2016) and the second one at early spring (cold period, March 2017). In order to investigate the phytoplankton community structure in the complex geo-biochemical conditions of the area, samples were collected from stations near the CO2 hydrothermal vents, at the hydrothermal sulfur and radioactive springs and at a fishery nearby Methana town. Three major phytoplankton groups, Bacillariophyceae, Dinophyceae, and Prymnesiophyceae, were studied, using inverted microscopy. In early autumn, Dinophyceae were dominant in the majority of the stations with cell concentrations of Prorocentrum spp. up to ~ 35.5 × 103 cells l-1. In early spring, the dominant class was Bacillariophyceae with dominant genus Nitzschia/Pseudo-nitzschia presenting cell concentrations up to ~ 33.9 × 103 cells l-1. Furthermore, Prymnesiophyceae appeared in both spring and autumn samples with small fluctuations. Total phytoplankton cell concentrations followed a seasonal trend, presenting slightly lower values in the hydrothermal-effected area in comparison with the broader Saronikos Gulf, confirming the prevalence of oligotrophic conditions. Seasonal variation was very strong, revealing an association with water temperature and nutrient content. Those environmental variables proved to have a strong effect that was reflected in the phytoplankton community structure.

Influences on Water Quality and Abundance of Cladophora, a Shore-Fouling Green Algae, over Urban Shoreline in Lake Ontario

Urban centers border western Lake Ontario, the terminus of the Laurentian Great Lakes, impacting water quality on the shores of this oligotrophic lake. The green algae Cladophora proliferates on the shallow lakebed and fouls the shoreline, presenting an eutrophication concern. The conditions over a typical urbanized shoreline were studied to assess linkages between Cladophora and area nutrient sources. The most pervasive of the mixing areas of varying extent identified using field sensor measurements was associated with the discharge of treated sewage from a Water Pollution Control Plant (WPCP). Phosphorus and nitrogen were enriched at times near the WPCP diffuser and also in shallow water along the shoreline. Dissolved nutrients were also measured directly above the lakebed in close proximity to Cladophora. Dissolved phosphorus and inorganic nitrogen were higher in proximity to the WPCP diffuser at times, however, spatial patterns were not as clearly aligned with external inputs as the patterns of enrichment in the water column. Biomass distribution of Cladophora was unrelated to nutrient levels in the water column or at the lakebed. Yet, concentrations of phosphorus in Cladophora, an indicator of nutrient sufficiency, were higher near the WPCP diffuser. This disparity, while possibly an artifact of variable loss rates of biomass among locations, may in part stem from variable water clarity among areas. Abundant dreissenid mussels also potentially obscure the effects of nutrient loading on algal biomass if the sequestering of phosphorus from offshore plankton, suggested by periods of onshore circulation, approaches the extent of external loading. Further study of phosphorus flux at the lakebed is required to establish clear linkages between external nutrient loading and algal growth in order to manage the proliferation of Cladophora over urban coastline.

A study case of bioluminescence potential dynamics in the Delaware Bay with observations and modeling

Results from first observational program of bioluminescence (BL) potential in the Delaware Bay area are presented. During the field program July 30–August 1, 2015, the satellite Visible Infrared Imaging Radiometer Suite (VIIRS) imagery shows the development of the submesoscale filament with elevated chlorophyll-a in the area of interaction of lighter water masses of the Bay outflow with denser upwelled water. We have shown that ageostrophic secondary circulation (ASC) cells contributed to the development of this filament. Analysis of BL potential observations have shown elevated values of BL potential in the area of the submesoscale filament. Analysis of observed temperature, salinity, sigma-t, and BL potential along the stations crossing the Bay mouth have shown a presence of a strong frontal structure separating colder, more saline, denser offshore water from the bay water masses. Over 3 days of sampling, this frontal structure moved onshore to the entrance of the bay, and brought offshore BL plankton communities with higher values of BL potential. We compared two surveys (at the end of July and in the middle of August) of water masses located in the area where the buoyant outflow of the Delaware Coastal Current is turned around and taken to the north up the shelf by upwelling favorable winds. Analysis of observations shows that the survey at the end of July has fresher water masses (due to higher river runoff before and during survey) and higher values of BL potential in comparison to the survey at the middle of August.

