Distribution patterns of Baltic herring larvae, Clupea harengus L., in the coastal waters off Helsinki, Finland

Changes in the spatial distribution of Baltic herring larvaethrough time were investigated in the waters off Helsinki, thenorthern Baltic Sea. The average larval densities were comparableto the relatively low levels (mean around 2 larvae m^–2)found in other studies in the Gulf of Finland. In shallow baysvirtually no herring larvae were found in early June. In lateJune and early July the densities of larvae longer than 10 mmincreased to 1–4 larvae m^–2 in shallow areas (depth<2 m), i.e. to levels higher than those recorded in deeperwaters. In combination with data on the size distributions ofthe larvae, these observations suggest that a significant proportionof the herring larvae are found in shallow water after the yolksac stage. We conclude that shallow coastal waters are potentiallyimportant nursery areas for herring larvae and that this featureof herring larval biology should be taken into account in planningsampling programs for larval herring.     

Growth and ingestion rates of larval fish populations in the coastal waters of Israel

Clupeoid larvae were collected on eight cruises between February1984 and February 1985 in the coastal waters of Israel. Fromanalysis of daily growth increments of otoliths, growth ratesof the abundant clupeoids, Engraulis encrasicolus, Sardina pilchardusand Sardinella aurita were found to be 0.55 mm day^–1,0.67 mm day^–1 and 0.60 mm day^–1, respectively, duringthe first month after hatching. Ingestion rates were estimatedusing an equation from the literature relating ingestion andgrowth of larval fish. Ingestion calculated for populationsof fish larvae in pelagic waters ranged from 0 to >23 mgC m^–2 day^–1 with maximum rates observed in April.Annual ingestion by larval fish at a pelagic station near Haifawas calculated to be 2.2 g C m^–2 year^–1, 10–20%of annual primary production estimated from ¹⁴C uptake.     

A comparison of seasonal growth and development of the copepods Calanus marshallae and C. pacificus in the northern Gulf of Alaska

The juvenile growth rates and development times of subarcticCalanus marshallae and temperate/sub-tropical C. pacificus wereinvestigated during nine cruises (May through October, 2001–04)in the northern Gulf of Alaska. The artificial cohort methodbased on a length-weight regression was used for growth estimatesand the reciprocal of the molting rate for developmental time.The copepodite stage duration ranged from 3 to 16 days for C.marshallae (C1–C4) and 3–23 days for C. pacificus(C1–C5). Seasonally, copepodid growth rates increasedfrom May to October, ranging between 0.055 and 0.291 day^–1(mean ± SE: 0.176 ± 0.008 day^–1) for C.marshallae, while growth rates increased from August to Octoberbetween 0.018 and 0.296 day^–1 (mean ± SE: 0.142± 0.016 day^–1) for C. pacificus. After standardizationto 5°C (Q₁₀ of 2.7), growth rate averaged 0.118 ±0.007 day^–1 and 0.075 ± 0.009 day^–1 for C.marshallae and C. pacificus, respectively. Calanus marshallaegrowth rate is satisfactorily described by a Michaelis–Mentenmodel using chlorophyll-a concentration (r² = 0.33) after temperaturecorrection, but the prediction improves with a composite nonlinearmodel combining body weight into the Michaelis–Mentenfunction (r² = 0.55). Considering the limited range of dataavailable for C. pacificus, the combination of the data forboth species suggests that C. pacificus has a similar functionalresponse to growth despite the differences in the geographicand temporal distributions with C. marshallae. Measured juvenilegrowth rates of the two Calanus species in this study were comparableto other calanoid species in the same area and showed reasonableagreement to Calanus growth models but less with global copepodgrowth models.     

