Importance of biotic interactions in large rivers: an experiment with planktivorous fish,dreissenid mussels and zooplankton in the St. Lawrence River

Physical conditions are usually considered pre‐eminent in controlling river plankton, but biotic interactions may be important in slackwater areas. To begin testing this general hypothesis, we conducted a 12‐day, predator–prey experiment in 3500 litre mesh enclosures in a slackwater area of the St. Lawrence River using planktivorous, juvenile yellow perch (Perca flavescens) and Dreissena mussels. Results generally supported our hypotheses that: (1) perch would directly suppress large zooplankton via predation but benefit microzooplankton through indirect interactions; and (2) dreissenids would directly depress rotifer densities via predation and have indirect negative effects on macrozooplankton. Based on gut contents of experimental fish, cladocera were the principal prey of smaller yellow perch (c. 46–50 mm), followed by copepods, ostracods, and rotifers. Larger juvenile perch (c. 67–73 mm) fed almost exclusively on copepods and ostracods. Densities of calanoid copepodids, nauplii, and some cyclopoid copepods (Diacyclops thomasi) were significantly depressed by perch, and adult Eurytemora affinis (99% of adult calanoids) essentially disappeared from fish enclosures. Despite being a favourite prey item of small perch, densities of the small cladoceran Bosmina (Sinobosmina) spp. were significantly higher when fish were present (150% greater than control densities on Day 12). Densities of the very abundant rotifer Polyarthra were >300% greater in fish enclosures than controls by Day 12, and the rotifers Synchaeta and predaceous Ploesoma were significantly more abundant in the presence of perch. Increases in rotifers and cladocera suggest indirect, positive effects of fish related to significantly higher phytoplankton biomass or decreased densities of predaceous copepods. Densities of eight of ten zooplankton groups examined declined significantly when mussels were present, and calanoid copepodids also declined but not significantly. Chlorophyll‐a concentrations were slightly lower in mussel enclosures. This evidence suggests biotic interactions play important roles among potamoplankton in slackwater habitats, but river‐wide implications of these findings require further study. Copyright © 2003 John Wiley & Sons, Ltd.     

Recent changes in the offshore crustacean zooplankton community of Lake Ontario

Crustacean zooplankton communities in the offshore of Lake Ontario have undergone substantial changes between 1997 and 2011. A shift was apparent in 2004 from an initial assemblage dominated by cyclopoid copepods (mostly Diacyclops thomasi), Daphnia retrocurva and bosminids, and with Cercopagis pengoi (which invaded the lake in 1998) the dominant predatory cladoceran, to one characterized by reduced numbers of cyclopoids, a more varied predatory cladoceran community, and increased biomass of calanoid copepods. These changes represented a shift from a community which has been relatively stable in the offshore of the lake for at least 40 years. A further change was seen in 2008, marked by increased biomass of Leptodiaptomus sicilis, Bythotrephes longimanus and Daphnia mendotae. Unlike the somewhat similar changes seen recently in the zooplankton communities of Lakes Huron and Michigan, the shifts in Lake Ontario have not been accompanied by a trend towards increased oligotrophy. A more likely explanation for the observed shifts in Lake Ontario is decreased vertebrate predation, likely due to alewife declines, and changes in the predatory invertebrate community.     

Microinvertebrate dynamics in riverine slackwater and mid‐channel habitats in relation to physico‐chemical parameters and food availability

Microinvertebrates play a critical role in riverine food webs, and recent studies have hypothesized that slackwaters, non‐flowing regions associated with the main channel, are important for their reproduction and recruitment. However, little is known regarding the population and community dynamics of microinvertebrate communities in slackwater regions, or how they compare with those in mid‐channel regions. This study examined microinvertebrate communities in the epibenthic and pelagic zones of slackwater and mid‐channel regions (i.e. four habitats) of an Australian floodplain river in relation to physico‐chemical parameters and food availability (as estimated by chlorophyll‐a concentration) between September 2005 and November 2006. Results from this study indicate that microinvertebrate abundance and diversity were greater in slackwater habitats than mid‐channel habitats overall, corresponding with the slower current velocities associated with the former. Nevertheless, communities in all four habitats were most abundant and diverse in late spring (coinciding with an increase in water temperature), and followed similar seasonal trajectories in terms of density, taxon richness and community structure. These findings support the view that slackwaters are important for in‐channel microinvertebrate production, and suggest that animals frequently disperse (either actively or passively) among slackwater and other main channel regions year round. Given the critical importance of microinvertebrates in riverine food webs, rivers should be managed with a view to maintaining a natural variety of accessible slackwater regions in order to support the production and survival of microinvertebrate communities. Copyright © 2009 John Wiley & Sons, Ltd.     

