Effects of ultraviolet radiation and dissolved organic carbon on the survival of subarctic zooplankton

High intensities of ultraviolet radiation are known to be harmful to aquatic biota, especially for species living in shallow, clear water bodies. Zooplankton species from such habitats are good model organisms to study the effect of changes in UV radiation, and how animals deal with this. We tested experimentally the effect of natural UV radiation, which was controlled by different filters and varying concentrations of UV-screening dissolved organic carbon (DOC), on the survival of the cladocerans, Daphnia longispina and D. pulex, and a calanoid copepod, Eudiaptomus graciloides. All species originated from subarctic Fennoscandia where underwater UV intensity is influenced by ozone depletion, changes in DOC and timing of ice break-up. Measured as mortality, all species were affected by both UVB and UVA radiation. Survival was highest and similar between species in the dark controls and photosynthetically active radiation exposures. Under each UV filter, the highest DOC concentration provided significant and best shelter from UV radiation and led to best survival. Variation in survival was observed between species. E. graciloides responded more readily to changes in UV radiation than did the daphnids. In natural environments, species' previous exposure to light and different protection strategies (pigmentation, vertical migration) are probably as important factors controlling the survival of zooplankton as radiation intensity and optical properties of water.     

Food sources and lipid retention of zooplankton in subarctic ponds

1. Subarctic ponds are seasonal aquatic habitats subject to short summers but often have surprisingly numerous planktonic consumers relative to phytoplankton productivity. Because subarctic ponds have low pelagic productivity but a high biomass of benthic algae, we hypothesised that benthic mats provide a complementary and important food source for the zooplankton. To test this, we used a combination of fatty acid and stable isotope analyses to evaluate the nutritional content of benthic and pelagic food and their contributions to the diets of crustacean zooplankton in 10 Finnish subarctic ponds. 2. Benthic mats and seston differed significantly in total lipids, with seston (62.5 μg mg^?1) having approximately eight times higher total lipid concentrations than benthic mats (7.0 μg mg^?1). Moreover, the two potential food sources differed in their lipid quality, with benthic organic matter completely lacking some nutritionally important polyunsaturated fatty acids (PUFA), most notably docosahexaenoic acid and arachidonic acid. 3. Zooplankton had higher PUFA concentrations (27–67 μg mg^?1) than either of the food sources (mean benthic mats: 1.2 μg mg^?1; mean seston: 9.9 μg mg^?1), indicating that zooplankton metabolically regulate their accumulation of PUFA. In addition, when each pond was evaluated independently, the zooplankton was consistently more ¹³C‐depleted (δ¹³C ?20 to ?33‰) than seston (?23 to ?29‰) or benthic (?15 to ?27‰) food sources. In three ponds, a subset of the zooplankton (Eudiaptomus graciloides, Bosmina sp., Daphnia sp. and Branchinecta paludosa) showed evidence of feeding on both benthic and planktonic resources, whereas in most (seven out of 10) ponds the zooplankton appeared to feed primarily on plankton. 4. Our results indicate that pelagic primary production was consistently the principal food resource of most metazoans. While benthic mats were highly productive, they did not appear to be a major food source for zooplankton. The pond zooplankton, faced by strong seasonal food limitation, acquires particular dietary elements selectively.     

A simple and inexpensive trap-tube sampler for zooplankton collection in shallow waters

The paper describes a new type of zooplankton sampler, which combines the concepts of the Pennak core sampler and the Schindler-Patalas plankton trap. The new sampler, called Trap Tube Sampler, consists of a PVC water pipe (1.5–2.0 m, long; 10 cm diameter) provided, at the bottom end, of a filtering unit and closing mechanism which alternatively closes the mouth of the tube and the mouth of the filtering unit. The new device is particularly suitable for collecting samples from the entire water column in shallow vegetated water bodies, fish ponds and mesocosm tanks.     

Observations on microbial percent respiration values in arctic and subarctic marine waters and sediments

Percent respiration was measured in over 1,100 arctic and subarctic marine water and sediment samples using¹⁴C-labeled glucose and glutamate. These measurements were made at different times of the year in 4 regions. Percent respiration values were typically lower in regions where the waters of large rivers mixed with seawater. They were also lower in sediments and in waters collected near the bottom than in surface waters. They were higher in winter arctic waters than water samples collected in the summer; however, a similar seasonal trend was not observed in subarctic waters. There were a number of studies in which there were significant positive rank correlations between percent respiration and salinity and between percent respiration and temperature. From what is known about the range of temperature and salinity encountered in samples collected during these studies and the results of temperature and salinity effects experiments, it was concluded that changes in these 2 variables did not explain the variation observed in percent respiration. Correlations between percent respiration and the inorganic nutrients PO₄ ^–3, NH₄ ⁺ and NO₃ ^– showed that of the 3 variables, only NO₃ ^– showed relatively high correlations with all the same sign. From this it was concluded that there may be situations in which NO₃ ^– levels may influence percent respiration in nearshore marine waters. It is also likely that qualitative characteristics of the available organic nutrients may also influence percent respiration levels. Although no organic nutrient data is available for statistical analysis, the patterns of percent respiration near river plumes and the relatively strong negative correlation often observed between uptake rates (heterotrophic activity) and percent respiration suggests that organic nutrients may be a factor in controlling percent respiration. It is suggested that there are situations in which percent respiration measurements may be used to document stress in natural microbial populations due to nutrient deficiencies.     