Growth and development of Neocalanus flemingeri/plumchrus in the northern Gulf of Alaska: validation of the artificial-cohort method in cold waters

In situ growth and development of Neocalanus flemingeri/plumchrusstage C1–C4 copepodites were estimated by both the artificial-cohortand the single-stage incubation methods in March, April andMay of 2001–2005 at 5–6°C. Results from thesetwo methods were comparable and consistent. In the field, C1–C4stage durations ranged from 7 to >100 days, dependent ontemperature and chlorophyll a (Chl a) concentration. Averagestage durations were 12.4–14.1 days, yielding an averageof 56 days to reach C5, but under optimal conditions stage durationswere closer to 10 days, shortening the time to reach C5 (fromC1) to 46 days. Generally, growth rates decreased with increasingstage, ranging from 0.28 day^–1 to close to zero but weretypically between 0.20 and 0.05 day^–1, averaging 0.110± 0.006 day^–1 (mean ± SE) for single-stageand 0.107 ± 0.005 day^–1 (mean ± SE) forartificial-cohort methods. Growth was well described by equationsof Michaelis–Menten form, with maximum growth rates (G_max)of 0.17–0.18 day^–1 and half saturation Chl a concentrations(K_chl) of 0.45–0.46 mg m^–3 for combined C1–3,while G_max dropped to 0.08–0.09 day^–1 but K_chl remainedat 0.38–0.93 mg m^–3 for C4. In this study, in situgrowth of N. flemingeri/plumchrus was frequently food limitedto some degree, particularly during March. A comparison withglobal models of copepod growth rates suggests that these modelsstill require considerable refinement. We suggest that the artificial-cohortmethod is the most practical approach to generating the multispeciesdata required to address these deficiencies.     

Feeding and metabolism of the siphonophore Sphaeronectes gracilis

The in situ predation rate of the siphonophore Sphaeronectesgracilis was estimated from gut content analysis of hand-collectedsiphonophores and from laboratory data on digestion rates ofprey organisms. At daytime prey densities of 0.25 copepods 1^–1,S. gracilis was estimated to consume 8.1 – 15.4 prey day^–1siphonophore^–1. From data on abundances of siphonophoresand copepods, S. gracilis was estimated to consume 2–4%of the copepods daily. In laboratory experiments, ingestionrates averaged 13.8 prey day^–1 siphonophore^–1 atprey densities of 5 copepods 1^–1 and 36.9 at 20 copeods1^–1. This was equivalent to a specific ingestion rate(for both carbon and nitrogen) of –17% day^–1 and45% day^–1, respectively, while specific ingestion in situwas only 2% day^–1. Ammonium excretion averaged 0.095 µg-atsiphonophore^–1 day^–1 at 5 prey 1^–1, and 0.162at 20 prey 1^–1. The specific respiration (carbon) andspecific excretion (nitrogen as ammonium) were calculated tobe 3% day^–1 at the lower experimental food level, and5% day^–1 at the higher food level. ¹Contribution from the Catalina Marine Science Center No. 66. ²Present address: Dept. of Biology, University of Victoria,Victoria, B.C., Canada V8W 2Y2.     

Larval age, growth and mortality in the oceanic squid Sthenoteuthis pteropus (Cephalopoda, Ommastrephidae) from the eastern tropical Atlantic

An investigation was carried out on larvae of the oceanic tropicalsquid Sthenoteuthis pteropus in the equatorial Atlantic (230'N–7S;12W–830'E) The age of the larvae was calculated from thestatolith microstructure of 20 larvae; mortality was estimatedfrom the size structure of 1128 larvae. The larval stage lasts32–38 days. At ages ranging from 14 to 38 days. the dailyrelative growth rates of mantle length decrease from 7.5 to2.8% day^–1 and from 14–16 to 5.8% of body weightday^–1 At age 12–24 days, mortality rates were estimatedusing both raw catch data and corrected data accounting fornet avoidance. The mean value of raw mortality rates was 0.189,the corrected value was 0.158. During the proboscis division(transformation of the larva into juvenile) at age 25–35days, a sharp decrease in larval growth rates and a simultaneousincrease in mortality rates (raw 0.443, corrected 0.379) wereobserved.     

