The effect of river runoff and phytoplankton production on the seasonal variation of the chemical composition of coastal waters of the Amursky Bay, Sea of Japan

The factors that control the seasonal variability of the concentration of suspended matter, nutrients, dissolved forms of metals and phytoplankton biomass in coastal waters of the eastern part of the Amurskii Bay were determined. It was shown that the runoff of the Razdolnaya River determined the seasonal variability in suspended matter content, values of the chemical oxygen demand (COD), silicates, Fe, Mn, Cu, and Ni significantly affected the concentration of nitrates. The advection of sea water is the main source of phosphates and dissolved forms of Zn and Cd. In the autumn and winter seasons, the gradual decrease in the concentration of silicates and phosphates was related to phytoplankton activity. In the spring and summer seasons, the growth of phytoplankton determined the variability of the phosphate and nitrate contents and compensated for the effect of the Razdolnaya River runoff. It is concluded that the phytoplankton biomass, rather than variations in riverine flow, caused the long-term variability of the nitrate and phosphate concentrations in the coastal waters of the Amurskii Bay.     

Effects of vessel-induced waves on the YOY-fish assemblage at two different habitat types in the main stem of a large river (Danube,Austria)

The effects of navigation on young-of-the-year (YOY) fish were investigated for the Danube River based on an integrative approach. During YOY growing season, wave height, wave frequency and water currents were recorded. Synchronously, fish drift and fish abundance of YOY were monitored in two distinct inshore nursery habitats—a gravel bar and a groyne field. The characteristics of vessel-induced waves were correlated to YOY-drift. In both habitats, an increase in drift densities during ship passages was observed. At the same time, assemblage composition and drift densities differed between habitat types. At the gravel bar, drift densities of larval cyprinids—representing many keystone species of the Danube—were higher during ship passages. In the groyne field, drift was low and percids dominated the YOY-assemblage along the shore. Banks with gentle slopes in the main channel seem to be particularly affected by vessel-induced wave wash. Precisely such habitats are often established within the scope of modern river restoration projects because they provide suitable nursery habitats for riverine fish species. Beside the positive effects on biota, the establishment of these habitats in navigable reaches may create problems for YOY-fish due to displacement effects induced by ship-generated waves.     

模拟水流条件下初级生产力及光动力学参数

为探讨水动力作用及其他物理因子改变对湖泊初级生产力的影响 ,1999年 5月 8日～ 6月 2 4日在中国科学院太湖湖泊生态系统研究站大型生态实验槽内进行模拟水动力实验 ,分 3种水流状态 2种光强测定初级生产力及其他相关参数。分析了初级生产力、光合速率的垂直分布 ,光合速率随光强的变化 ,并借助光动力学模型拟合得到光动力学参数。结果表明 ,在静止状态下 ,当水表面光强大于 5 0 0μmol/ (m2 · s)时 ,0～ 0 .4 m处存在光抑制现象 ,最大初级生产力出现在 0 .4～ 0 .6 m,此后由于动力作用使水体悬浮物增加 ,改变了水下光照条件 ,致使最大初级生产力呈向上移动的趋势 ,出现在 0～ 0 .2 m间 ;光合速率在静止状态下随深度递减缓慢 ,而到大水流状态则递减极为迅速 ,大水流状态下的平均光合速率明显低于静止状态和小水流状态 ;基于 2种类型的光动力学模型进行非线性拟合得到的 P- I曲线相关性很好 ,2种模型模拟的结果比较接近 ,基本上能够反映太湖光合速率随光强变化的实际情况 ;在太湖这种大型浅水湖泊 ,水动力的作用使得水体中悬浮物增加 ,造成光强的迅速衰减 ,这可能会大大降低湖泊的初级生产过程     

Temporal patterns and variations in phytoplankton community organization and abundance in Narragansett Bay during 1959-1980