DIEL DIFFERENCES IN 0+ FISH SAMPLES: EFFECT OF RIVER SIZE AND HABITAT

Studies of diel patterns in 0+ fish assemblages in rivers are scarce and produce contradictory results. We compared day and night electrofishing samples at 29 sites on rivers of different sizes in the Czech Republic. Diel patterns of 0+ fish differed among sites, but were consistent within each river stretch. River size was negatively correlated with both the day : night ratio of abundance (day samples produced more 0+ fish in smaller streams, night samples produced more 0+ fish in larger rivers) and similarity between day and night assemblage. A higher day : night ratio of abundance and lower similarity between day and night samples were observed at shelter‐lacking habitats (beach) compared with shelter‐providing habitats (boulder bank and flooded terrestrial vegetation). Differences in 0+ fish size were not dependant on diel period, river size, habitat or species. Day‐only or night‐only samples provided only two‐third of the information provided by a combination of day and night samples. The effect of habitat type and river size on diel differences in electrofishing catches should be considered when deciding on 0+ fish sampling protocol. Copyright © 2011 John Wiley & Sons, Ltd.     

Creating a disturbance: manipulating slackwaters in a lowland river

The dynamic nature of habitat patches in rivers is driven primarily by flow regime. Altered hydrology, through river regulation, can limit the size and distribution of slackwater patches; important areas for the development of young fish and for shrimp in lowland rivers. Between late October 2002 and late January 2003, we investigated responses of fish, shrimp and their potential prey to the experimental creation of slackwaters and, conversely, to the experimental creation of flowing patches, by diverting water away from flowing patches and into slackwater patches, respectively. A pre‐experimental survey indicated that slackwaters contained many more fish than flowing patches, and fish larvae were flushed out of slackwaters during the construction of flowing patches. Creation of slackwaters resulted in increased abundance of fish and shrimp, with the opposite occurring when slackwaters were changed into flowing patches. Converting slackwaters into flowing patches, and vice versa, altered the species composition of zooplankton and microbenthic assemblages but did not change their densities. Thus, standing crop of potential prey alone could not explain the differences in fish or shrimp abundance found between patch types. We hypothesize that slackwaters primarily act as refuges from current and provide energetic advantages to the young stages of fish and to shrimp. River regulation has the potential to affect the recruitment success of fish and shrimp by affecting the size, arrangement and availability of slackwater patches. Copyright © 2006 John Wiley & Sons, Ltd.     

Influence of Hydrological Connectivity on Plankton Communities in Natural and Reconstructed Side‐Arms of a Large New Zealand River

We sampled natural and reconstructed side‐arms during different stages of hydrological connectivity with a large floodplain river in northern New Zealand, to determine whether re‐establishment of connectivity would be an effective strategy for restoring plankton communities in former side‐arms. Connectivity between side‐arms and the river was moderated by water level and influenced flow rates and closure of inlets and outlets. Physicochemical conditions were more strongly related to the connectivity phase than to habitat type (river, natural or reconstructed side‐arm), except during low connectivity when natural side‐arms in particular were characterised by higher ammonium (NH₄‐N) and total phosphorus (P) concentrations, as well as specific conductivity. Dissolved reactive phosphorus (PO₄‐P), water temperature, conductivity and dissolved oxygen were identified as explanatory variables of phytoplankton and zooplankton community composition, which along with total nitrogen (phytoplankton) or total suspended solids (zooplankton) explained 44–52% of variation. Phytoplankton community composition and the abundance of several dominant or discriminatory taxa were affected by connectivity but not habitat type, whereas habitat and connectivity both had significant effects on zooplankton communities and abundances of the cladoceran Bosmina meridionalis. Significant interactions between connectivity and paired habitat types occurred for abundances of the diatom Asterionella, the cryptophyte Cryptomonas, the rotifer Synchaeta oblonga and cyclopoid copepods, reflecting differential responses to connectivity among habitats by these taxa. Overall, these results underscore the importance of hydrological connectivity between side‐arms and rivers in moderating plankton community composition, and highlight unpredictable trajectories of community development and alternative transient states that can occur soon after side‐arm reconnection. Copyright © 2016 John Wiley & Sons, Ltd.     

