Correspondence among methods of zooplankton biomass measurement in lakes: effect of community composition on optical plankton counter and size-fractionated seston data

The effectiveness of the optical particle counter (OPC) to estimatezooplankton biomass depends on the variability in zooplanktonshape and the presence of interfering particles. In marine environmentswhere zooplankton are composed of similarly shaped copepods,an average shape is relatively easily obtained. However, infreshwater environments, spheroid cladocerans mix with ellipsoidcopepods and make the application of a single morphometric modeldifficult. To expand the use of the OPC to freshwater environments,we developed new ellipsoid models for three common lake types(eutrophic, mesotrophic, and oligotrophic). In addition, weassessed how closely different size fractions of seston correspondedto zooplankton biomass. When expressed in common dry mass units,OPC- and seston-derived zooplankton biomass estimates showeda 1:1 correspondence with taxonomically derived estimates inproductive lakes (r > +0.70, P < 0.001) but not in oligotrophicsystems. OPC ellipse models differed among lake sets (major-to-minoraxis ratio: 1.5 to 2.7) but were not a simple function of thecladoceran-to-copepod ratio. The seston size fraction that providedthe best estimates of zooplankton biomass was smaller in mesotrophiclakes (>200 µm) than in eutrophic or oligotrophic lakes(>500 µm). The presence of algae and rotifers had nodetectable influence on OPC and size-fractionated seston estimates.Overall, these analyses suggest that OPC and seston providereliable estimates of lacustrine zooplankton biomass as longas region-specific ellipse models and size fractions, respectively,are used.     

Contribution of ciliated protozoa to the planktonic biomass in a series of Ontario lakes: quantitative estimates and dynamical relationships

The plankton of nine Ontario lakes spanning several physiographicregions was sampled every two weeks during the ice-free periodof 1981, and one lake was studied in the three previous years.Phytoplankton, zooplankton, and ciliated protozoa were sampled,counted and sized. The size data were converted to biomass estimatesto yield quantitative comparisons of the relative allocationof biomass among different functional compartments. This isthe first study to look simultaneously and quantitatively atthe total plankton system of lakes (including ciliates, pbytoplanktonand net zooplankton) over a broad physiographic region. Ciliatesconstitute –10% of the non-algal biomass and 5% of thetotal planktonic biomass of these lakes. Ciliate standing cropsamong lakes are significantly corrrelated with total organicand total inorganic carbon concentrations in the water column,while the dynamics of ciliate biomass fluctuations are significantlycorrelated with variations in total phosphorus concentration,in conductivity, in Kjeldahl nitrogen concentration, and ininorganic carbon content. There appears to be a significantdynamical relationship between ciliates as a proportion of thetotal planktonic biomass, exclusive of filamentous and large(>30 µm) spherical algae, and the relative biomassof small algae (2–5 µm) as a fraction of total algalbiomass, again exclusive of filaments and large (>30 µm)algae. The hypothesis is advanced that ciliates primarily functionas bacterial grazers in planktonic ecosystems and that theirprimary competitors in this role are rotifers.     

Zooplankton community structure in southwestern Quebec lakes: the rOles of acidity and predation

The zooplankton community structure of 22 lakes with varyingacidity and fish biomass, located southwest of the mining andsmelting region of Rouyn/Noranda, Quebec, was examined in July1987. Lakes with dominant piscivorous fish communities couldbe discriminated from non-piscivorous communities using solelypH and Chaoborus abundance, demonstrating that modificationsin the zooplankton community occurred concurrently with changesin the fUh communities. Strong discontinuities in zooplanktonspecies distributions were discerned in lakes with pH values<5.3; abundance ranges for several species could be semi-quantitativelyrelated to lake morpho-metry or chemistry. Small shallow kettlelakes with elevated heat budgets had high biomasses of smallherbivorous organisms. The presence of >5 mg m^–3 ofadvanced Chaoborus instars (III and IV) was associated withreduced microcrustacean biomass in many lakes, particularlythose with low fish biomass. There was little evidence for size-selectivepredation by fish in these oligotrophic lakes. Fish biomass/effortcould be semi-quantitatively related to the biomass of Leptodorakindtii and the ratio of adult Diaptomus nunutus to copepoditestage IV.     