Incident irradiance, surface water temperature and phytoplanktonspecies abundances were measured at weekly intervals in NarragansettBay from 1959 through 1980. Stepwise discriminant analyses (SDA)of this 22-year data set indicate that fundamental ecosystemchanges occurred between the 1960s and 1970s, with 1969 beingthe key transitional year in these decadal shifts in phyto planktontaxonomic structure and seasonal abundance. This decadal shiftwas accompanied by the increased summer abundance of small Thalassiosiraspp., which first appeared in 1966 and by 1969 became the sixthmost important phytoplankton component in this bay. Decadaltrends in phyto plankton community organization and abundancewere also accompanied by distinct long-term climatological gradientsof temperature and light. The 1960s were generally colder andbrighter than the 1970s. Prior to 1969, the annual phytoplanktonmaximum occurred most commonly during winter; in the 1970s,the annual maximum generally shifted to a summer event. Three5-year phytoplankton cycles occurred between 1959 and 1974.During each pentade, the phytoplankton community returned toa similar taxonomic organization and abundance cycle after divergingin the intervening years. Pentade cycles did not occur after1974; the phytoplankton community thereafter diverged significantlyfrom each preceding year. Five species [Skeletonema costatum,Detonula confervacea, Asterionellopsis glacialis, Hererosigmaakashiwo (= Olisthodiscus lureus) and Thalassiosira nordenskioeldii]dominated the phytoplankton over the 22-year period. SDA revealeda high degree of similarity and constancy in the annual occurrencepatterns of these taxa. The decadal shifts revealed by SDA weremore directly related to the considerable interannual variabilitythat characterized the abundance and seasonality of the lessabundant species.     

Seasonal variations of size-fractionated phytoplankton along the salinity gradient in the York River estuary, Virginia (USA)

The dynamics of phytoplankton size structure were investigatedin the freshwater, transitional and estuarine zones of the YorkRiver over an annual cycle. The contribution of large cells(microplankton, >20 µm) to total concentrations ofchlorophyll a increased downstream during winter, whereas thatof small cells (nanoplankton, 3–20 µm; picoplankton,<3 µm) increased downstream during summer. In the freshwaterregion, the contribution of micro phytoplankton to total concentrationsof chlorophyll a was significant during warm seasons (springand summer) but not during colder seasons (winter), whereasthe contribution of small-sized cells (especially picoplankton)increased during cold seasons. Temperature, light and high flushingrate appear to control phytoplankton community structure inthe freshwater region. In the transitional region, nano-sizedcells dominated the phytoplankton population throughout allseasons except during the spring bloom (April) when the chlorophylla concentration of micro phytoplankton increased. Size structurein the transitional region is most likely regulated by lightavailability. In the mesohaline region, nano- and pico-sizedcells dominated the phytoplankton population during the summerbloom, whereas micro-sized cells dominated during the winterbloom. Factors controlling phytoplankton community size structurein the mesohaline zone may be riverine nitrogen input, temperatureand/or advective transport from up-river. Based on these results,the spatial and seasonal variations in size structure of phytoplanktonobserved on the estuarine scale may be determined both by thedifferent preferences for nutrients and by different light requirementsof micro-, nano- and picoplankton. The results suggest thatanalyses of phytoplankton size structure are necessary to betterunderstand controls on phytoplankton dynamics and to bettermanage water quality in river-dominated, estuarine systems.     

Interannual variability in the distribution of the phytoplankton standing stock across the seasonal sea-ice zone west of the Antarctic Peninsula