Utilization of lichens in the arctic and subarctic

Economic Botany - The principal economic importance of lichens lies in their providing sustenance for hundreds of thousands of reindeer in circumpolar regions, animals upon which the Eskimos, Lapps...     

Seasonal variations in biomass, abundance and composition of zooplankton in the subarctic waters north of Iceland

The seasonal variations in biomass, abundance and species composition of zooplankton in relation to hydrography and chlorophyll a were studied in the subarctic waters north of Iceland. The sampling was carried out at approximately monthly intervals from February 1993 to February 1994 at eight stations arranged along a transect extending from 66°16′N–18°50′W to 68°00′N–18°50′W. The mean temperature at 50 m depth showed a clear seasonal pattern, with lowest water temperatures in February (∼1.1°C) and the highest in July (∼5.4°C). The spring growth of the phytoplankton began in late March and culminated during mid-April (∼7.0 mg Chl a m⁻³). Both the biomass and the abundance of total zooplankton were low during the winter and peaked once during the summer in late May (∼4 g m⁻² and ∼38,000 individuals m⁻²). A total of 42 species and taxonomic groups were identified in the samples. Eight taxa contributed ∼90% of the total zooplankton number. Of these Calanus finmarchicus was by far the most abundant species (∼60% of the total zooplankton). Less important groups were ophiuroid larvae (∼9%), Pseudocalanus spp. (∼8%), Metridia longa (∼4%), C. hyperboreus (∼3%), Acartia longiremis (∼2%), chaetognaths (∼2%) and euphausiid larvae (∼2%). The dominant copepods showed two main patterns in seasonal abundance: C. finmarchicus, C. hyperboreus and C. glacialis had one annual peak in numbers in late May, while Pseudocalanus spp., M. longa and A. longiremis showed two maxima during the summer (July) and autumn (October/November). Ophiuroid larvae and chaetognaths (mainly Sagitta elegans) peaked during the middle of July, while the number of euphausiid eggs and larvae was greatest from May to July. The succession in population structure of C. finmarchicus indicated its main spawning to be in April and May, coincident with the phytoplankton spring bloom. A minor spawning was also observed sometime between August and October. However, the offspring from this second spawning contributed only insignificantly to the overwintering stock of C. finmarchicus. Received: 12 September 1997 / Accepted: 1 March 1998     

Impact of common tern colonies upon the zooplankton in Rybinsk Reservoir shallow waters

The comparison of the zooplankton communities developing in the semiprotected shallow waters of Rybinsk Reservoir under the impact of common tern (Sterna hirundo Linnaeus) colonies with communities in sheltered overgrowing habitats situated apart from colonies, has reveled some peculiar features of the former: higher biomass, prevalence of cladocerans, lower share of rotifers, and decrease in species diversity.     

Fish-mediated zooplankton community structure in shallow turbid waters: a mesocosm study

Wetlands Ecology and Management - Turbidity influences the success of fish feeding, but little is known about the difference in the effects of turbidity as a result of organic and inorganic matter....     

Sedimentation in arctic Canada: Particulate organic carbon flux to a shallow marine benthic community in Frobisher bay

Summary Sediment traps designed and constructed by the authors were deployed on the bottom during short periods of time at a depth of 33 m in upper Frobisher Bay. When the results were compared with data obtained from a trap set at the compensation depth (20 m), resuspension of particulate organic carbon was estimated at 25% of the sediment. The annual sedimentation cycle was typified by consistently low winter rates, with variable maximum rates and loads occurring in August following the peak of primary production in the water column. Mean annual sediment flux was found to be 26 g C m^-2 y^-1, representing 31–53% of the range of estimates of carbon fixed in total annual primary production. Relating this to estimated zoobenthic production indicated a benthic conversion efficiency of 53%, not accounting for loss to sediment or benthic primary production. Increased sedimentation following the spring bloom in August appears to stimulate reproduction in some benthic species.     