A comparison of performance of WP2 and MOCNESS

Zooplankton biomass in the Barents Sea was monitored during1988–97 using WP2 and MOCNESS plankton nets. These twosampling gears differ in their size and mode of operation. Theplankton samples were size fractionated into three categoriesand the dry weight per square metre was calculated. The smallestand the medium size fractions (< 2000 µm) representedmainly copepods, and the larger size fraction (> 2000 µm)consisted mainly of macrozooplankton such as krill and amphipods.WP2 biomass values were higher for the smallest size fraction,whereas the MOCNESS tended to give higher values for the largestsize fraction However, the total amount of zooplankton biomass(g m^–1) obtained by these two methods was not significantlydifferent.     

Composition,size, and biomass of zooplankton in large productive Florida lakes

Crustacean zooplankton data were compiled from long-term observational studies at seven large shallow Florida lakes, to determine whether there are general characteristics in regard to species composition, body size, and biomass. In particular, we examined whether patterns in body size and species richness fit empirical models developed by Stanley Dodson. The lakes included range in size from 125 to 1730 km² and encompass mesotrophic to hyper-eutrophic conditions. We found that zooplankton biomass was strongly dominated by one species of calanoid copepod—Arctodiaptomus dorsalis. Large daphnids were absent, and Cladocera assemblages were dominated by small taxa such as Ceriodaphnia, Chydorus, and Eubosmina. The total number of species of pelagic cladocerans (8–12) was consistent with Dodson’s predictions based on lake area. The average size of crustacean zooplankton in Florida lakes is small in comparison with temperate communities. A. dorsalis is the smallest calanoid copepod in North America, and the mean length of Cladocera (0.6 mm) is consistent with Dodson’s results that size decreases from temperate to tropical zones. Total biomass of crustacean zooplankton was very low, ratios of zooplankton to phytoplankton biomass (0.01–0.1) are among the lowest reported in the literature, and the zooplankton displayed short-lasting early spring peaks in biomass. Cladocera were almost entirely absent in spring and summer. Factors known to occur in Florida lakes, which appear to explain these characteristics of biomass, include intense fish predation and high summer water temperature.     

Microzooplankton and macrozooplankton glutamate dehydrogenase as indices of the relative contribution of these fractions to ammonium regeneration in the Gulf of Maine

Ammonium regeneration by micro- (35–153 µm) andmacrozooplankton (> 153 µm) was determined in the Gulfof Maine by measuring the activity of the excretory enzyme glutamatedehydrogenase (GDH) in various size fractions. GDH maxima weregenerally observed to correspond to the depth of the chlorophyllmaximum as previously reported in the Gulf of Mexico and inthe vicinity of the Nantucket Shoals. GDH activity of the microzooplanktonwas considerably lower than the macrazooplankton, suggestingthe microzooplankton made only a minor contribution (1–11%) to the total ammonium regenerated. These results were confirmedby biomass estimates made from counts of individual species.Ammonium excretion by both zooplankton fractions was estimatedto supply 5–31 % of the nitrogen requirements for primaryproduction, with an estimated 59–63% supplied by the verticaltransport of nitrate (new nitrogen) into the euphoric zone.     

Copepods as environmental indicator in lakes: special focus on changes in the proportion of calanoids along nutrient and pH gradients

Copepods are important contributors to the zooplankton community in lakes. Being “sandwiched” between predators and resources, they are sensitive to changes in the environment. It has been proposed that the proportion of calanoids of total copepod abundance or biomass could be a valuable indicator of eutrophication. We investigated relationships between environmental factors and the abundance, biomass and size of calanoid and cyclopoid copepods as well as their proportions in summer in 68 Danish freshwater lakes (587 lake years) with contrasting nutrient levels and pH. When lake pH was?<?6.0, mean lake depth and trophic state were the most important factors and calanoids completely dominated the copepod community. In shallow lakes with a mean depth?<?2.5 m and with pH?>?6.0, the proportion of calanoids in terms of biomass decreased substantially with increasing phosphorus and chlorophyll a concentrations but stayed around 50% at?>?2.5 m depth irrespective of nutrient level. Time series of the lakes, recovering from eutrophication, confirmed this multi-lake pattern although the trajectory varied from lake to lake. We conclude that the proportion of calanoids in terms of biomass might be a valuable indicator of trophic state in shallow but not deep lakes and only when pH?>?6.

