Sedimentation in Arctic Canada: Species composition and biomass of phytoplankton contributed to the marine sediments in Frobisher Bay

Summary Sedimentation of phytoplankton provides food and energy for zoobenthic communities. In this study the rates, species composition and biomass of phytoplankton input to Frobisher Bay sediments were examined during ice (late November to July) and open water (late July to October) periods from 1982 to 1985. The rates were higher on the sea bed than at 20 m. The minimum rate (3x10⁵ cells·m^-2·day^-1) of sedimentation occurred during the early part of the ice period. It increased as the ice thickened and reached a maximum of 2.8x10⁸ cells·m^-2·day^-1 after the phytoplankton bloom at the beginning of the open water period in the first two weeks of August. The sedimented phytoplankton was dominated by diatoms, with a great majority of pennate species during the spring (April to June) and centric forms during the summer (July to August). Green flagellates, dinoflagellates and chrysophytes occurred as a low percentage of the total population in all seasons. Other indicators (chlorophyll a and phaeopigments) showed highest biomass levels in the deepest traps. They were consistently low during the winter (December to March) and reached their maxima during the open-water period of summer. Their abundance was correlated with the seasonal cycle of the phytoplankton in the water column.     

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.     

Abundance and seasonal migrations of the penaeid shrimp Metapenaeus affinis (H. Milne-Edwards) within Iraqi waters

Migration of Metapenaeus affinis (H. Milne-Edwards) from the Arabian Gulf to nursery grounds in the inland waters of Iraq extends from May/June to January/February. Shrimp ranging in size from 3–125 mm total length were found in inland waters. In the shallow waters of the Al-Assaflya small-sized shrimp only were caught, while in the Marshes large-sized shrimp were abundant. Maximum numbers of recruits were recorded when temperatures reached 23–25 °C. However, the discharge of the Shatt Al-Arab may also be an important factor regulating recruitment.The growth rates of small shrimp were higher than those of large ones. There mostly was, a preponderance of females over males. Spawning at sea appears to occur immediately after emigration. Gonad development was not observed. Commercial landings, at the two main fish markets at Basrah, during September–November 1985 averaged 1000 kg day^–1.     

Rotifers as predators on small ciliates

Clearance rates of Synchaeta pectinata, Brachionus calyciflorus and Asplanchna girodi on Tetrahymena pyriformis (46 µm in length) at a density of 10 cells ml^–1, in the presence of algal food, were 2.5 to 6.1 ml rot.^–1 day^–1. Clearance rates of these rotifers were, respectively, about 2, 3, and 13 times lower on Strobilidium gyrans (58 µm in length) than on T. pyriformis, indicating that the saltations of S. gyrans are an effective escape response. Clearance rates of S. pectinata were considerably lower on Colpidium striatum (81 µm) than on S. gyrans, suggesting that S. pectinata may not be able to ingest ciliates of this size. S. pectinata had a clearance rate of 19 ml rot.^–1 day^–1 on S. gyrans at a density of 1.2 cells ml^–1, in the absence of edible algal food. Rotifers may prey extensively on ciliates in natural plankton communities, ingesting 25 to 50 individuals in the 45–60 µm size range day^–1.     

Growth and habitat separation in eight cohorts of three species of cyprinids in a subalpine lake

Synopsis Distribution and growth of the embryos, larvae and juveniles of Rutilus rutilus (roach), Scardinius erythrophthalmus (rudd) and Leuciscus cephalus (chub) from an oligotrophic subalpine lake in Tyrol, Austria, were studied during the first three to four months after hatching. R. rutilus was the first to spawn, a single cohort hatching around May 23rd. Four cohorts of S. erythrophthalmus hatched between June 19 and August 1. Three cohorts of L. cephalus hatched between July 3 and 25. The length/weight relationship of all species changed at a length of approximately 15–16 mm. R. rutilus, hatching at the lowest temperature, also showed the lowest growth rate during early life (maximum 10.4 per cent fresh body weight day^–1). In the other two species relative growth rates up to 20% day^–1) were measured. Rudd and chub remained in the shallow littoral during the whole period of observation, whereas roach left the littoral a few weeks after hatching and migrated into deeper water. A subtle shift in vertical distribution was observed for the first cohort of rudd which moved into slightly deeper water when the second cohort made its appearance.To whom correspondence should be addressed     