Microzooplankton, vertical mixing and advection in a larval fish patch

A large ({small tilde}30 ? 75 km) patch of larval walleye pollock,Theragra chalcogramma, was located south of the Alaska Peninsuladuring May 1986. A drifter deployed in this patch followed ananticyclonic path consistent with dynamic topography. Changesin community composition and vertical distribution of microzooplankton>40 µm were sampled for 4 days alongside this drifterto examine feeding conditions for larvae. Biological and physicalchanges during the first 2 calm days revealed substantial small-scalevariability within the larger circulation pattern. Changes duringthe last 2 days were dominated by vertical mixing due to strongwinds. Despite mixing, prey concentrations remained adequatefor feeding by larval pollock as determined by laboratory studies.A satellite-tracked drifter replaced the first drifter and wasstill located within the patch 6 days later. Overall distributionsof larvae and movements of the drifters show a net translationof 7.8 km day^–1 south-westward, but details of the studyreveal complex interactions between coastal waters and a coastalcurrent. During the 10-day period there was an increase in standardlength of the larval fish population of 0.13 mm day^–1and a decline in abundance of {small tilde}7.6% day^–1.Both calculated rates must be underestimates due to continuingrecruitment of small larvae from hatching eggs.     

Growth and development of Metridia pacifica (Copepoda: Calanoida) in the northern Gulf of Alaska

Juvenile growth and development rates for Metridia pacifica,one of the dominant larger copepods in the subarctic Pacific,were investigated from March through October of 2001–2004in the northern Gulf of Alaska. The relationship between prosomelength (PL, µm) and dry weight (DW, µg) was determined:log₁₀ DW = 3.29 x log₁₀ PL – 8.75. The stage durationsof copepodites ranged from 3 to 52.5 days but were 8–15days under optimal condition. Seasonally, growth rates increasedfrom March to October and typically ranged between 0.004 and0.285 day^–1, averaging 0.114 ± 0.007 day^–1(mean ± SE). After standardization to 5°C (Q₁₀ of2.7), growth rates averaged 0.083 ± 0.005 day^–1and were significantly correlated to chlorophyll a, with saturatedgrowth rates of 0.149 day^–1 for C1–C3, 0.102 day^–1for C4–C5 and 0.136 day^–1 for all stages combined.Measured juvenile growth rates were comparable with specificegg production rates in this species. The comparisons of ourrates in this study with those predicted by the global modelsof copepod growth rates suggested that further refinement ofthese models is required.     

Factors Inducing Successful Anhydrobiosis in the African Chironomid Polypedilum vanderplanki: Significance of the Larval Tubular Nest

The African chironomid Polypedilum vanderplanki exhibits anhydrobiosis,i.e., the larvae can survive complete desiccation. Recoveryrate and trehalose content were investigated in larvae desiccatedslowly or at a rate more than 3 times faster. Upon slow desiccation(evaporation rate 0.22 ml day^–1) larvae synthesized 38µg trehalose/individual before complete desiccation, andall of them recovered after rehydration, whereas larvae thatwere dehydrated quickly (evaporation rate 0.75 ml day^–1)accumulated only 6.8 µg trehalose/individual and noneof them revived after rehydration. In the pools that are theirnatural habitat P. vanderplanki larvae make tubes by incorporatingdetritus or soil with their sticky saliva. This tubular structureis a physical barrier not only to protect the larva from naturalenemies but also induces successful anhydrobiosis by reducingthe dehydration rate. When larvae were dehydrated with 100 µldistilled water (DW) in soil tubes, they accumulated 37 µgtrehalose/individual and more than half of them could reviveafter rehydration, whereas larvae without tubes accumulatedlower level of trehalose and none recovered after rehydration.     