     

Trophic structure in the pelagial of 25 shallow New Zealand lakes: changes along nutrient and fish gradients

To gain better insight into the importance of predator and resourcecontrol in New Zealand lakes we surveyed the late summer trophicstructure of 25 shallow South Island lakes with contrastingnutrient levels (6–603 µg TP l^–1) and fishdensities. Total catch of fish per net (CPUE) in multi-meshgillnets placed in the open water and the littoral zones waspositively related with the nutrient level. Trout CPUE was negativelycorrelated with total phosphorus (TP) and total nitrogen (TN).Zooplankton seemed largely influenced by fish, as high fishCPUE coincided with low zooplankton and Daphnia biomass, lowaverage weight of cladocerans, low contribution of Daphnia tototal cladoceran biomass, low ratio of calanoids to total copepodbiomass and low ratio of zooplankton biomass to phytoplanktonbiomass. However, chlorophyll a was only slightly negativelyrelated to Daphnia biomass and not to zooplankton biomass ina multiple regression that included TN and TP. Ciliate abundancewas positively related to chlorophyll a and negatively to Daphniabiomass, but not to total zooplankton biomass, while no relationshipswere found between heterotrophic nanoflagellates and zooplankton.The relationships between fish abundance and nutrients and fishabundance and zooplankton:phytoplankton ratio and between chlorophylla and TP largely followed the pattern obtained for 42 northtemperate Danish lakes. We conclude that fish, including trout,have a major effect on the zooplankton community structure andbiomass in the pelagial of the shallow oligotrophic to slightlyeutrophic New Zealand lakes, but that the cascading effectson phytoplankton and protist are apparently modest.     

Seasonal fluctuations in macrophyte cover and water transparency of four brown-water lakes: implications for crustacean zooplankton in littoral and pelagic habitats

The dynamics of crustacean zooplankton in the littoral and pelagic zones of four forest lakes having variable water qualities (colour range 130–340 mg Pt l⁻¹, Secchi depth 70–160 cm) were studied. The biomass of zooplankton was higher in the littoral zone than in the pelagic zone only in the lake having the highest transparency. In the three other lakes, biomass was significantly higher in the pelagic zone than in the littoral zone. In the two lakes with highest transparency, the littoral biomass of cladocerans significantly followed the development of macrophyte vegetation, and cladoceran biomass reached the maximum value at the time of highest macrophyte coverage. In lakes with lowest transparency, littoral zooplankton biomass developed independently of macrophyte density and decreased when macrophyte beds were densest. The seasonal development of the littoral copepod biomass did not follow the development of macrophytes in any of the lakes. The mean size of cladocerans in the pelagic zone decreased with increasing Secchi depth of the lake, whereas in the littoral zone no such phenomenon was detected. Seasonally, when water transparency increased temporarily in two of the lakes, the mean size of cladocerans in the pelagic zone decreased steeply. For copepods, no relationship between water transparency and body size was observed. The results suggested that in humic lakes the importance of the littoral zone as a refuge decreases with decreasing transparency of the water and that low water transparency protects cladocerans from fish predation. All the observed between-lake differences could not be explained by fish predation, but were probably attributed to the presence of chaoborid larvae with variable densities. Feeding efficiency of chaoborids is not affected by visibility and thus they can obscure the relationship between water quality, fish density, and the structure of crustacean zooplankton assemblages. Handling editor: S. I. Dodson     

Zooplankton biomass gradient off south-western Nova Scotia: nearshore ctenophore predation or hydrographic separation?

A persistent large-scale cross-shelf gradient in zooplanktonbiomass >1050 µm was evident off south-western NovaScotia during annual spring surveys between 1985 and 1987, withrelatively low levels inshore and higher levels offshore. Conversely,the abundance of the tentaculate ctenophore Pleurobrachia pileuswas the greatest inshore, and distributed reciprocally to zooplankton>1050 µm. The principle prey of both adult ctenophoresand post-larval cod is zooplankton >1050 µm (primarilycalanoid copepods), and cod growth rates are strongly influencedby prey biomass. Ctenophore predation appears to have been responsiblefor the low nearshore zooplankton biomass, whereas the influenceof hydrographic factors on the zooplankton gradient was minimal.On a smaller scale, persistent, abrupt changes in zooplanktonbiomass >1050 µm and ctenophore density existed 3–30km from shore, in contrast to linear gradients in water density(₁) during a 5 week sampling period in spring 1987. Ctenophoreswere confined to depths <55 m and zooplankton >1050 µmpredominantly occurred at depths >55 m. High concentrationsof chlorophyll and phaeopigment were evident at depths <55m also suggesting intense predation by ctenophores on largeherbivores. The relatively high proportion of smaller zooplankton(153–308 –m) in the nearshore is also consistentwith the predation hypothesis. The reduced growth experiencedby post-larval cod inshore appears generated by ctenophore predationof a common prey resource.     