Regional and interannual productivity of biogenic components and planktonic foraminiferal fluxes in the northwestern Pacific Basin

Time-series sediment trap experiments at subtropical (WCT-1) and subarctic (WCT-2) stations in the northwestern Pacific indicate seasonal, latitudinal and depth variations in total particulate, biogenic and foraminiferal fluxes. At the subtropical station, the average total mass flux was 19.4 mg m⁻² day⁻¹ in the shallow trap (1060 m) and 21.5–26.1 mg m⁻² day⁻¹ in the deep trap (3930 m) during the sampling period. At subarctic station, these values were 91.5–176.9 mg m⁻² day⁻¹ in the shallow and 68.6–112.3 mg m⁻² day⁻¹ in the deep trap. We recognized 12 and 15 planktonic foraminiferal species at Station WCT-1 and Station WCT-2, respectively. The planktonic foraminiferal flux and species turnover are related to seasonal and interannual changes in source water and water column conditions at both stations. At Station WCT-1, the highest flux was recorded during the summer, with a peak in mid to late June associated with similar flux patterns of the dominant species, Globigerinoides ruber and Globigerinita glutinata. The total flux of foraminiferal tests at the shallow and deep traps is similar in numbers and magnitude. At Station WCT-2, the peaks of total flux of foraminiferal tests at the two trap depths differ in number, and their magnitude in the deep trap is almost half of that in the shallow trap. A distinctive seasonal pattern occurred in the shallow and the deep trap, with a peak in total foraminiferal flux in mid June to mid July. Globigerina quinqueloba, Neogloboquadrina pachyderma and Neogloboquadrina dutertrei dominate the planktonic population throughout the year.Subtropical Station WCT-1 was characterized by low total foraminiferal fluxes and low total mass flux, which is dominated by calcium carbonate and depleted in opal, whereas high foraminiferal fluxes and a high total mass flux dominated by high biogenic opal, and less calcium carbonate and organic matter characterize subarctic Station WCT-2. The foraminiferal carbonate that reaches the seafloor accounts for an average 20–27% and 22–23% of the total calcium carbonate at Station WCT-1 and Station WCT-2, respectively. The primary reason for the difference in flux at both stations thus lies in the different contributions of siliceous and calcareous planktonic assemblages. The seasonal variation in biogenic particulate flux at both stations implies that temporal changes in biological productivity are governed by large-scale seasonal climatic variability and local hydrography.     

Short-term variability in the flux of rapidly sinking particles in the Antarctic marginal ice zone

A sediment trap deployment was made at a station (64°42'S, 139°59'E) at five depths (537, 796, 1,259, 1,722, 2,727 m) in the marginal ice zone (MIZ) of the Antarctic Ocean during a summer productive period from 26 December 1994 to 20 January 1995. This aim of the study was to reveal a possible occurrence of a sporadic bloom in surface layers and to evaluate the role of fast-sinking particles in transportation processes of bloom-derived material down to mesopelagic and bathypelagic layers. During the observation, a marked flux increase (70.5 mg C m^-2 day^-1, 7.7 mg N m^-2 day^-1) was observed at the depth of 537 m on 7-9 January. The increased flux at 537 m decreased with depth and time. The same mass of sinking particles forming the flux maximum at each depth sank down from the shallowest trap (537 m) to the deepest trap (2,727 m) within 4-11 days, indicating that these particles were transported downward to the bottom with the sinking rate of >200 m day^-1. Collected particles were composed of two major particle fractions; one was dominated by fecal pellets of macrozooplankton (mainly Euphausia superba) with relatively fast sinking rates (FSP; fast-sinking particles) and the other by minute diatoms of Fragilariopsis curta with slow sinking rates (SSP; slowly sinking particles). According to the comparison of time depth changes of these two fractions, the SSP had unexpectedly faster sinking rates comparable with the FSP during the periods of maximum fluxes, probably indicating the SSP were transformed from the FSP during sinking. The present result strongly suggests that a local bloom of F. curta and intensified zooplankton grazing activities occurred in surface layers in a few days in the MIZ, and then the egested fecal pellets were rapidly transported downward with fragmentation processes into small-sized minute particles in mesopelagic and bathypelagic layers.     