First record of Palaemon macrodactylus Rathbun, 1902 (Decapoda, Palaemonidae) in the western Mediterranean

The Mediterranean Sea is one of the world’s hotspots for marine bio-invasions. Most invasions are first documented based on an initial record of occasional adult captures. However, reports of larval stages could indicate that there is an adult population that is reproducing and therefore well established in the area. The spread of the oriental shrimp, Palaemon macrodactylus, from its native estuarine waters of southeast Asia to new regions worldwide is well documented. We report the first record of this species in the Mediterranean based on the presence of its larval stages in plankton samples. Decapod larvae were collected in five offshore plankton surveys performed off the Balearic Islands (western Mediterranean), and zoeae III and VI of the oriental shrimp werefound among them. Taking into account the duration of the successive developmental stages, and the hydrodynamic characteristics of the study area, these larvae were most probably spawned by adult populations not yet documented. The larvae were found in marine waters despite the fact that adults usually inhabit brackish waters. Our study is a good example of how plankton studies can help to detect larval stages of invasive species before the adult populations are detected.

Nucleopolyhedrovirus Detection and Distribution in Terrestrial, Freshwater, and Marine Habitats of Appledore Island, Gulf of Maine

Viruses in aquatic ecosystems comprise those produced by both autochthonous and allochthonous host taxa. However, there is little information on the diversity and abundance of viruses of allochthonous origin, particularly from non-anthropogenic sources, in freshwater and marine ecosystems. We investigated the presence of nucleopolyhedroviruses (NPV) (Baculovirus), which commonly infect terrestrial lepidopteran taxa, across the landscape of Appledore Island, Gulf of Maine. PCR and qPCR primers were developed around a 294-bp fragment of the polyhedrin (polH) gene, which is the major constituent protein of NPV multivirion polyhedral occlusion bodies. polH was successfully amplified from several aquatic habitats, and recovered polH sequences were most similar to known lepidopteran NPV. Using quantitative PCR designed around a cluster of detected sequences, we detected polH in Appledore Island soils, supratidal freshwater ponds, nearshore sediments, near- and offshore plankton, and in floatsam. This diverse set of locations suggests that NPVs are widely dispersed along the terrestrial--marine continuum and that free polyhedra may be washed into ponds and eventually to sea. The putative hosts of detected NPVs were webworms (Hyphantria sp.) which form dense nests in late summer on the dominant Appledore Island vegetation (Prunus virginiana). Our data indicate that viruses of terrestrial origin (i.e., allochthonous viruses) may be dispersed widely in coastal marine habitats. The dispersal of NPV polH and detection within offshore net plankton (>64 μm) demonstrates that terrestrial viruses may interact with larger particles and plankton of coastal marine ecosystem, which further suggests that viral genomic information may be transported between biomes.

Birth-date selection in early life stages of plaice Pleuronectes platessa in the eastern Irish Sea (British Isles)

For species with extended spawning seasons, short periods of beneficial conditions often lead to disproportionate survival of sub-sets of the offspring. This has been demonstrated for freshwater fish and for marine pelagic species, but has been less frequently tested for other teleost groups. Using offshore plankton surveys, we constructed egg production curves for plaice Pleuro- nectes platessa L. in the eastern Irish Sea. Data from 5 yr showed that spawning began sometime before January and was completed by the end of April. Over these years the timing of peak spawn- ing varied between mid-February and mid-March. In 2001, 2002 and 2003, we also sampled post- settlement plaice from inshore nursery grounds. Otolith micro-increment analyses and temperature- dependent egg development rates were used to back-calculate the birth dates of the settled fish. By June, immigration to the nursery grounds was largely completed and the shape of the reconstructed birth-date distributions corresponded closely to the relevant egg production curves. In contrast to many other studies, there was little evidence for significantly disproportionate survival of specific portions of the egg production curve between spawning and the recently post-settled stages. Such a survival pattern could be generated by a gradual switch from starvation to predation as the principal cause of larval mortality as the spawning season progressed.

Stable Isotope Analysis of Amphidromous Hawaiian Gobies Suggests Their Larvae Spend a Substantial Period of Time in Freshwater River Plumes

We employed stable isotope analysis (δ13C, δ15N) to evaluate the sources of nutrients used by amphidromous gobiid fishes (Lentipes concolor, Sicyopterus stimpsoni, Awaous guamensis) caught migrating into and living in Hakalau Stream, Hawaii. Although considerable variation amongst the stable isotope values of stream items was noted across all 4 years of our study, the relationships between the fishes were relatively constant. Stable isotope values of recruiting gobies were consistently closer to those of both inshore plankton and freshwater adults than those of offshore plankton, suggesting that the larvae of these species derive much of their nutrition from inshore environments influenced by fresh water. Small differences between the stable values of these species further suggested that their larvae come from different inshore locations. After entering fresh water all species appear to swim rapidly upstream without feeding. Finally, once well upstream, adult L. concolor and A. guamensis appear to assume an omnivorous diet while adult S. stimpsoni rely upon autochthonous production within streams. We propose that freshwater food webs play an integral yet complex role in the lives of both larval and adult amphidromous Hawaiian fishes.