Trophodynamics of the plankton community at Dogger Bank: predatory impact by larval fish

The trophodynamics of a coastal plankton community were studied,focusing on fish larvae and their copepod prey. The major objectiveswere to describe distributional overlap and evaluate the predatoryimpact by larval fish. The study was carried out across DoggerBank in the North Sea, August-September 1991. Sampling transectscrossed tidal fronts off the Bank and plankton at all trophiclevels showed peak abundance within frontal zones. Also Verticallythere was a significant overlap in distributional patterns ofthe plankton. Seven species of fish larvae were abundant, ofthese sprat (Sprattus sprattus) dominated. The abundance ofone group of fish larvae peaked in the shallow water close tothe Bank, whereas other species, including sprat, were foundin deeper water. Prey preference and predation pressure of fishlarvae were assessed using information on prey sizes and growthrates of larvae and the copepod prey. We estimated larval removalof preferred prey sizes to 3–4% day^–1, counterbalancedby a 3–7% day^–1' replenishment from copepod productionand growth. Additional predation pressure on copepods by aninvertebrate predator was estimated to 1–3%day^–1.In conclusion, the dynamics of fish larvae and other zooplankterswere closely linked. At peak abundances of fish larvae (>35mg dry weight m^–2), the accumulated predation on specificsize ranges of copepods, exerted by larvae and other predators,could exceed the ability of copepod replenishment and intra-/interspecificcompetition among predators might take place.     

The Kinetics of Tracheid Development in Tsuga canadensis Carr. and its Relation to Tree Vigour

Cell counts from samples taken at weekly intervals, from 14May to 22 October 1969, in a T. canadensis stand in Massachusetts,U.S.A., showed that the width of the annual ring was correlatedwith the rate of cell production and that only the least vigoroustrees (c. 20 tracheids year^–1) had a shorter growing season.The time required for completion of a cell-division cycle inthe cambial zone decreased during the course of the season,from 35 to 20 days for the less vigorous trees (25–45tracheids year^–1) and from 28 to 10 days for the morevigorous trees (45–100 tracheids year^–1). The timerequired for the completion of radial growth of the tracheidsdecreased from 18 to 9 days, with no evidence of any changeswith tree vigour. The actual radial growth-rate of the tracheidswas constant within the range 1.5–3 um day^–1. Thetime required for deposition of the secondary cell wall increasedfrom 10 to 50 days, with little evidence of any changes withtree vigour. The actual rate of deposition of cell wall materialwas about 0.15 µ² µ^–1 day^–1 and seemedto show little change during the course of the season. The timeperiod required for lysis of the cytoplasm was about 4 days,with no evidence of any changes with tree vigour and littleevidence of any changes during the course of the season.     

Photosynthetic organic carbon production and respiratory organic carbon consumption in the trophogenic layer of Lake Biwa

Measurement of the photosynthetic production rate in Lake Biwawas camed out from May 1985 to September 1987. In the light-saturatedlayer, the seasonal variation in the photosynthesis rate perchlorophyll a was regulated by water temperature. The depth-integratedphotosynthetic production rate was 0.21-1.48 g C m^–2 day^–1and the maximum value was observed in midsummer when the watertemperature of the mixed surface layer was highesL The criticalnutrient for photosynthesis may be dissolved reactive phosphorus,which was generally <1 µg P 1^–1 throughout theobservation period. In the trophogenic layer, respiratory organiccarbon consumption, estimated from measurement of respiratoiyelectron transport system activity, was 0.35-1.07 (mean 0.66)g C m^–1 day^–1 and corresponded, on average, to 79%of the photosynthetic carbon production rate. This implies thatthe major part of photosynthetic fixed organic matter mightbe recycled in the trophogenic layer. The estimated settlingorganic carbon flux at 20 m depth, from calculation of theseparameters and changes in the particulate organic carbon concentration,was 0.01 (-0.09 to 0.13) g C m^–1 day^–1 The meansettling organic carbon flux measured by sediment trap at 20m was 0.19 (0.09-0.31) g C m^–1 day^–1 higher thanthe estimated value. It seemed that organic matter collectedby sediment trap may contain allochthonous matter and resuspendedepilimnetic sediment matter.     