Zooplankton dynamics in a Great Lakes connecting channel: Exploring the seasonal composition within the St. Clair-Detroit River System

The connecting channels linking the Laurentian Great Lakes provide important migration routes, spawning grounds, and nursery habitat for fish, but their role as conduits between lakes for zooplankton is less understood. To address this knowledge gap in the St. Clair–Detroit River System (SCDRS), a comprehensive survey of crustacean zooplankton was performed in both riverine and lacustrine habitats from spring to fall 2014, providing the first system-wide assessment of zooplankton in the SCDRS. Zooplankton density and biomass were greatest in northern reaches of the system (southern Lake Huron and the St. Clair River) and decreased downstream towards Lake Erie. The composition of zooplankton also changed moving downstream, transitioning from a community dominated by calanoid copepods, to more cyclopoids and cladocerans in the Detroit River, and to cladocerans dominant in western Lake Erie. Coincidentally, species richness increased as sampling progressed downstream, and we estimated that our single-year sampling regime identified ~88% of potential taxa. Other species assemblages have responded positively to recent water quality and habitat restoration efforts in the SCDRS, and this survey of the zooplankton community provides benchmark information necessary to assess its response to continued recovery. In addition, information regarding the lower trophic levels of the system is integral to understanding recruitment of ecologically and economically valuable fish species targeted for recovery in the SCDRS.     

Mesozooplankton community structure in a degrading mangrove ecosystem of the Cochin coast,India

The diversity, abundance and community structure of mesozooplankton from the hydrographically differing mangrove habitats in Cochin, Kerala (India), were investigated. Eleven zooplankton groups were identified, with calanoid copepods (62%) comprising the predominant population, followed by crustacean nauplii (22%) and mysids (12%). Nitrogen limiting (N: P ratio 5.13 ± 7.03) with a mixo‐mesohaline salinity (8.96 ± 3 psu) and relatively low temperatures (30.28 ± 0.9 °C) was a characteristic feature of the study zones. Non‐metric multidimensional scaling based on Bray–Curtis similarity (PRIMER) analysis revealed there was little variation in the mesozooplankton structure among sampling sites (stress 0.05; ANOSIM test Global R:0.325). BEST analysis indicated that temperature and salinity exhibited the best correlation (ρ = 0.701) with fauna. The results of this study indicated a severe reduction in zooplankton abundance, biomass and diversity. It is clear that the zooplankton communities are depleting, along with the mangrove vegetation, because of the shallowness and unsuitable environmental conditions of the water bodies attributable to anthropogenic stresses.     

Spatial variation in zooplankton community structure is related to hydrologic flow units in the Missouri river,USA

Zooplankton are a vital link in the food webs of large rivers, and their communities are shaped by both local environmental features and advection. In the Missouri River, flow characteristics naturally change along its length, but human modifications to facilitate commercial transport have altered natural flow in many sections of the river. We evaluated the effect of flow on zooplankton community structure at multiple spatial scales, and used multivariate analyses to evaluate the relative importance of flow and local abiotic environment on these communities. During July–September 2005, zooplankton samples and physico‐chemical measures were collected from the Missouri River main channel at 78 sites over a 2831 km range (Montana to Missouri). We identified a total of 30 cladoceran species, 22 copepod species and 27 rotifer genera, and we detected highly significant differences in zooplankton community structure among hydrologically distinct flow units and larger spatial zones. At the local scale, crustacean zooplankton and rotifers responded differently in the analyses. For copepods and cladocerans, distance from the nearest upstream reservoir explained more of the overall community pattern of the river than any other combination of environmental factors, reflecting the influence of dams on the zooplankton community of the Missouri River. For rotifers, a combination of flow characteristics due to impoundment and channelization and local environment (temperature) was important. Our study indicates that, because of the overwhelming effect of flow on zooplankton communities, hydrology must first be taken into account before zooplankton can be used as bioindicators of other environmental stresses. Copyright © 2009 John Wiley & Sons, Ltd.     