On the importance of dissolved organic matter in the nutrition of zooplankton in some lake waters

Summary A steady state, radiotracer technique was used to study the original source of the carbon in zooplankton. The experiments were started in filtered lake water with added inorganic radiocarbon. At the beginning of the experiments, a proportionally insignificant volume of unfiltered water was introduced into the culture, together with some ovigerous zooplankton individuals. Since the radioactivity: carbon ratio in the dissolved inorganic carbon was kept constant, a similar ratio would be expected to develop in the autotrophic phytoplankton. The same ratio would then be expected to develop in the zooplankton, if its sole carbon source was autotrophic phytoplankton.According to the results of this approach dissolved organic matter seems to be an important food resource for zooplankton, particularly in highly humic lakes. This conclusion was confirmed by the finding that zooplankton from these lakes was able to grow and reproduce in experiments started with filtered lake water and conducted in complete darkness.The development of algae was followed over the course of one experiment in highly humic water. The same micro-flagellates reproduced equally well in both light and darkness, which indicates the importance of heterotrophic metabolism in their nutrition. Although there are no direct observations about the food of zooplankton in our experiments, it appears likely that heterotrophic flagellates play an important role as a food of zooplankton in humic waters.The importance of dissolved organic matter in the nutrition of aquatic organisms would seem to be much greater than has generally been recognized. Consequently the prevailing concepts of the structure and functioning of planktonic ecosystem should be thoroughly re-evaluated.     

Isotopic analysis of cyanobacterial nitrogen fixation associated with subarctic lichen and bryophyte species

Dinitrogen fixation by cyanobacteria is of particular importance for the nutrient economy of cold biomes, constituting the main pathway for new N supplies to tundra ecosystems. It is prevalent in cyanobacterial colonies on bryophytes and in obligate associations within cyanolichens. Recent studies, applying interspecific variation in plant functional traits to upscale species effects on ecosystems, have all but neglected cryptogams and their association with cyanobacteria. Here we looked for species-specific patterns that determine cryptogam-mediated rates of N₂ fixation in the Subarctic. We hypothesised a contrast in N₂ fixation rates (1) between the structurally and physiologically different lichens and bryophytes, and (2) within bryophytes based on their respective plant functional types. Throughout the survey we supplied ¹⁵N-labelled N₂ gas to quantify fixation rates for monospecific moss, liverwort and lichen turfs. We sampled fifteen species in a design that captures spatial and temporal variations during the growing season in Abisko region, Sweden. We measured N₂ fixation potential of each turf in a common environment and in its field sampling site, in order to embrace both comparativeness and realism. Cyanolichens and bryophytes differed significantly in their cyanobacterial N₂ fixation capacity, which was not driven by microhabitat characteristics, but rather by morphology and physiology. Cyanolichens were much more prominent fixers than bryophytes per unit dry weight, but not per unit area due to their low specific thallus weight. Mosses did not exhibit consistent differences in N₂ fixation rates across species and functional types. Liverworts did not fix detectable amounts of N₂. Despite the very high rates of N₂ fixation associated with cyanolichens, large cover of mosses per unit area at the landscape scale compensates for their lower fixation rates, thereby probably making them the primary regional atmospheric nitrogen sink.     

Dissolved organic carbon fluctuations in Dal Lake waters

Dissolved organic carbon (DOC) fluctuations were studied in the surface water of Dal Lake from May 1984 to April 1985, at monthly intervals. Concentration of DOC varied from 0.140 mg l^–1 to 0.725 mg l^–1. A minimum was recorded in May and September, while an increase was observed during December and January. The fluctuations during the observation period showed a relationship with temperature and pH.     

Variability of macroinvertebrate community composition in an arctic and subarctic stream

The macroinvertebrate community composition was compared in two Alaskan streams (USA) for numeric and species constancy during the ice-free period from 1981 to 1983. Imnavait Creek is a first order arctic stream (60° 39 N, 149° 21 W) draining upland tundra in the foothills of the Brooks Range. Caribou-Poker Creek is a 4th order subarctic stream (65° 08 N, 147° 28 W) draining the taiga forest north of Fairbanks, Alaska. The aquatic insect larvae and other macroinvertebrates were sampled with drift nets and Hess bottom samplers for four periods, each 1 week long in the ice free season of three years. We found 112 species in the arctic stream and 138 species in the subarctic stream in a chironomid-dominated community. In any sample period the communities contained 51–60 species in the arctic and 49–92 species in the subarctic. Between the four sample periods on average 39% and 50% of the species were present in two sequential samples in the arctic and subarctic stream, respectively. New immigrants, never before found in the system, averaged 37% and 31% of the community, respectively. These systems are exposed to several intermediate disturbances: prolonged and variable freeze-up, extreme variation in discharge, wide diel and seasonal changes in temperature, and erosion by frazil and anchor ice. The dipterans that compose the most numerous and variable taxa must have variable diapause, ability to grow in cold waters, and good dispersal powers, even migrating across drainages in the arctic. Much of the seasonal dominance pattern appears therefore to be stochastic.     