The spatial distribution of phytoplankton cell abundance, carbon(C) biomass and chlorophyll a (Chl a) concentration was analysedduring three summers (1996, 1997 and 1999) in a seasonal sea-icearea, west of the Antarctic Peninsula. The objective of thestudy was to assess interannual variability in phytoplanktonspatial distribution and the mechanisms that regulate phytoplanktonaccumulation in the water column. Phytoplankton C biomass andChl a distributions were consistent from year to year, exhibitinga negative on/offshore gradient. The variations in C concentrationhad a close and non-linear relationship with the upper mixedlayer depth, suggesting that the vertical mixing of the watercolumn is the main factor regulating phytoplankton stock. Themagnitude of C gradients was 5-fold higher during 1996 thanduring 1997 and 1999. This was ascribed to interannual variationsin the concentration of diatom blooms in the region influencedby sea-ice melting. Vertical distribution of the phytoplankton,as estimated from Chl a profiles, also varied along an on/offshoregradient: Chl a was distributed homogeneously in the upper mixedlayer in coastal and mid-shelf stations and concentrated inthe deep layer (40–100 m) occupied by the winter waters(WW, remnants of the Antarctic surface waters during summer)in more offshore stations. The region with a deep Chl a maximumlayer (DCM layer) was dominated by a phytoplankton assemblagecharacterized by a relatively high concentration of diatoms.The extent of this region varied from year to year: it was restrictedto pelagic waters during 1996, extended to the shelf slope during1997 and occupied a major portion of the area during 1999. Itis hypothesized that iron depletion in near surface waters dueto phytoplankton consumption, and a higher concentration inWW, regulated this vertical phytoplankton distribution pattern.Furthermore, we postulate that year-to-year variations in thespatial distribution of the DCM layer were related to interannualvariations in the timing of the sea-ice retreat. The similaritybetween our results and those reported in literature for otherareas of the Southern Ocean allows us to suggest that the mechanismsproposed here as regulating phytoplankton stock in our areamay be applicable elsewhere.     

Modelling the habitat preferences of preadult and adult fishes on the shoreline of the large, lowland Elbe River

From the years 1997 to 2000, fish assemblages from 15 groyne fields and training walls (a current guiding dyke, protecting the groyne field) of the Middle Elbe River (Germany) were examined by electrofishing. The aim of the study was to detect abiotic environmental key variables which determine preadult and adult fish assemblage in typical riverbank structures using multivariate ordination techniques (canonical correspondence analysis) and univariate methods (logistic regression analysis). Habitat preferences of preadult and adult stages of selected fish species were also estimated. Fish sampling was carried out at 190 river stretches; a further 1615 samples were taken by the point‐abundance sampling method. In total, 21 732 preadult and adult individuals belonging to 30 fish species were caught; perch, eel, ide, roach, chub, gudgeon, bleak and white bream were the most frequent. Eurytopic species clearly dominated the fish assemblage with 72% total abundance, followed by rheophilic species (26%). Limnophilic fish species (rudd, crucian carp, stickleback and tench) were relatively rare (2% of total abundance), with frequencies of occurrence between 1 and 10%. The highest species diversities were estimated in structurally diverse training walls and groyne fields with defective groynes, whereas low species diversity was found to occur in poorly structured, strongly silted groyne fields with intact groynes. Additionally, typical seasonal and annual changes of the fish assemblage were observed. From the mesoscalic point of view, the structure of the preadult and adult fish assemblage was affected by both spatial (type of groyne field, river stretch) and temporal factors (year, season). Regarding the microhabitat, hydromorphological parameters (slope, current velocity, water level, predominant and secondary substrate) followed by physicochemical water values [dissolved oxygen (DO), temperature and pH‐value] are of significance for the structure of the fish community. Furthermore, hiding places (shelter) are of subordinate but significant importance for the fish assemblage in the poorly structured main channel of the Elbe River. Importance of hydromorphological factors on the microhabitat scale was confirmed by multiple logistic regression in 12 of 14 preference models, conducted for different ontogenetic stages of roach, bream, white bream, bleak, ide, chub, gudgeon, asp, perch and eel. Habitat preferences of adult bleak and preadult roach were first determined by physicochemical water values and seasonal influences. Univariate models were generated to describe specific habitat preferences of different species and age‐stages regarding the most important environmental variables (depth, velocity, predominant substrate, slope and shelter). Intraspecific differences were noticed between preadult and adult chub regarding preferences for water velocity, or between preadult and adult perch concerning slope of shore. Interspecific differences were observed, e.g. between adult roach and white bream, with regard to the use of shelters.     