Lake Ontario’s nearshore zooplankton: Community composition changes and comparisons to the offshore

In large lake systems the nearshore habitat is an intermediate zone between the shoreline and offshore, is an important nursery for larval fish, and is highlighted as an area in need of research in the Laurentian Great Lakes. In this study, we used two long-term monitoring programs to characterize the nearshore zooplankton community composition using seasonal data (May – October) and to compare the nearshore and offshore zooplankton community composition changes over time (1998 – 2019) to determine if the changes were synchronized. In the nearshore, we found the highest zooplankton biomass during the late summer/early fall (August 27th – Oct 6th), compared to mid-summer (July 1st – Aug 26th) and late spring (May 20th – June 30th). In the summer, the nearshore zooplankton community was dominated by cladocerans while copepods dominated the offshore community. From 1998 to 2019, both nearshore and offshore copepods shifted from a cyclopoid to a calanoid-dominated state, but the details of this change were different. For example, taxon-specific analysis revealed that despite reduced cyclopoids in both habitats, Mesocyclops edax increased in the nearshore. Additionally, taxon-specific analysis suggested the changes occurred an average of three years earlier in the nearshore. Using Analysis of Similarity, the nearshore and offshore summer zooplankton community compositions became increasingly distinct over time. Results from this study highlight the uniqueness of the nearshore in large lake systems, the importance of seasonal and long-term monitoring, and the potential of the nearshore as an early indicator of offshore changes.     

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.     

A Decade of Predatory Control of Zooplankton Species Composition of Lake Michigan

From 1983 to 1992, 71 species representing 38 genera from the Calanoida, Cladocera, Cyclopoida, Mysidacea, Rotifera, Mollusca and Harpacticoida comprised the offshore zooplankton community of Lake Michigan. Our data demonstrate that the composition and abundance of the calanoid community after 1983 is not unlike that of 1960s and that species diversity of the calanoid community is more diverse than the cladoceran community in the 1990s as compared to the early 1980s. Even though the relative biomass of the cladocerans has remained similar over the 1983–1993 period, the species diversity and evenness of the Cladocera community in the early 1990s is unlike anything that has been previously reported for Lake Michigan. Cladocera dominance is centered in one species, Daphnia galeata mendotae, and only three species of Cladocera were observed in the pelagic region of the lake in 1991 and 1992. Nutrient levels, phytoplankton biomass, and the abundance of planktivorous alewife and bloater chub and Bythotrephes are examined as possible causes of these changes in zooplankton species composition. The increase in Rotifera biomass, but not Crustacea, was correlated with an increase in relative biomass of unicelluar algae. Food web models suggest Bythotrephes will cause Lake Michigan's plankton to return to a community similar to that of the 1970s; that is Diaptomus dominated. Such a change has occurred. However, correlational analysis suggest that alewife and bloater chubs (especially juveniles) are affecting size and biomass of larger species of zooplankton as well as Bythotrephes.     

A cross-lake comparison of crustacean zooplankton communities in the Laurentian Great Lakes, 1997–2016

Spring and summer open-water crustacean zooplankton communities were examined across all five Laurentian Great Lakes from 1997 to 2016. Spring communities were dominated by calanoid (lakes Superior, Huron and Michigan) or cyclopoid (lakes Erie and Ontario) copepods. Volumetric biomass of summer communities increased along an assumed trophic gradient (Superior, Huron, Michigan, Ontario; eastern, central and western Erie), as did dominance by cyclopoids and cladocerans. Over the time series of the study, summer communities in lakes Michigan, Huron and Ontario shifted towards greater dominance by calanoids and greater similarity with Lake Superior. Trajectories of changes were different; however, reductions in cladocerans accounted for most of the change in lakes Michigan and Huron while reductions in cyclopoids and increases in Leptodiaptomus sicilis were behind the changes in Lake Ontario. Shifts in the predatory cladoceran community in Lake Ontario from Cercopagis pengoi to occasional dominance by Bythotrephes longimanus, a species much more vulnerable to planktivory, as well as the appearance of Daphnia mendotae in a daphnid community previously consisting almost exclusively of the smaller Daphnia retrocurva, suggest impacts of reduced vertebrate predation. In contrast, strong correlations between cladocerans and chlorophyll in lakes Michigan and Huron point to the possible importance of bottom-up forces in those lakes. Large interannual shifts in cladoceran community structure in the central and eastern basins of Lake Erie suggest intense but variable vertebrate predation pressure. The zooplankton communities of lakes Huron, Michigan and Ontario may be approaching a historic community structure represented by Lake Superior.     