Development, growth and egg production of Centropages abdominalis in the eastern subarctic Pacific

Centropages abdominalis is a neritic, omnivorous, temporallyabundant copepod present throughout the subarctic Pacific andits marginal seas. The two main objectives of this study wereto determine how temperature influences the development of C.abdominalis and whether growth rates of in situ populationsmay be limited by available food. At 6.9°C, median developmenttime from eggs laid to 50% adults was 42 days and the averageweight-specific somatic growth rate was 0.17 day^–1. At4.6°C, median development time to adult was 59 days (projected)and growth rate averaged 0.08 day^–1, suggesting that 4.6°Cmay be approaching the lower temperature for development andgrowth in this species. The functional relationship betweendevelopment time and temperature was established over the temperaturerange in which this species occurs. The in situ adult growthrates between 10 and 13°C averaged 0.14 day^–1 andwere generally lower than the laboratory-reared juvenile growthrates, which may indicate that adult C. abdominalis are foodlimited in the field during summer and autumn.     

Reproductive response of the copepods Calanoides carinatus and Calanus agulhensis to varying periods of starvation in the southern Benguela upwelling region

Egg production by the calanoid copepods Calanoides carinatusand Calanus agulhensis fed excess Thalassiosira weissflogiiwas monitored in the laboratory following starvation periodsof 1, 3, 5, 7 and 9 days. Following short (1–3 day) periodsof starvation, egg production by C.agulhensis returned to thesatiated rate (51.1 eggs {female} day^–1) more rapidly(after 0.9–2.4 days of excess food) than that of Ca. carinatus(after 2.8–3.1 days). However, following longer (5–9day) periods of starvation, Ca. carinatus regained satiatedlevels of egg production (55.8 eggs {female}^–1 day^–1)more rapidly (after 3.1–4.0 days of excess food) thanC. agulhensis (after 3.8–5.2 days of feeding following5–7 days of starvation). Moreover, many C. agulhensisfemales did not regain normal rates of egg production after9 days of starvation. For both species, the time required foregg production to recover was proportional to the starvationperiod, although only up to 7 days for C. agulhensis, and wasthe same following 4.25 days of starvation. Previously fed Ca.carinatus terminated egg production more rapidly than C. agulhensiswhen starved. The ability of Ca. carinatus to tolerate, andrecover rapidly from, prolonged periods of starvation, combinedwith a comparatively fast development time and high rate ofegg production, provides this species with a strong competitiveadvantage over C. agulhensis in the highly pulsed food environmentof the southern Benguela upwelling region.     

Growth and production of Euphausia lucens in the southern Benguela Current

The growth rate of a population of Euphausia lucens from thewest coast of South Africa was estimated from laboratory studiesand from monthly size-frequency distributions of samples collectedover a 1-year period. Laboratory studies indicated that growthrates ranged from 0.131 (larvae) to 0.047 mm day^–1 (juveniles),while size-frequency distributions suggested a growth rate of{small tilde}0.026 mm day^–1 for the adults. The mean annualbiomass from the inshore, intermediate and offshore regionsranged from 9.75 to 47.29 mg dry wt m^–3 with the highestbiomass being found in the inshore region. Calyptopis larvaewere present for most months of the year, indicating continuousrecrwtment. The relative contribution of flesh, moults and eggsto the total annual production was estimated separately forall three regions. Production due to growth (P_g) was estimatedto be 92.71–185.60 mg dry wt m^–3 year^–1, whileexuviai production (P_e) varied between 60.01 and 281.38 mg drywt m year Production of eggs (P_r) was estimated to range from5.07 to 12.39 mg dry wt m year the lowest value being obtainedin the inshore region. Moult production represented {small tilde}6times the mean biomass in each region, while the P/B ratio forflesh production varied from 3.92 to 8.91, the highest ratiobeing obtained in the offshore region. Total P/B ratios rangedfrom 10.14 to 16.01.     