Cultivation of Lemna gibba under desert conditions. I: Twelve months of continuous cultivation in open ponds

Duckweed, Lemna gibba, was grown in 12 m² shallow ponds in the Negev desert, during 12 months of continuous cultivation, beginning April 1984. Average monthly growth rates varied with the season of the year. The lowest daily yield, 2·6±0·4 g dry weight m⁻² day⁻¹, was obtained during January. Highest daily yields, 7·9±2·6 g dry weight m⁻² day⁻¹ and 7·0±1·2 g dry weight m⁻² day⁻¹, were obtained during September and May. A 35% decline of the yield was seen during midsummer (July), 4·8±1·2 g dry weight m⁻² day⁻¹. The average rate for the year was 5·15±1·7 g dry weight m⁻² day⁻¹. The protein content of the plants ranged from 30 to 38% per unit dry weight.Growth performance is discussed in relation to the prevailing climatic conditions.     

Light effects on leaf development and photosynthetic capacity of Hydrocotyle bonariensis Lam.

Rates of net CO₂ uptake were examined in developing leaves of Hydrocotyle bonariensis. Leaves that developed under high photosynthetically active radiation (48 mol m^-2 day^-1 PAR) were smaller, thicker, and reached maximum size sooner than did leaves that developed under low PAR (4.8 mol m^-2 day^-1). Maximum net CO₂ uptake rates were reached after 5 to 6 days expansion for both the low and the high PAR leaves. Leaves grown at high PAR had higher maximum photosynthetic rates and a higher PAR required for light saturation but showed a more rapid decline in rate with age than did low PAR leaves. To assess the basis for the difference observed in photosynthetic rates, CO₂ diffusion conductances and the mesophyll surface available for CO₂ absorption were examined for mature leaves. Stomatal conductance was the largest conductance in all treatments and did not vary appreciably with growth PAR. Mesophyll conductance progressively increased with growth PAR (up to 48 mol m^-2 day^-1) as did the mesophyll surface area per unit leaf area, but the cellular conductance exhibited most of its increase at low PAR (up to 4.8 mol m^-2 day^-1).     

Seasonal occurrence of coccoliths in sediment traps from West Caroline Basin, equatorial West Pacific Ocean

Coccolith fluxes were investigated by sediment trap studies in the West Caroline Basin, which is located in the equatorial western Pacific. The investigation was conducted from June 1991 to March 1992 at two water depths, 1592 and 3902 m, as part of the Northwest Pacific Carbon Cycle Study (NOPACCS) program. Two seasonal maxima of coccolith fluxes were observed during September–early October and late December–January. The average coccolith and coccosphere fluxes at the depth of the shallow trap were 1800×10⁶ coccoliths m⁻² day⁻¹ and 1.9×10⁶ coccospheres m⁻² day⁻¹, respectively. The flux of coccoliths followed the same trend as the total flux, and was closely correlated with the flux of organic matter flux. Florisphaera profunda, Gladiolithus flabellatus, Gephyrocapsa oceanica, Umbilicosphaera sibogae var. sibogae, Emiliania huxleyi, and Oolithotus fragilis were the most abundant species together comprising more than 85% of the total flora. Observed seasonal changes of the species composition of coccolith flora, as well as analysis of the R-mode cluster, revealed that during the summer, the assemblage was marked by the dominance of G. oceanica and U. sibogae. However, during the winter, the assemblage was dominated by E. huxleyi and O. fragilis. These assemblage changes were influenced by monsoonal events, which were observed off the New Guinea coast. F. profunda dominated the community in the shallow trap throughout most of the year; peak values of this species were recorded during the winter. The coccosphere assemblage was dominated by G. oceanica at both water depths. In the deep trap, the sedimentation pattern was similar to that observed at the shallow depth. Mean coccolith and coccosphere fluxes at the deep trap were 2000×10⁶ coccolith m⁻² day⁻¹ and 0.08×10⁶ coccospheres m⁻² day⁻¹, respectively. The increase in coccolith flux with water depth suggests a lateral influx. The estimated average daily mass of CaCO₃ flux in coccoliths and coccospheres was 16.6 mg m⁻² day⁻¹ at the 1592 m trap and 17.9 mg m⁻² day⁻¹ at the 3902 m trap, respectively. These calculated values contributed only 23.3% to the total CaCO₃ flux at the shallow trap and 27.9% at the deep trap.     