Estimating chlorophyll a abundance from the 'phytoplankton colour' recorded by the Continuous Plankton Recorder survey: validation with simultaneous fluorometry

The green colour (measured with reference to standard colourcharts) of sections of the Continuous Plankton Recorder (CPR)filtering silk was compared with estimates of chlorophyll aconcentration derived from a fluorometer mounted on the CPRduring seven tows in the North Sea between February and May1991. After the green colour was assessed, the abundance ofphytoplankton cells on the filtering silks was quantified bymicroscope analysis. Data were collected for 115 10-nautical-milesamples over a total of seven cruises. For these 115 samples,there was only a weak (F_1.113 = 3.8, P = 0.05, r² = 0.03) positiverelationship between the colour of the filtering silk and thechlorophyll a concentration. However, when this comparison wasrestricted to four tows (68 10-nautical-mile samples) wherethe recorded phytoplankton cell abundance on the silks was verylow, there was a highly significant (F_1.66 /,69.1, P < 0.001,r² = 0.51) positive relationship between the silk colour andthe chlorophyll a concentration. By measuring the relative colourintensity of CPR standard colour categories and quantifyingthe individual variation in the assessment of colour, a theoreticalmodel was developed which pedicted that if the silks were colouredin direct proportion to the chlorophyll a concentration in thewater, then the expert r² for the relationship between silkcolour and chlorophyll a concentration would be 0.62. The greencolour recorded by the CPR survey was therefore identified asa quantitative index of chlorophyll a concentration, but onlywhen numbers of phytoplankton cells on the CPR silks are nothigh.     

Formation,Transport and Retention of Aggregates in a River‐Lake System (Spree,Germany)

The formation, particle size distribution, structure and retention of aggregates were studied in a river‐lake system of the lowland River Spree and factors influencing these processes were evaluated. Samples were taken from the inflow and outflow of a flushed lake, as well as along the adjacent flowing reach of the River Spree, between April 2001 and May 2002. The aim was to record the influence of the seasons and different turbulence intensities on the size, shape, composition, abundance and transport properties of the particles. As additional parameters, the phytoplankton biomass, the concentration of suspended particulate matter, the concentrations of nutrients and the discharge were measured. The abundance of the particles showed a strong seasonality in the investigated period, which closely correlated with the concentrations of suspended particulate matter and particulate organic matter in the water column. The organic carbon content of the suspended matter varied between 37% (dry weight) in summer and about 14% in winter. The concentration of particles ranged between 500 and 2500 particles/ml. The number of particles was significantly lower upstream from the lake than at the downstream transects. The majority of the aggregates in the River Spree were smaller than 500 µm, most of the particles even showing a diameter less than 50 µm. Many particles, above all algae, were washed out of the lake Neuendorfer See into the adjacent flowing reach of the River Spree. Algae influenced the abundance of the particles and contributed to the formation of aggregates with their stickiness. In the river flowing section large particles were formed by turbulence structures, whilst simultaneously the number of smaller particles decreased in the water column caused by aggregation processes. In general, an increasing flow velocity and discharge rate resulted in a larger amount of aggregates in the water column. On the other hand, particle abundance was reduced in the flow direction when the flow velocity was low. Under these conditions particles were retained by sedimentation and benthic filter‐feeding. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Scaling-up from nutrient physiology to the size-structure of phytoplankton communities

In many community assemblages, the abundance of organisms isa power-law function of organism size. In phytoplankton communities,changes in size structure associated with increases in resourceavailability and total biomass have often been interpreted asa release from grazer control. A metapopulation-like approachis used to scale up from the individual physiological responsesto environmental conditions to community size structure assumingthe community taxonomic composition reflects the species pool.We show that the size scaling of cellular nutrient requirementsand growth can cause (1) the power-law relationship betweencell size and abundance, (2) dominance of small phytoplanktoncells under oligotrophic conditions, and (3) relative increasein abundance of larger phytoplankton cells under eutrophic conditions.If physiological differences associated with the taxonomic compositionof different community size fractions are considered, then themodel can replicate detailed field observations such as theabsence of small, slow-growing Prochlorococcus spp. and therelative dominance of large diatom species in nutrient-rich,upwelling regions of the ocean. This paper was presented in a session on "Size Structure ofPlankton Communities", at the ASLO Summer International Meeting,held in Santiago de Compostela, Spain, between 19 and 24 June,and coordinated by Xabier Irigoien, Roger Harris and Angel Lopez-Urrutia.     