Zooplankton behavioral responses to solar UV radiation vary within and among lakes

Zooplankton taxa exhibit varying tolerances to ultraviolet radiation(UVR), with rotifers and copepods tending to be more UV tolerantthan cladocerans, regardless of lake UV transparency. On thebasis of these differences, we hypothesized that UV avoidancebehavior would be greatest in zooplankton with low UV tolerance,particularly in high-UV systems. Both downward and upward movementsof zooplankton were examined in cylindrical acrylic columnsthat either transmitted full sunlight or blocked UV-B and shortwavelength UV-A radiation. Columns were suspended verticallyin the surface waters of a high- and low-UV lake at varyingtimes of day and cloud cover. In the high-UV lake, the cladoceranDaphnia catawba displayed a distinct avoidance of the surfacewaters in the UV+ treatment but often showed a preference forthe surface in the UV– treatment. Copepods were less responsiveto UVR with calanoids displaying a somewhat greater UV avoidancethan cyclopoids. In the low-UV lake, no behavioral differenceswere observed among UV treatments. These results suggest thatUVR may influence the vertical distribution and habitat partitioningof certain zooplankton taxa in high-UV lakes while other factors,such as predation and food limitation, may be more importantin low-UV lakes.     

Size-fractionated chlorophyll a biomass and picophytoplankton cell density along a longitudinal axis of a temperate estuary (Southampton Water)

Size-fractionated chlorophyll a biomass and picophytoplanktoncell number distributions were investigated along a longitudinalaxis of Southampton Water estuary during autumn. Chlorophylla concentration in the >5µm and the 1–5 µmsize fractions was highest midway down the estuary, and decreasedboth in the landward and seaward directions. In contrast, chlorophylla biomass in the 0.2–1 µm size fraction showed nodecline towards the seaward end of the estuary. In agreementwith this observation, phycoerythrin-containing picocyanobacteriacell concentration showed a positive exponential-like relationshipwith salinity and eukaryotic picophytoplankton were also highestat high salinities. Expressed as a percentage of total, chlorophylla standing stock in both the 1–5 µ.m (4.4–28.7%)and the 0.2–1 µm size fractions (1.7–8.6%)was inversely correlated with total chlorophyll a concentration.Both these two fractions made a greater input to the total phaeopigmentconcentration than to the total pool of active chlorophyll a.     

Annual Zooplankton Succession in Coastal NW Mediterranean Waters: The Importance of the Smaller Size Fractions

Zooplankton abundance, biomass (biovolume) and taxonomic compositionwere studied within an annual cycle (August 1995–October1996) in the Bay of Blanes (northwest Mediterranean). Weeklyzooplankton sampling included oblique tows made with a 200 µmJuday–Bogorov net, and vertical tows made with a 53 µmnet, to adequately sample both mesoplankton and the smallerzooplankton fractions. Total zooplankton abundance showed highvariability, lacking any clear seasonal pattern. However, thedifferent species within the zooplankton community displayeda clear succession throughout the year. In general, cyclopoidcopepods (Oithona spp.) and cladocerans (Peniliaavirostris)dominated the summer and autumn communities, whereas in winterand spring, calanoid copepods (Clausocalanus spp., Paracalanussp. and Centropages typicus) were predominant. The zooplanktonannual cycle in the Bay of Blanes does not resemble those ofother Mediterraneanlittoral areas, probably due to the inherentparticularity and variability associated with open coastal environments.On average, the abundance of organisms estimated with a traditional200 µm Juday–Bogorov net was 8.1 times lower thanthe values obtained with a 53 µm net. Even if only organisms>200 µm collected in the 53 µm tows were considered,the total abundance within the 53 µm net was still 4.4times higher than the estimates from the Juday–Bogorovnet. These results suggest the need for accurate samplings ofthe entire zooplankton assemblage when characterizing the structureand dynamics of zooplanktonic communities.     