Cage culture ofKappaphycus alvarezii var.tambalang (Gigartinales,Rhodophyceae)

Kappaphycus alvarezii var.tambalang was cultured in a 3 × 3 m bamboo raft installed inside a 4 × 4 m floating net cage ofLates calcarifer (sea bass) broodstock at SEAFDEC Sub-station Igang, Guimaras, Philippines, from December 1989 to May 1990. Growth and production ofK. alvarezii var.tambalang was influenced by the culture months. The highest growth rate and production were recorded in January and May, respectively, while the lowest growth rate and production were observed in March. Average growth rate (wet weight) ranged from 3.72 to 7.17% day^-1, while production ranged from 575.5 to 2377 g m^-1 line^-1. A total production of approximately 123 t (fresh) or 37 t (dried) ha^-1 in the 5-month harvest season was produced from this culture system. Cultivation ofK. alvarezii var.tambalang in cages is possible, which indicates that seaweeds can be grown with carnivorous finfish, a practice which is still untapped.     

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.     

Effects of water level fluctuation on the growth ofNelumbo nucifera Gaertn. in Lake Kasumigaura,Japan

Investigations were made of the growth ofNelumbo nucifera, an aquatic higher plant, in a natural stand in Lake Kasumigaura. A rise of 1.0 m in the water level after a typhoon in August 1986 caused a subsequent decrease in biomass ofN. nucifera from the maximum of 291 g d.w. m⁻² in July to a minimum of 75 g d.w. m⁻². The biomass recovered thereafter in shallower regions. The underground biomass in October tended to increase toward the shore. The total leaf area index (LAI) is the sum of LAI of floating leaves and emergent leaves. The maximum total LAI was 1.3 and 2.8 m² m⁻² in 1986 and 1987, respectively. LAI of floating leaves did not exceed 1 m² m⁻². The elongation rates of the petiole of floating and emergent leaves just after unrolling were 2.6 and 3.4 cm day⁻¹, respectively. The sudden rise in water level (25 cm day⁻¹) after the typhoon in August 1986 caused drowning and subsequent decomposition of the mature leaves. Only the young leaves were able to elongate, allowing their laminae to reach the water surface. The fluctuation in water level, characterized by the amplitude and duration of flooding and the time of flooding in the life cycle, is an important factor determining the growth and survival ofN. nucifera in Lake Kasumigaura.     

Attenuation of the vertical flux of copepod fecal pellets under Arctic sea ice: evidence for an active detrital food web in winter

A variable fraction of fecal pellets produced in the epipelagic layer is intercepted and retained before reaching the bottom. We assessed fecal pellet retention in the ice-covered Beaufort Sea in early February by comparing the shape and size-frequency distribution of pellets collected by a sediment trap moored at 210 m to that produced in vitro. Appendicularian ellipsoidal and copepod cylindrical pellets made up 75 and 24% of the flux (165 μg C m⁻² day⁻¹). In contrast, production (135 μg C m⁻² day⁻¹) was dominated by cylindrical pellets (93%). The vertical flux of cylindrical pellets at 210 m was attenuated by 70%. Pellets >120 μm in width, represented 42% of the production, but were not detected in the trap. Retention most likely resulted from coprorhexic feeding by copepods such as Metridia longa. Our observations suggest that the detritivore food web prevailing under the ice of the Arctic Ocean in winter is dominated by appendicularians feeding on pellets fragmented by copepods.     

Isolating the components of activity-density for the carabid beetle Pterostichus melanarius in farmland

The activity and density of the carabid beetle, Pterostichus melanarius, were studied over 10 weeks in a continuous mark-recapture experiment using a grid of pitfall traps spanning a hedgerow and extending approximately 30 m into two cereal fields; 1777 beetles were individually marked. The recapture rate was approximately 60% and 40% for males and females, respectively. Activity-density rose and fell four times between early June and mid August. Jolly-Seber estimates of density showed population density increasing to a single peak in late July. The mean population density in late July and August was relatively stable at approximately 0.26 m⁻². Activity varied over the 10 weeks and was significantly higher during August than in June or July. The daily displacement distance frequencies, calculated from 750 male and 485 female recaptures of individually marked beetles, were distributed exponentially. Mean displacement distances were approximately 2.6 m day⁻¹ during June and July, and 5.3 m day⁻¹ during August. A diffusion model provided diffusion coefficients of 23.7 and 27.9 m² day⁻¹ for female and male beetles, respectively. The spatial distribution of P. melanarius was aggregated in patches. Spatial analysis by distance indices showed the spatial distribution of counts between successive periods of activity-density to be significantly associated. Approximately 5.75% of recaptures were from releases on the opposite side of the hedgerow. Approximately 20% of recaptures were from releases in opposite halves of the grid within the same field. The hedgerow acted as a significant barrier to dispersal between fields, with implications for the metapopulation structure of the species. Received: 26 January 1998 / Accepted: 1 April 1998     