Crustacean zooplankton available for larval walleyes in a Lake Michigan embayment

Adequate densities of zooplankton prey are critical for growth and survival of larvae of many fish species. Little information exists on the density of zooplankton in Great Lakes inshore areas during early spring, when larvae of important fishes rely on zooplankton. Reduced age-0 walleye recruitment and the absence of data on zooplankton availability for larval walleyes in northern Green Bay, Lake Michigan, led us to assess zooplankton densities during this critical spring period. We conducted biweekly vertical plankton tows in 2014–2016 near reefs and river plumes used by spawning walleyes for periods when larval walleyes were expected to be relying on zooplankton prey. Densities of zooplankton were well below literature values identified for good growth and survival of larval walleyes, averaging 1.5 individuals L⁻¹ for all taxa and 0.12 individuals L⁻¹ for large-bodied taxa across all sites and sampling dates. Various factors could contribute to the low density of zooplankton observed. We found low but significantly higher densities of cyclopoid copepods, nauplii, Bosmina, and total zooplankton at river mouth sites compared to open water sites. These results suggest that food availability for larval walleye in our study area was severely limiting which is consistent with the paucity of strong year classes observed since 2000. We suspect northern Green Bay has limited potential for producing strong year classes of walleyes under such conditions. Fishery managers working in unproductive waters should consider assessing the zooplankton community during critical periods to identify potential bottlenecks to reproductive success and larval fish survival.     

Assessing the Relationship Between River Mobility and Habitat

Human interventions that limit channel mobility such as bank stabilization are frequent in riparian zones in urban or agricultural environments. This is potentially problematic because channel mobility is an important geomorphological and ecological agent that structures natural instream and riparian ecosystems. This study aims to (1) quantify the relationship between mobility and three types of habitat‐related features, namely bars, oxbow lakes and log jams, for a 54‐km‐long reach of the Yamaska Sud‐Est River (Quebec, Canada), which runs from the forested Appalachian Mountains to the agricultural St‐Lawrence lowlands, and (2) evaluate the impact of human interventions and geomorphological characteristics on these three features. Channel mobility was measured from historical aerial photos for the period 1950–2009. A combination of high‐resolution aerial photos, LiDAR digital elevation model, and field observations was used to measure and map sediment bars, oxbow lakes and log jams, as well as several geomorphological characteristics (channel width, slope, sinuosity and floodplain width). A strong link between the mobility and the presence of habitat features is revealed, but local geomorphological contexts result in different mobility patterns responsible for specific habitats. Floodplain to channel width ratio appears as the best geomorphological factor predicting habitat diversity. Human intervention, mostly through bank stabilization, also appears to be a key factor limiting mobility and its related habitats. These results highlight the importance of defining a protected mobility corridor along rivers where geomorphic processes such as bank erosion can freely occur, as it is an essential process that should be integrated in land use planning and river management. Copyright © 2015 John Wiley & Sons, Ltd.     

Ecology of an estuarine mysid shrimp in the Columbia River (USA)

The estuarine mysid, Neomysis mercedis, has colonized John Day and other run‐of‐the‐river Reservoirs of the Columbia River, over 400 km from the estuary. In John Day Reservoir N. mercedis numbers peaked (2 m^?3) in August in areas near the dam in association with lower water velocity and softer bottom than at the upstream sampling sites. Neomysis broods were primarily released in late spring and early fall. Gut content analysis showed that Neomysis feeds mostly on cladoceran zooplankton and rotifers in John Day Reservoir. Diel vertical migration was documented, with daytime distribution restricted to the bottom and preferentially to the soft‐textured sediments in the deepest areas. Common pelagic fishes in the reservoir, especially juvenile American shad (Alosa sapidissima) and chinook salmon (Oncorhynchus tshawytscha), are daytime zooplankton feeders that cannot prey on Neomysis owing to mysid diel vertical migration. Thus, Neomysis has become an important food web component in John Day Reservoir. We also collected N. mercedis further upstream in Lower Granite Reservoir, where another estuarine crustacean, Corophium salmonis, also is reported, underscoring the need to better understand the role of these estuarine invertebrates in the trophic ecology of the Columbia River. Copyright © 2006 John Wiley & Sons, Ltd.     

Diel patterns in spatial distribution of fish assemblages in lentic and lotic habitat in a regulated river

Fish assemblages in large rivers are governed by spatio‐temporal changes in habitat conditions, which must be accounted for when designing effective monitoring programmes. Using boat electrofishing surveys, this study contrasts species richness, catch per unit effort (CPUE), total biomass, and spatial distribution of fish species in the Saint John River, New Brunswick, Canada, sampled during different diel periods (day and night) and macrohabitats (hydropower regulated river and its reservoir) in the vicinity of the Mactaquac (hydropower) Generating Station. Taxa richness, total CPUE, and total biomass were significantly higher during night surveys, resulting in marked differences in community composition between the two diel periods. Furthermore, the magnitude of diel differences in catch rate was more pronounced in lentic than in lotic macrohabitats. The required sampling effort (i.e., number of sites) to increase accuracy and precision of CPUE estimates varied widely between fish species, diel periods, and macrohabitats and ranged from 15 to 185 electrofishing sites. Determining a correction factor to contrast accuracy and precision of day‐ with night‐time surveys provide useful insights to improve the design of long‐term monitoring programmes for fish communities in large rivers. The study also shows the importance of multihabitat surveys to detect differences in the magnitude of diel changes in fish community metrics.     