Convection in ice-covered lakes: effects on algal suspension

Convection occurs in ice-covered lakes if solar radiation warmsnear-surface water from the freezing point towards the temperatureof maximal density. One effect of convective mixing may be tosuspend non-motile phytoplankton in the upper water column,providing cells with enough light for growth during ice-coveredperiods. Observations of the diatom Aulacoseira baicalensisunder the ice cover of Lake Baikal, Siberia, support the hypothesisthat convective mixing causes net suspension of cells. Thispaper presents a theoretical examination of the conditions underwhich convective flow fields can suspend algae in the photiczone of the upper water column. It is shown that the efficiencyof algal suspension depends on the ratio of the still-wateralgal sinking rate, W_p, to convective updraft speed, W_u. Thesuspension efficiency is also shown to be affected by asymmetriesin the flow field and night-time cessation of convection, butonly if W_p and W_u are comparable in value. It is concluded thatconvection in Lake Baikal should be vigorous enough to increasethe mixed-layer residence time of A.baicalensis from a few daysto over a month, at least during years with thin snow cover.     

Zooplankton-phytoplankton interactions in a eutrophic lake

Enclosure experiments were made in a cyanobacteria dominatedlake (Lake Rotongaio) to assess the impact of zooplankton (>150µm) grazing on algal growth rates and determine the effectof diel and vertical changes in zooplankton grazing intensityand nutrient (NH₄-N) regeneration upon abundance of phytoplankton.The filamentous cyanobacterium Anabaena minutissima var. attenuataand diatom Cyclotella meneghiniana showed a negative linearchange in abundance with a gradient in zooplankton grazing intensity.Phytoflagellates were not grazed and showed a positive linearchange in abundance with increasing zooplankton biomass. Theseeffects, as well as shortening of filament length of Anabaena,were caused by raptorial feeding by the alanoid copepod Boeckellapropinqua which dominated the zooplankton. Phytoplankton growthwas not stimulated by addition of nutrients, suggesting nutrientregeneration was not important. Diel and vertical changes infeeding and NH₄-N regeneration rates were measured in Marchand June 1988. Diel differences were more pronounced in Marchwhen the water column was stratified. Specific feeding rateswere more important than vertical changes in zooplankton biomassin determining community grazing rates in March, but in Junewhen the water column was mixed, vertical distribution of zooplanktonbiomass was important. Zooplankton grazing was an importantloss process for phytoplankton in the lower part of the epilimnionin Lake Rotongaio.     

Response of phytoplankton and bacteria to nutrients and zooplankton: a mesocosm experiment

Although both nutrient inputs and zooplankton grazing are importantto phytoplankton and bacteria in lakes, controversy surroundsthe relative importance of grazing pressure for these two groupsof organisms. For phytoplankton, the controversy revolves aroundwhether zooplankton grazers, especially large cladocerans likeDaphnia, can effectively reduce phytoplankton populations regardlessof nutrient conditions. For bacteria, little is known aboutthe balance between possible direct and indirect effects ofboth nutrients and zooplankton grazing. However, there is evidencethat bacteria may affect phytoplankton responses to nutrientsor zooplankton grazing through direct or apparent competition.We performed a mesocosm experiment to evaluate the relativeimportance of the effects of nutrients and zooplankton grazingfor phytoplankton and bacteria, and to determine whether bacteriamediate phytoplankton responses to these factors. The factorialdesign crossed two zooplankton treatments (unsieved and sieved)with four nutrient treatments (0, 0.5, 1.0 and 2.0 µgphosphorus (P) l^–1 day^–1 together with nitrogen(N) at a N:P ratio of 20:1 by weight). Weekly sieving with 300µm mesh reduced the average size of crustacean zooplanktonin the mesocosms, decreased the numbers and biomass of Daphnia,and increased the biomass of adult copepods. Nutrient enrichmentcaused significant increases in phytoplankton chlorophyll a(4–5x), bacterial abundance and production (1.3x and 1.6x,respectively), Daphnia (3x) and total zooplankton biomass (2x).Although both total phytoplankton chlorophyll a and chlorophylla in the <35 µm size fraction were significantly lowerin unsieved mesocosms than in sieved mesocosms, sieving hadno significant effect on bacterial abundance or production.There was no statistical interaction between nutrient and zooplanktontreatments for total phytoplankton biomass or bacterial abundance,although there were marginally significant interactions forphytoplankton biomass <35 µm and bacterial production.Our results do not support the hypothesis that large cladoceransbecome less effective grazers with enrichment; rather, the differencebetween phytoplankton biomass in sieved versus unsieved zooplanktontreatments increased across the gradient of nutrient additions.Furthermore, there was no evidence that bacteria buffered phytoplanktonresponses to enrichment by either sequestering P or affectingthe growth of zooplankton.     