Biofiltering efficiency, uptake and assimilation rates of Ulva clathrata (Roth) J. Agardh (Clorophyceae) cultivated in shrimp aquaculture waste water

The growth, biofiltering efficiency and uptake rates of Ulva clathrata were studied in a series of outdoor tanks, receiving waste water directly from a shrimp (Litopenaeus vannamei) aquaculture pond, under constant aeration and two different water regimes: (1) continuous flow, with 1 volume exchange a day (VE day^-1) and (2) static regime, with 1 VE after 4 days. Water temperature, salinity, pH, dissolved inorganic nitrogen (DIN), phosphate (PO₄), chlorophyll-a (chl-a), total suspended solids (TSS), macroalgal biomass (fresh weight) and tissue nutrient assimilation were monitored over 12 days. Ulva clathrata was highly efficient in removing the main inorganic nutrients from effluent water, stripping 70–82% of the total ammonium nitrogen (TAN) and 50% PO₄ within 15 h. Reductions in control tanks were much lower (Tukey HSD, P < 0.05). After 3 days, the mean uptake rates by the seaweed biomass under continuous flow were 3.09 mg DIN g DW day⁻¹ (383 mg DIN m⁻² day⁻¹) and 0.13 mg PO₄ g DW day⁻¹ (99 mg PO₄ m⁻² day⁻¹), being significantly higher than in the static regime (Tukey HSD, P < 0.05). The chl-a decreased in seaweed tanks, suggesting that U. clathrata inhibited phytoplankton growth. Correlations between the cumulative values of DIN removed from the water and total nitrogen assimilated into the seaweed biomass (r = 0.7 and 0.8, P < 0.05), suggest that nutrient removal by U. clathrata dominated over other processes such as phytoplankton and bacterial assimilation, ammonia volatilization and nutrient precipitation.     

Response on scallop culture to enhanced nutrient supply by experimental fertilisation of a landlocked bay

The effect of an enhanced nutrient supply to coastal waters of a landlocked bay, Hopavågen in Central Norway, on the phytoplankton production and biomass, and on growth of scallops (Pecten maximus) was studied in 1997–1999. Nitrogen, silicon and phosphorous (N:Si:P = 16:8:1, atomic) were added daily between May and October in 1998 at a level of 0.4 mg P m^–3 day^–1. The concentration of nutrient addition was doubled in 1999 during the same period. High addition of nutrients (1999) resulted in a significantly higher phytoplankton biomass in the summer period, expressed as chlorophyll a content, than without nutrient (1997) and low nutrient (1998). The respective mean chlorophyll a levels were 2.4 in 1999, 1.6 in 1998 and 1.2 g l^–1 in 1997. The mean primary production during the summers generally increased with the addition of nutrients from an average level of 320 mg carbon m^–2 day^–1 in 1997 to 1200 mg carbon m^–2 day^–1 in 1999. Scallops placed at 10 m depth in Hopavågen showed an increase in growth rate of the outer scallop shell in the period July–September from 0.16% day^–1 in 1997 to 0.53% day^–1 in 1998. Scallops grown in an unfertilised control station in the fjord outside Hopavågen had a significantly lower growth rate than those grown in the fertilised water of Hopavågen. The results showed decreased growth rate with increasing shell sizes. However, for all size groups studied a higher growth rate of the scallops was observed when nutrients were added to the bay. The tissue dry weight content of scallops grown in Hopavågen was 2–4 times higher than in the control scallops.     