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.     

First report of the cyanobacterium Aphanizomenon ovalisporum Forti in two Greek lakes and cyanotoxin occurrence

Aphanizomenon ovalisporum is reported for the first time inGreece, in two warm, monomictic lakes. Aphanizomenon ovalisporumwas dominant constituting 99 and 58% of the total cyanobacterialbiomass in lakes Lysimachia and Trichonis, respectively. Trichomeswere solitary (length 60–700 µm), were narrowedslightly at the ends, had a few terminal hyaline cells and hadcells containing gas vesicles (length 2.5–6.9, width 2.4–5.1µm). Heterocytes, spherical or ellipsoidal (length 4.4–10.5,width 2.41–5.1 µm) and akinetes (length 16.0–27.8,width 6.0–15.9 µm) were located in the middle ofthe trichome. High performance liquid chromatography (HPLC)analysis detected microcystin–LR (MC–LR) and a putativeanabaenopeptin in the L. Lysimachia sample. The sestonic MC–LRconcentration was 0.9 µg L^–1. The origin of MC–LRin L. Lysimachia is discussed. The other cyanobacteria presentwere Pseudanabaena sp. and Planktothrix mougeotii (1% of thetotal cyanobacterial biomass).     

Carbon dynamics in the 'grazing food chain' of a subtropical lake

Studies were conducted over a 13 month period at four pelagicsites in eutrophic Lake Okeechobee, Florida (USA), in orderto quantify carbon (C) uptake rates by size-fractionated phytoplankton,and subsequent transfers of C to zooplankton. This was accomplishedusing laboratory ¹⁴C tracer methods and natural plankton assemblages.The annual biomass of picoplankton (<2 µm), nanoplankton(2–20 µm) and microplankton (<20 µm averaged60, 389 and 100 µg C 1^–1 respectively, while correspondingrates of C uptake averaged 7, 51 and 13 µg C1^–1h^–1. The biomass of microzooplankton (40–200 µm)and macrozooplankton (<200 µm averaged 18 and 60 µgC 1^–1, respectively, while C uptake rates by these herbivoregroups averaged 2 and 3 µg C 1^–1 h^–1. Therewere no strong seasonal patterns in any of the plankton metrics.The ratio of zooplankton to phytoplankton C uptake averaged7% over the course of the study. This low value is typical ofthat observed in eutrophic temperate lakes with small zooplanktonand large inedible phytoplankton, and indicates ineffectiveC transfer in the grazing food chain. On a single occasion,there was a high density (<40 1^–1) of Daphnia lumholrzii,a large-bodied exotic cladoceran. At that time, zooplanktoncommunity C uptake was <20 µg C 1^–1 h^–1and the ratio of zooplankton to phytoplankton C uptake was near30%. If D.lumholrzii proliferates in Lake Okeechobee and theother Florida lakes where it has recently been observed, itmay substantially alter planktonic C dynamics.     

Community structure, biomass and productivity of size-fractionated summer phytoplankton populations in lower Narragansett Bay, Rhode Island

Seventeen size-fractionation experiments were carried out duringthe summer of 1979 to compare biomass and productivity in the< 10, <8 and <5 µm size fractions with that ofthe total phytoplankton community in surface waters of NarragansettBay. Flagellates and non-motile ultra-plankton passing 8 µmpolycarbonate filters dominated early summer phytoplankton populations,while diatoms and dinoflagellates retained by 10 µm nylonnetting dominated during the late summer. A significant numberof small diatoms and dinoflagellates were found in the 10–8µm size fraction. The > 10 µm size fraction accountedfor 50% of the chlorophyll a standing crop and 38% of surfaceproduction. The <8 µm fraction accounted for 39 and18% of the surface biomass and production. Production by the< 8 µm fraction exceeded half of the total communityproduction only during a mid-summer bloom of microflagellates.Mean assimilation numbers and calculated carbon doubling ratesin the <8 µm (2.8 g C g Chl a^–1 h^–1; 0.9day^–1)and<5 µm(1.7 g C g Chl a^–1h^–1; 0.5day^–1)size fractions were consistently lower than those of the totalpopulation (4.8 g C g Chl a^–1 h^–1; 1.3 day^–1)and the <10 µm size fraction (5.8 g C g Chl a^–1h^–1; 1.4 day ^–1). The results indicate that smalldiatoms and dinoflagellates in fractionated phytoplankton populationscan influence productivity out of proportion to their numbersor biomass. ¹Present address: Australian Institute of Marine Science, P.M.B.No. 3, Townsville M.S.O., Qld. 4810, Australia.     