Stable isotope analyses and demographic responses counter prospects of planktivory by Caridina (Decapoda: Atyidae) in Lake Victoria.

Caridina nilotica, a freshwater atyid prawn, is a vital component of the Lake Victoria ecosystem. Despite its important role in the food web leading to Nile perch, the diet of Caridina is not well understood. Caridina freshly collected from the inshore littoral and offshore plankton of Lake Victoria were cultured individually under laboratory conditions on (A) decomposing hydrophytes, (B) living hydrophytes, (C) planktonic algae, (D) zooplankton and (E) 35- microm filtered lake water (a 'starvation' control). Inter-moult intervals (IMI, days), size-standardized moult intervals (MI, days mm(-1)), per moult growth increments (PMI, mm) and survivorship (%) were monitored daily for up to 5 weeks. Significant effects of both food type and shrimp source on MI were revealed by ANOVA. MI increased progressively from treatment A to D, and was shorter in offshore than littoral shrimps. Food influence on IMI was confirmed by ANCOVA. PMI values were close to the limits of detection, but were generally in line with MI responses. PMI values were marginal in treatments A and B, and negligible or negative in treatments D and especially E. Survivorship values, although confounded by non-dietary factors, were generally consistent with dietary influences on MI, although values obtained for treatment E were inconsistently high for true starvation. Disparate responses between inshore and offshore shrimps hint at possible ecotypic differentiation, or perhaps the existence of cryptic species. Stable isotope analyses (SIA, delta13C and delta15N signatures) of cultured shrimps were further consistent with their utilization of food type A but not D. SIA signatures of feral shrimps maintained in situ in enclosure bags with three separate potential fresh hydrophyte food sources (Vossia cuspidata, Cyperus papyrus, and Eichhornia crassipes) reflected Caridina's probable dietary reliance on decomposed organic matter with accompanying bacterial exudates. Collections of feral shrimps from various locations yielded parallel SIA results. No support for zooplanktivory by shrimps occupying either inshore littoral/benthic or offshore planktonic habitats is provided by the delta15N signatures obtained from our data, which support Caridina's primary role as a detritivore.

Zooplankton from the Maracajaú Reefs, Northeastern Brazil.

This study was carried out to assess the diversity and the abundance of the reef zooplankton in the Maracajaú reefs. Samples were collected with two plankton nets (65 and 300 micrometers mesh size, each) from February to June/2000 at three stations in a transect perpendicular to coast. Plankton biomass in the 65 µm fraction varied from 45.19 mg.m-3 to 316.45 mg.m-3. In the 300 µm fraction biomass varied from 9.33 mg.m-3 to 43.16 mg.m-3. Zooplankton presented 61 taxa. Copepods were the most diverse taxa with 23 species. Euterpina acutifrons, Oithona oswaldocruzi, Oithona hebes, Oithona sp., Parvocalanus crassirostris, Acartia lilljeborgi, Bivalvia and Gastropoda larvae, Trilocularina sp., Balanus sp. larvae, Spirillina sp., Polychaeta larvae, Brachyura (zoea) and Oikopleura longicauda occurred in more than 75% of the samples. The zooplankton followed a seasonal pattern, with the highest numbers occurring during the rainy season (February). A high density of microzooplankton was found which varied from 1,917 ind.m-3 to 47,620 ind.m-3. The macrozooplankton presented lower densities with a minimum of 92.4 ind.m-3 and a maximum of 1,617 ind.m-3. Species diversity was intermediate (~2 bits.ind-1) and eveness relatively high (~0.78). Samples could be joined into 3 groups, mainly influenced by rainfall. Species similarity presented 5 groups: oceanic, neritic, a mixture of neritic and oceanic, reef and estuarine. Our data suggest a dynamic relationship between the zooplankton communities characteristic of inshore water trapped by the coastal boundary layer and more offshore plankton communities. Key words: Zooplankton, reefs, distribution, diversity

The Contributions of Sea Ice Algae to Antarctic Marine Primary Production1

The seasonally ice-covered regions of the Southern Ocean have distinctive ecological systems due to the growth of microalgae in sea ice. Although sea ice microalgal production is exceeded by phytoplankton production on an annual basis in most offshore regions of the Southern Ocean, blooms of sea ice algae differ considerably from the phytoplankton in terms of timing and distribution. Thus sea ice algae provide food resources for higher trophic level organisms in seasons and regions where water column biological production is low or negligible. A flux of biogenic material from sea ice to the water column and benthos follows ice melt, and some of the algal species are known to occur in ensuing phytoplankton blooms. A review of algal species in pack ice and offshore plankton showed that dominance is common for three species: Phaeocystis antarctica, Fragilariopsis cylindrus and Fragilariopsis curta. The degree to which dominance by these species is a product of successional processes in sea ice communities could be an important in determining their biogeochemical contribution to the Southern Ocean and their ability to seed blooms in marginal ice zones.