Trophodynamics of the scyphomedusae Aurelia aurita. Predation rate in relation to abundance, size and type of prey organism

Ephyra larvae and small medusae (1.7–95 mm diameter, 0.01–350mg ash-free dry wt, AFDW) of the scyphozoan jellyfish Aureliaaurita were used in predation experiments with phytoplankton(the flagellate Isochrysis galbana, 4 µm diameter, {smalltilde}6 x 10^–6 µg AFDW cell^–1), ciliates (theoligotrich Strombidium sulcatum, 28 µm diameter, {smalltilde}2 x 10^–3 µg AFDW), rotifers (Synchaeta sp.,0.5 µg AFDW individual^–1) and mixed zooplankton(mainly copepods and cladocerans, 2.1–3.1 µg AFDWindividual^–1). Phytoplankton in natural concentrations(50–200 µg C I^–1) were not utilized by largemedusae (44–95 mm diameter). Ciliates in concentrationsfrom 0.5 to 50 individuals ml^"1 were consumed by ephyra larvaeand small medusae (3–14 mm diameter) at a maximum predationrate of 171 prey day^–1, corresponding to a daily rationof 0.42%. The rotifer Synchaeta sp., offered in concentrationsof 100–600 prey I^–1, resulted in daily rations ofephyra larvae (2–5 mm diameter) between 1 and 13%. Mixedzooplankton allowed the highest daily rations, usually in therange 5–40%. Large medusae (>45 mm diameter) consumedbetween 2000 and 3500 prey organisms day^"1 in prey concentrationsexceeding 100 I^–1. Predation rate and daily ration werepositively correlated with prey abundance. Seen over a broadsize spectrum, the daily ration decreased with increased medusasize. The daily rations observed in high abundance of mixedzooplankton suggest a potential ‘scope for growth’that exceeds the growth rate observed in field populations,and this, in turn, suggests that the natural populations areusually food limited. The predicted predation rate at averageprey concentrations that are characteristic of neritic environmentscannot explain the maximum growth rates observed in field populations.It is therefore suggested that exploitation of patches of preyin high abundance is an important component in the trophodynamicsof this species. ¹Present address: University of Bergen, Department of MarineBiology, N-5065 Blomsterdalen, Norway     

On temperature-independent growth of phytoplankton

Growth rates of two freshwater diatoms and three chlorophyteswere compared under 3 h days at 10 µmol m^–2 s^–1.Specific growth rates ranged between µ=0.03 day^–1and µ=0.055 day^–1 for the different species andwere in every species independent of temperature between 8 and16°C.     

Pigment-specific rates of phytoplankton growth and microzooplankton grazing in a subtropical lagoon

Phytoplankton growth and microzooplankton grazing rates wereevaluated in one station in Bahía Concepción,located in the middle region of the Gulf of California, México.We used high-performance liquid chromatography (HPLC) estimationsof phytoplankton pigment signatures to evaluate the annual variationof taxon-specific grazing and growth rates obtained with thedilution technique. Chlorophyll-a (Chl-a) concentrations variedwidely (0.34–3.32 µg L^–1) and showed two maxima,during late spring and autumn, associated with the transitionbetween mixed and stratified conditions. Phytoplankton growthrates varied seasonally with the lowest rates during summer(range: 0.01–2.55 day^–1 for Chl-a; 0.00–3.84day^–1 for Chl-b; 0.26–3.29 day^–1 for fucoxanthin;0.00–6.27 day^–1 for peridinin; 0.00–4.35 day^–1for zeaxanthin). Microzooplankton grazing was an important lossprocess (range: 0.0–1.89 day^–1 for Chl-a; 0.00–3.12day^–1 for Chl-b; 0.26–3.29 day^–1 for fucoxanthin;0.00–2.03 day^–1 for peridinin; 0.00–3.51 day^–1for zeaxanthin). Average grazing rates accounted 68–89%of estimated average phytoplankton pigment-specific growth rates.The analysis of pigment signatures indicates that diatoms anddinoflagellates were the dominant groups, and contrary to expectationfor typical subtropical lagoons, the specific growth rates inBahía Concepción showed a pronounced seasonalvariability, linked to transitional hydrographic conditions.Our results indicate a close coupling between the communitymicrozooplankton grazing and phytoplankton growth rates, withoutselective feeding behavior. These results suggest that microzooplanktonplay a critical role and may significantly modify the availabilityand efficiency of transfer of energy to higher trophic levels.     