Trophic structure and productivity of the lagoonal communities of Tikehau atoll (Tuamotu Archipelago,French Polynesia)

Carbon standing stocks and fluxes were studied in the lagoon of Tikehau atoll (Tuamotu archipelago, French Polynesia), from 1983 to 1988.The average POC concentration (0.7–2000 µm) was 203 mg C m^–3. The suspended living carbon (31.6 mg C m^–3) was made up of bacteria (53%), phytoplankton < 5 µm (14.2%), phytoplankton > 5 µm (14.2%), nanozooplankton 5–35 µm (5.7%), microzooplankton 35–200 µm (4.7%) and mesozooplankton 200–2000 µm (7.9%). The microphytobenthos biomass was 480 mg C m^–2.Suspended detritus (84.4% of the total POC) did not originate from the reef flat but from lagoonal primary productions. Their sedimentation exceeded phytobenthos production.It was estimated that 50% of bacterial biomass was adsorbed on particles. the bacterial biomass dominance was explained by the utilisation of 1) DOC excreted by phytoplankton (44–175 mg C m^–2 day ^–1) and zooplankton (50 mg Cm^–2 day^–1)2) organic compounds produced by solar-induced photochemical reactions 3) coral mucus.50% of the phytoplankton biomass belongs to the < 5 µm fraction. This production (440 mg C m^–2 day^–1) exceeded phytobenthos production (250 mg C m^–2 day^–1) when the whole lagoon was considered.The zooplankton > 35 µm ingested 315 mg C m^–2 day^–1, made up of phytoplankton, nanozooplankton and detritus. Its production was 132 mg C m^–2 day^–1.     

Distribution and Seasonal Abundance of Trematode Parasites (Trematoda: Allocreadiidae: <Emphasis Type="Italic">Crepidostomum</Emphasis> spp.) in burrowing-mayfly nymphs (Ephemeroptera: Ephemeridae: <Emphasis Type="Italic">Hexagenia</Emphasis> spp.) from Connecting Rivers of the Laurentian Great Lakes

Burrowing-mayfly nymphs such as Hexagenia spp. have been used extensively in North America and Europe as a biomonitoring tool to indicate mesotrophic water quality, yet infestation by associated parasites has not been well documented. We performed laboratory analysis of archived samples of Hexagenia spp. nymphs collected in 1985 and 1986 to provide base-line data on the distribution (1985) and seasonal infestation (1986) of the trematode parasite Crepidostomum spp. in Hexagenia spp. nymphs in connecting rivers between Lakes Superior and Erie of the Laurentian Great Lakes. In May and June 1985, frequency of occurrence of metacercariae was widely distributed throughout the connecting rivers (63% of 203 stations with nymphs), except in areas where nymph densities were relatively low (i.e.,≤69 nymphs/m²). Distribution was probably underestimated in the present study because of low probability (mean = 31%, range = 0–57%) of detecting infestation in a small number of collected nymphs ( ≤10) at nymph densities ≤69/m². In 1986, seasonal infestation between April and October occurred in 3.3% (627) of 18696 nymphs. Overall prevalence, mean intensity, and mean abundance of parasites at one station in the St. Marys River indicate parasite transmission occurred between June and September. This period of transmission is dependent on the life-cycle of the parasite. In addition, the life-cycle of Hexagenia spp. determines which annual cohort of nymphs is infested and therefore, the duration of infestation. Although, no impacts of infestation on Hexagenia spp. nymphs were observed in the present study, infestation intensities were high enough (≥25 metacercariae per nymph) at one station in the St. Marys River to potentially cause tissue damage in a high proportion (53%) of infested nymphs.     

How to increase juvenile shrimps in mangrove waters?

The distribution, density and species composition of juvenile shrimps were studied in three mangrove zones of Pichavaram, situated on the southeast coast of India. Nine species of shrimps dominated with Metapenaeus monoceros, M. brevicornis and Penaeus indicus were recorded. This shrimp resource was higher in mangrove-lined waters than that in neritic waters. This field observation was experimentally proved using litter bags. The average catch around the litter bags was 0.80 individuals day^–1 for Rhizophora leaves, 1.6 ind. day^–1 for Avicennia leaves and 0.16 ind. day^–1 in control. In general, abundance of juvenile shrimps was the highest around the leaves that were decomposed for 40 days and colonized with high counts of nitrogen-fixing azotobacters. Finally, a vegetation trap technique using mangrove leaves has been proposed for efficient schooling of juvenile shrimps. There was about 3-fold increase in number of shrimp juveniles around the trap than at control waters.