Instar-specific mortalities of coexisting Daphnia species in relation to food and invertebrate predation

Instar-specific mortalities of Daphnia hyalina and D.cucullatawere studied from May 19 to September 29, 1988 in combinationwith invertebrate predator and phytoplankton abundance. Simultaneouslife-table experiments were conducted under semi-natural conditionsin the laboratory to estimate juvenile mortality in a predator-freeenvironment. Juvenile mortality by predation was calculatedas the difference between juvenile mortality in the field andin the experiments and was the most important factor for thedifferences in abundance of the two species. For D.hyalina juvenilemortality was higher in early summer and probably caused byselective predation by Chaoborus flavicans. Predation by Leptodorakindtii was probably more important during the rest of the summer.Estimated mortality by predation adequately explained juvenilemortality, except for a 3-week period in August. Decreasingflagellate densities in July were accompanied by increased juvenilemortalities of D.hyalina and D.cucullata in the life-table experimentsin August and coincided with a Daphnia population decrease.     

Bias in satellite-derived pigment measurements due to coccolithophores and dinoflagellates

Satellite-derived estimates of phytoplankton pigments are thoughtto be affected by the phytoplankton species composition. Measurementsof surface algal chlorophyll and satellite-derived pigment werecompared for waters containing coccolithophores and dinoflagellates.Satellite-derived chlorophyll concentration was underestimatedby a factor of 2–3 in a patch of the large coccolithophore,Umbilicosphaera sibogae, and also in a bloom of the dinoflagellate,Gonyaulax polyedra. Overall abundance and species-specific propertiessuch as light scatter and vertical migration probably causedthese results.     

Relations between bacterioplankton and phytoplankton abundance in three lentic ecosystems in the Colombian Andes

We analyzed relations among phytoplankton and total bacterioplankton fractions in three lentic ecosystems (Neusa and Prado dams, and Fúquene lagoon) with different physicochemical characteristics, in the Andes of Colombia. Samplings were made in three sites of each water body during three surveys. Neusa dam (meso to oligotrophic) had the lowest bacterial concentration; Prado dam (eutrophic) had a high bacterial and algal abundance, and the Fúquene lagoon (mesotrophic) had lower concentrations of phytoplankton but a high relative concentration of bacteria, probably because of its particular conditions: high organic matter and low nutrient levels in the water. There was a negative correlation of total bacterioplankton with the phytoplankton (Pearson = -0.4479, p = 0.019, n=27) and a positive correlation between phytoplankton and heterotrophic bacteria (Pearson = 0.3866, p = 0.062, n=24) and between total bacterioplankton and DBOs (Pearson = 0.4088, p = 0.034, n=27). Apparently, total bacterioplankton and phytoplankton were not coupling, but cultivable bacteria and the phytoplankton had some degree of relationship.     

The structure and dynamics of the phytoplankton assemblages in Lake Kinneret, Israel

The phytoplankton of Lake Kinneret, a warm monomictic lake,is dominated by a Pyrrhophyta-Chlorophyta assemblage. Four stagesof succession of planktonic algae occur in the lake, startingwith thermal and chemical destratification and ending with stratification. The index of diversity of the phytoplankton communities is highduring the destratification and mixed periods. The index reachesminimal values during late summer, when the ecosystem is subjectto strong physical, chemical and biological stresses. The diversityin Lake Kinneret increases with the increase in nutrients andnot with the increase in temperature. During most of the year, the nanoplanktonic forms are in greaternumbers than the netplankton species. This fact is correlatedwith the amounts of available nutrients in the lake. The annual averages of the wet autotrophic biomass in Lake Kinneretare very high in comparison with other warm lakes. The contributionof the nanoplanktonic species to the total algal biomass isvery small during the Peridinium bloom, but represents approximatelyhalf of the total algal biomass during the rest of the year. The concentration of nutrients in the water, together with theadverse competitive effect of Peridinium on other algae, areto a large extent responsible for the composition, successionand abundance of the phytoplankton assemblages in Lake Kinneret.     