Trophic links in the lowland River Meuse (Belgium): assessing the role of bacteria and protozoans in planktonic food webs

Trophic interactions within the plankton of the lowland RiverMeuse (Belgium) were measured in spring and summer 2001. Consumptionof bacteria by protozoa was measured by monitoring the disappearanceof ³H-thymidine-labelled bacteria. Metazooplankton bacterivorywas assessed using 0.5-µm fluorescent microparticles (FMPs),and predation of metazooplankton on ciliates was measured usingnatural ciliate assemblages labelled with FMPs as tracer food.Grazing of metazooplankton on flagellates was determined throughin situ incubations with manipulated metazooplankton densities.Protozooplankton bacterivory varied between 6.08 and 53.90 mgC m^–3 day^–1 (i.e. from 0.12 to 0.86 g C^–1bacteria g C^–1 protozoa day^–1). Metazooplankton,essentially rotifers, grazing on bacteria was negligible comparedwith grazing by protozoa (1000 times lower). Predation of rotiferson heterotrophic flagellates (HFs) was generally low (on average1.77 mg C m^–3 day^–1, i.e. 0.084 g C^–1 flagellatesg C^–1 rotifers day^–1), the higher contribution ofHF in the diet of rotifers being observed when Keratella cochleariswas the dominant metazooplankter. Predation of rotifers on ciliateswas low in spring samples (0.56 mg C m^–3 day^–1,i.e. 0.014 g C^–1 ciliates g C^–1 rotifers day^–1)in contrast to measurements performed in July (8.72 mg C m^–3day^–1, i.e. 0.242 g C^–1 ciliates g C^–1 rotifersday^–1). The proportion of protozoa in the diet of rotiferswas low compared with that of phytoplankton (<30% of totalcarbon ingestion) except when phytoplankton biomass decreasedbelow the incipient limiting level (ILL) of the main metazooplantonicspecies. In such conditions, protozoa (mainly ciliates) constituted50% of total rotifer diet. These results give evidence thatmicrobial organisms play a significant role within the planktonicfood web of a eutrophic lowland river, ciliates providing analternative food for metazooplankton when phytoplankton becomesscarce.     

The fate of discarded appendicularian houses: degradation by the copepod, Microsetella norvegica, and other agents

Despite the potential importance of zooplankton in degradationof marine snow, the association of colonising zooplankton withdiscarded appendicularian houses has not been investigated innorthern areas. We sampled the vertical distributions of appendicularians,houses and potential zooplankton colonisers at two stationsin the central North Sea during late summer. In addition, grazingexperiments were performed with the copepod Microsetella norvegica,which was assumed to be the main contributor to house degradation.The results were used in (i) inverse modelling, to estimatethe factors which were significant in shaping the vertical distributionof houses and (ii) calculations to estimate potential housedegradation rates. M. Norvegica was able to feed on appendicularianhouses, with feeding rates up to 0.42 g C (g C)^–1 day^–1(0.14 µg C ind.^–1 day^–1). The model resultssuggested that the vertical distribution of houses was shapedby sinking of houses, bacterial degradation and feeding of M.norvegica and invertebrate larvae. The estimated community degradationrate by M. norvegica was low, whereas invertebrate larvae haddegradation rates close to bacterial degradation. We concludethat at the typical concentrations of M. norvegica in the NorthSea (10⁴ ind. m^–2), its role in marine snow degradationis likely to be small. Degradation by other zooplankton groups,such as invertebrate larvae, can, however, be substantial.