Zooplankton Distribution and Abundance in West Greenland Waters, 1950-1984

Abundance indices of zooplankton obtained from offshore plankton surveys carried out in June–July from 1950 to 1984 off West Greenland were examined for trends and relationships with sea temperature and salinity. The zooplankton displacement volume and most of the zooplankton taxa showed higher abundance indices in the generally warmer period 1950–68 compared to the more variable period 1969–84. Sandeel larvae (Ammodytes sp.) were generally more abundant during cold periods after 1969. Zooplankton displacement volume showed a positive correlation (r = 0.28) with temperature, whereas correlations of individual zooplankton taxa with temperature or salinity were weaker. The relationships of zooplankton abundance indices with temperature suggest changes in zooplankton productivity during 1950–84.

Variability in the inter-island environment of the Prince Edward Islands (Southern Ocean)

Variability in the oceanographic parameters and macrozooplankton and micronekton composition, densities and distributional patterns were investigated during a repeat survey conducted between the Prince Edward Islands in April 1998. Results of this study demonstrated the occurrence of pronounced water pulses along the inter-island trench. The location of the Subantarctic Front to the north of the island plateau, through its interactions with the island group, appeared to have a marked effect on the mesoscale dynamics of physical and biological parameters between and around the islands. Seawater temperature and salinity accounted for >40% of the variation in the zooplankton distribution during the trench studies. A total of 41 macroplankton and micronekton taxa, consisting of subantarctic, subtropical and Antarctic species, were identified. Numerical analyses revealed two major groupings of stations corresponding to an offshore and inshore region. Although there was no evidence for quantitative differences in macroplankton densities between the inshore surveys, offshore plankton biomass was at least three- to eightfold higher than during the trench surveys. The importance of water pulses in carrying stocks of large plankton between the islands appeared to be minimal, at least during the time when the investigation took place.

Acritarchs and microfossils from the Mesoproterozoic Bangemall Group, northwestern Australia.

Three microfossil assemblages occur in the Mesoproterozoic Bangemall Group (1625-1000 Ma) of northwestern Australia, each occupying a different environmental and taphonomic setting. In peritidal environments, benthic prokaryotic filaments and spheroids of matting habit and small size were permineralized by early diagenetic silicification of stromatolitic carbonates. In shallow subtidal environments, benthic filaments of large size and nonmatting habit and planktonic sphaeromorph acritarchs with thin walls and moderate dimensions were compressed in mildly kerogenous shale. In deeper subtidal environments, planktonic megasphaeromorph acritarchs with thick walls were initially entombed in concretionary nodules in highly kerogenous shale and then permineralized by silica during later diagenesis. Taxonomic diversity and numerical abundance evidently decrease offshore. The three assemblages have typical Mesoproterozoic aspects: peritidal benthic habitats were dominated by Siphonophycus-Sphaerophycus-Eosynechococcus-Myxococcoides-Palaeopleurocapsa, shallow subtidal settings were occupied by Siphonophycus-Leiosphaeridia-Pterosphermopsimorpha-Satka, and offshore plankton consisted solely of very large chuarid acritarchs. Because of its taphonomic restriction to mid-intertidal stromatolites, the peritidal assemblage can be equated in microenvironment with a similar assemblage in the Neoproterozoic Draken Conglomerate, suggesting that ecological stasis at the community level can last for intervals up to 900 million years. In the deeper subtidal assemblage, the common chuarid has an unusual mode of preservation, in three dimensions in early diagenetic concretions, revealing that it possesses a thick multilamellate wall. Because of this distinctive ultrastructure, the new genus Crassicorium is erected for these fossils, which are among the oldest indubitable eukaryotes. Very large (34-55 micrometers in diameter) filaments from shallow subtidal habitats are assigned to the emended species Siphonophycus punctatum.