Influence of phytoplankton composition and stratification degree on gut pigment content of Ceriodaphnia sp. at dawn

Very short-term feeding activity of the cladoceran Ceriodaphniasp. was investigated in situ in a eutrophic reservoir in thesouth of Spain, using fluorimetric analysis of the gut pigmentcontent in periods when the water column was relatively mixedor strongly stratified. The results obtained in the mixed watercolumn showed a clear increase in gut pigment content at dawn,a period sampled with high frequency. The accumulation of thecladoceran at the depth of maximum concentration of phytoplankton,and the high gut pigment concentration in cladocerans at thatdepth just after dawn, suggested active feeding of Ceriodaphniaon phytoplankton at that time. During stratification, the abundanceof Ceriodaphnia was higher, but the gut pigment contents werevery low and they did not reflect any clear feeding patterns,with either time or depth. Changes in phytoplankton concentrationand composition between the relatively mixed and the stratifiedwater column suggest a shift in feeding activity from herbivorousto.     

Comparisons of zooplankton and phytoplankton in created shallow water habitats of the lower Missouri River: implications for native fish

Shallow water habitat (SWH) is important for riverine fish and their invertebrate prey, yet the availability of SWH has declined in many systems due to human impacts. We evaluated the potential ecological benefits of restoring SWH by comparing zooplankton and phytoplankton from created backwaters (a floodplain feature connected to the river on the downstream end but disconnected at the upstream end) and chutes (a side channel of the river that diverts flow from the main channel through the chute and back into the main channel) on the lower Missouri River. We tested the hypothesis that backwaters support higher abundances of zooplankton and phytoplankton than chutes using data that were collected during the summer of 2010. As predicted, backwaters had more diverse cladoceran communities and greater abundances of rotifers, copepod nauplii, adult copepods, and cladocerans than chutes. Total algal biovolume was the same in chutes and backwaters; however, phytoplankton taxa richness was higher in backwaters, and there was a greater biovolume of green algae (Chlorophyta), Crypotophyta, cyanobacteria, and Euglenophyta in backwaters than in chutes. Differences in zooplankton and phytoplankton between backwaters and chutes appeared to be related to slower current velocities, longer retention times, and lower levels of turbidity and total suspended solids in backwaters. While chutes have the potential to provide greater habitat diversity than the mainstem, there were no differences in water quality or phytoplankton abundance, community structure, or diversity between these two habitats. Combined, our results suggest that created backwaters initially provide a greater potential food resource for native fishes. However, additional research is needed to determine whether chutes can also develop beneficial shallow water features over a longer period of time.     

Coupling of nauplii release in barnacles with phytoplankton blooms: a parallel strategy to that of spawning in urchins and mussels

We examined the coupling between the release of nauplii in Semibalanusbalanoides and phytoplankton blooms, and compared it with themechanism synchronizing spawning with phytoplankton abundancein green sea urchins (Strongylocentrotus droebachiensis) andblue mussels (Mytilus edulis). Close contact with phytoplanktoncells is required to stimulate the release of nauplii in Semibalanus,whereas spawning in urchins and mussels is triggered by an extracellularmetabolite of phytoplankton. The response is concentration dependentand positively correlated with the frequency of moulting Themoulting rate in Semibalanus is reported to be directly correlatedwith food intake and thus larval release should be proportionalto feeding activity. The release of Semibalanus nauplii is morestrongly stimulated by the diatom Skeletonema costatum thanby Phaeodactylum tricornutum, whereas the highest response isobtained in treatments with nauplii of Artemia salina. We suggestthat these differences are related to size of the planktonicparticles (Artemia nauplii < Skeletonema < Phaeodactylum)Exposure to air, as occurs in the intertidal zone, does notinfluence significantly larval release Larval release in otherCrustacea may similarly be coupled to phytoplankton abundance.