Comparison of pelagic food webs in lakes along a trophic gradient and with seasonal aspects: influence of resource and predation

Composition and seasonal dynamics of phytoplankton, bacteria,and zooplankton (including heterotrophic flagellates, ciliates,rotifers and crustaceans) were studied in 55 lakes in NorthernGermany with different trophic status, ranging from mesotrophicto hypertrophic. Mean abundance and biomass of all groups increasedsignificantly with trophic level of the lake, but bacteria andmetazooplankton showed only a weak correlation and a slightincrease with chlorophyll concentration. Composition of phytoplanktonshowed a dominance of cyanobacteria in hypertrophic lakes, whereasthe importance of chrysophytes and dinophytes decreased withan increase in trophic status. Protozoans (heterotrophic flagellatesand ciliates) made up 24% (mesotrophic lakes) to 42% (hypertrophiclakes) of total zooplankton biomass on average, and were dominatedby ciliates (62–80% of protozoan biomass). Seasonally,protozoans can build up to 60% of zooplankton biomass in spring,when heterotrophic flagellates can contribute     

Factors affecting the bacteria-heterotrophic nanoflagellate relationship in oligo-mesotrophic lakes

The coupling between bacteria and heterotrophic nanoflagellates(HNF) was examined in nine lakes of low productivity for evidenceof the effects of various metazooplankton (i.e. rotifers, cladoceransand copepods) on this relationship. We considered the size ofcladocerans and, in contrast to most previous across-systemstudies, the three strata of the water column (i.e. epilimnion,metalimnion and hypolimnion). Rotifers were numerically dominantin all lakes and accounted for 45–84% of total metazooplanktonabundance, while the abundance of large cladocerans was relativelylow, ranging from 0.066 to 15.2 ind. L^–1. The across-lakerelationship between bacteria and HNF was significant in thedeeper strata (meta- and hypolimnion) but not in the epilimnionand in the two groups of lakes separated on the basis of theiraverage number of large cladocerans (<5 and >5 ind. L^–1,respectively). The results confirmed the negative impacts oflarge cladocerans on HNF, but also showed that rotifers, probablythrough grazing on HNF, may be an important factor causing variationin the bacteria–HNF relationship in unproductive waters.Quadratic models best described the relationships between metazooplanktonand the ratio of bacteria to HNF. This ratio seemed to be aresult of complex interactions between several factors, includingthe zooplankton composition and abundance and the depth of thelake. Indeed, this ratio significantly decreased across lakes,with increase in depth. In addition, shallower lakes (having<5 large cladocerans L^–1 and fewer Polyarthra vulgaris)tended to have more bacteria and HNF and a higher ratio of bacteriato HNF than deeper lakes (which had >5 large cladoceransL^–1 and substantial proportions of P. vulgaris). We suggestthat the epilimnion, metalimnion and hypolimnion of lakes betaken into account when analysing the bacteria–HNF relationshipas well as the cascading effects of zooplankton on microbialcommunities.     

The biogeochemistry and zoogeography of lakes and rivers in arctic Alaska

Water samples from 45 lakes and 8 rivers in arctic Alaska were analyzed for major anions, cations, nutrients, chlorophyll, zooplankton, and benthos. The waters were dilute (conductivities of 30 to 843 μS cm⁻¹), and their composition varied from Na-Ca-Cl waters near the Arctic Ocean to Ca-Mg-HCO₃ waters further inland. Sea salt input in precipitation was important in determining the chemistry of coastal lakes, partly because of low groundwater flow and less time for water to react with shallow unfrozen soils. Further inland, variations in water chemistry among sites were related mainly to differences in bedrock, the age of associated glacial drift, and the input of wind blown sediment. Variations in zooplankton species composition among the lakes were related more to latitude, lake morphometery, and biotic interactions than to water chemistry. The presence of fish as predators mostly determined the overall size structure of the zooplankton community. The chironomid taxa identified have been previously reported from the Neararctic, except for Corynocera oliveri which is a new record. The abundance of the widely distributed chironomid Procladius appears to be controlled by sculpin predation.