Plankton and Dissolved Inorganic Carbon Isotopic Composition in a River-Dominated Estuary: Apalachicola Bay, Florida

To characterize the isotopic composition of organisms at the base of the food web and the controls on their variability, the concentration and δ13C isotopic composition of dissolved inorganic carbon (DIC) and plankton δ13C, δ15N, and δ34S were measured. The measurements were made during periods of high and low river flow in Apalachicola Bay, Florida, United States, over 3 yr. DIC concentration and δ13C values were related to salinity, indicating that conservative mixing of riverine and marine waters was responsible for the overall distributions. The usefulness of DIC δ13C data for characterizing the trophic processes within the estuary was dependent upon the residence time of water within the season. Plankton δ13C values varied from −22‰ to −30‰ and were directly related to estuarine DIC δ13C, offset by a factor of roughly −20‰. This offset factor varied with salinity. Values of δ34S in estuarine plankton (station means ranged from 11.4‰ to 13.1‰) were depleted relative to marine plankton (17.7±0.4‰) possibly due to the admixture of34S-depleted sedimentary sulfide with estuarine samples. Values of δ34S in plankton were not related to δ13C values of plankton and were only weakly correlated to the salinity of the water from which the plankton were collected, indicating that marine sulfate was the primary source of planktonic sulfur. Values of δ15N in plankton varied from 5.5‰ to 10.7‰ and appeared related to dominance of the sample by phytoplankton or zooplankton. Estuarine plankton was15N enriched relative to offshore plankton and estuarine sediment.

Distribution and Growth of Young-of-the-Year Striped Bass in the Miramichi River Estuary, Gulf of St. Lawrence

Distribution and growth of young-of-the-year striped bass Morone saxatilis were determined at the northern edge of the species' range in the Miramichi River, Gulf of St. Lawrence, between May and December 1992, from samples obtained from ichthyoplankton, beach seine, trawling, and bycatch surveys. Shortly after yolk absorption, catches declined in offshore plankton surveys, and young-of-the-year appeared in nearshore areas. As summer progressed, young-of-the-year striped bass moved downstream into more saline waters. Downstream migration appeared to be size related. A logistic growth model closely fit field observations. Growth was relatively slow in spring (June: 0.32 mm/d), rapid in summer (July, August, and September: 0.75–1.15 mm/d), and reduced in fall (October and November: 0.17–0.46 mm/d). Growth was faster and length attained before winter was greater than reported for populations in the center of the species' geographic distribution.

Food‐web responses to species invasion by a predatory invertebrate: Bythotrephes in Lake Michigan

Several developments in the offshore plankton community accompanied the invasion of Bythotrephes cederstroemi Schoedler (Crustacea: Cladocera: Cercopagidae) into Lake Michigan. A native predatory cladoceran, Leptodora kindti, became significantly reduced in abundance and biomass in the presence of Bythotrephes. The offshore Daphnia assemblage, which had consisted of three species before the arrival of Bythotrephes, was reduced to dominance by only D. galeata mendotae. Abundances of Daphnia species exhibited reciprocal relationships to Bythotrephes abundance in both space and time. The surviving Daphnia populations offshore exhibited altered daytime vertical distributions which reduced spatial overlap with the invading predator. Model calculations indicate that energetic requirements by Bythotrephes equaled or exceeded replacement production by Daphnia during midsummer of the first years of species invasion. Decreases in midsummer Daphnia biomass did not produce significant increases in midsummer algal biomass, measured as particulate chlorophyll a. Instead, physical mixing depth and epilimnetic temperature, a correlate of the intensity of density stratification, exhibited the strongest statistical relationship to Chl a over the years studied.

Growth of Juvenile Pollock (Pollachius virensL.) Along the Atlantic Coast of Canada with Inferences of Inshore-offshore Movements

Data collected from tagging studies along the coast of Nova Scotia and offshore ichthyoplankton surveys were used to investigate growth and inshore-offshore migrations of juvenile pollock. Age zero pollock from 0.3 to 4.2 cm in length were found in the offshore plankton between November and June and small pollock at lengths of 7-11 cm were caught inshore in July. These small pollock appear to have moved inshore after leaving the offshore pelagic community at 3-6 months of age. They remain inshore until they reach approximately 30+ cm during their second year. Tagged inshore pollock were observed to grow 17-18 cm during their first year of life, an annual instantaneous growth rate (G) of4.1, and 27-30 cm by the end of their second year (G =1.8). Seasonal variability in growth was also observed in juvenile pollock. Mean absolute growth from May until August was 1.75 cm per month, from August until October 1.50 cm per month, and from November until April about 0.5 cm per month. A tagging related mortality experiment using winter tagged juveniles indicated a 11% mortality during the first 3 months after tagging.