The organic carbon budget of a shallow arctic tundra lake on the Tuktoyaktuk Peninsula,N.W.T., Canada

The organic carbon cycle of a shallow, tundra lake (mean depth 1.45 m) was followed for 5 weeks of the open water period by examining CO₂ fluxes through benthic respiration and anaerobic decomposition, photosynthesis of benthic and phytoplankton communities and gas exchange at the air-water interface. Total photosynthesis (as consumption of carbon dioxide) was 37.5 mmole C m^–2 d^–1, 83% of which was benthic and macrophytic. By direct measurement benthic respiration exceeded benthic photosynthesis by 6.6 mmole C m^–2 d^–1. The lake lost 1.4 × 10⁶ moles C in two weeks after ice melted by degassing C0₂, and 6.8 mmole C m^–2 d^–1 (1.5 × 10⁶ moles) during the remainder of the open water period; 2.2 mmole C m² d^–1 of this was release Of CO₂ stored in the sediments by cryoconcentration the previous winter. Anaerobic microbial decomposition was only 4% of the benthic aerobic respiration rate of 38 mmole C m^–2 d^–1. An annual budget estimate for the lake indicated that 50% of the carbon was produced by the benthic community, 20% by phytoplankton, and 30% was allochthonous material. The relative contribution of allochthonous input was in accordance with measurement of the ¹⁵N of sedimented organic matter.     

Effects of zooplankton on the sinking flux of organic carbon in Lake Biwa

To clarify the roles of zooplankton in the sedimentation of seston from the epilimnion, the sinking flux of particulate carbon was measured along with primary production rate and zooplankton biomass from July 1996 to October 1997 at a pelagic site in the north basin of Lake Biwa. During the study period, the flux varied seasonally from 66 to 510 mg C m⁻² day⁻¹ and was low in summer when zooplankton, composed mainly of Eodiaptomus japonicus and Daphnia galeata, were abundant. Simple correlation analysis revealed that the sinking flux correlated neither with the primary production rate nor with the amount of sestonic carbon above the sediment trap. However, the particle elimination rate, estimated as the difference between the primary production rate and the sinking flux, correlated positively with the zooplankton biomass. These results suggest that zooplankton play a substantial role in decreasing the sinking flux in Lake Biwa. Received: March 6, 2000 / Accepted: October 7, 2000     

Enhanced photochemical loss of organic carbon in acidic waters

Previous studies have shown that (a) a large portion of theannual total organic carbon (TOC) inputs to central Ontario lakes iseither lost to sediments or degraded and lost via evasion to theatmosphere, (b) the partitioning of organic carbon between sediments andthe atmosphere appears to be a function of acidity and (c) UVirradiation can account for observed long-term loss of TOC from thewater column. These findings were extended by examining whether acidityenhances photo-oxidative losses of TOC. Stream waters (initialalkalinities between –97 and 233 µeq l^–1)were incubated in UV-transparent containers under incident solarradiation for periods ranging from 14 to 23 days. The highestphoto-oxidation rates occurred when alkalinity was negative. Additionsof acid and base to stream waters increased and decreasedphoto-oxidation rates, respectively. The exceptional clarity ofatmospherically acidified lakes is usually attributed to increasedprecipitation of Al-organic carbon complexes but may instead be due tohigher photo-oxidation rates of allocthonous organic carbon leading tohigher evasion rates of CO₂.     

有机碳源和氮源对三角褐指藻生长的影响

研究了8种有机碳源和4种氮源对三角褐指藻(Phaeodactylum tricornutum)生长的影响.结果表明,三角褐指藻具有兼养生长的能力,碳浓度为5mmol/L和50mmol/L时,葡萄糖、果糖、乙酸钠和甘油对其生长有明显的促进作用,乳酸钠和乙醇抑制藻细胞生长,半乳糖和柠檬酸钠对其生长随有机碳浓度而异.甘油、葡萄糖和乙酸钠的适宜浓度范围分别为5 -800mmol/L、100 -400mmol/L和50 -300mmol/L.培养液中分别加入50mmol/L甘油、400mmol/L葡萄糖和150mmol/L乙酸钠,培养第14天,三角褐指藻的最大生物量分别为对照的1.45倍、1.25倍和1.11倍.甘油兼养生长的最优氮源是尿素,适宜浓度范围为0.88-8.8mmol/L.当尿素浓度为4.4mmol/L时,最高生物量可达1.31g/L.