Temporal patterns in phytoplankton,zooplankton and fish composition,abundance and biomass in Shirati Bay,Lake Victoria,Tanzania

Shirati Bay is among the important breeding and nursery sites for major fish species in Lake Victoria. Weekly samplings were conducted to assess the temporal patterns in phytoplankton, zooplankton and fish composition, abundance and biomass in relation to prevailing water quality parameters. The study also determined the influences of plankton dynamics and water quality on the fish catch composition and biomass. It was hypothesized that temporal patterns in the composition, abundance and biomass in the plankton in the bay are controlled by water quality parameters that, in turn, affect the composition and biomass of fish catches. The phytoplankton comprised mainly cyanophytes and bacillariophytes, while the zooplankton were dominated by copepods. The heavy rain season exhibited a significantly higher plankton abundance and biomass than the dry season. The plankton abundances in both seasons exhibited significant positive correlations with water temperature and transparency. The phytoplankton community was controlled by calanoid and cyclopoid species. At higher trophic levels, Lates niloticus juveniles, Oreochromis niloticus juveniles and haplochromines controlled Cladocera and Cyclopoid copepods, while Tilapia rendalli juveniles controlled the Rotifera. This study revealed that Cyanophyta and Bacillariophyta are the dominant phytoplankton, whereas cyclopoids dominate the zooplankton species in the bay. These dominant plankton groups are partly controlled by rainfall, water temperature and transparency. Fish biomass, zooplankton and phytoplankton exhibit a typical predator–prey inverse relationship. Thus, evaluation of the plankton composition, abundance and biomass should be mandatory during fisheries stock assessments to effectively manage the fishery resources in the bay.     

Zooplankton sampling in large riverine systems: A gear comparison

Large river systems create challenges when sampling zooplankton. The dynamics of large rivers, spatial heterogeneity of biota, and behaviours of zooplankton can all contribute to large variation in estimation. The gear utilized for zooplankton collections may also be a factor, and there is not a well‐studied gear, nor industry standard, for collection in riverine systems. The lack of consistent sampling methods makes it difficult to compare zooplankton within and across systems. A standard method for zooplankton collection would be valuable for comparison among studies. To optimize zooplankton assessment in large river habitats, we tested how community estimations varied between four common zooplankton sampling devices across three river habitat types. We tested four gears: the Schindler–Patalas trap, integrated tube sampler, powered water pump, and horizontal tow net. Each device was used to collect samples in thalweg, channel border, and backwater river habitats within Pool 8 of the Upper Mississippi River in June, July, and August of 2017. Our results support that there are qualitative and quantitative differences in zooplankton estimates among gear in different habitats. The powered pump most often yielded highest abundances of total and individual zooplankton taxa allowing for more reliable community comparisons. With some modification, the pump is recommended as the most appropriate sampling gear when performing quantitative studies of zooplankton composition and abundance in large river habitats.     

A Comparison of Total Phosphorus,Chlorophyll a,and Zooplankton in Embayment,Nearshore, and Offshore Habitats of Lake Ontario

Lower trophic levels were compared in embayment, nearshore, and offshore habitats of Lake Ontario, 1995 to 1997, in the context of oligotrophication and invasion of dreissenid mussels. Total phosphorus (TP), chlorophyll a (chl a), Secchi disk depth, temperature, and zooplankton were measured to spatially and temporally contrast these habitats and to test for recently hypothesized “decoupling” of chl a from TP (lower than predicted chl a per unit TP, consistent with dreissenid mussel grazing). The embayment habitat had higher concentrations of TP and chl a, greater volumetric zooplankton density and biomass, and higher springtime water temperatures than both nearshore and offshore habitats, while overall areal zooplankton biomass was highest in the offshore. Furthermore, concentrations of TP and volumetric zooplankton density in nearshore habitats are now more similar to the offshore pelagia than they were three decades ago. Finally, a lower yield of chl a per unit TP was found in nearshore habitat compared to offshore and embayment habitats. The current lower yield of chl a per unit TP in nearshore habitat can be attributed more to Dreissena than to erosion and/or resuspension of sediments.