Calanoid copepod grazing on phytoplankton: seasonal experiments on natural communities

Calanoid copepods are major components of most lacustrine ecosystems and their grazing activities may influence both phytoplankton biomass and species composition. To assess this we conducted four seasonal, in situ, grazing experiments in eutrophic Lake Rotomanuka, New Zealand. Ambient concentrations of late stage copepodites and adults of calanoid copepods (predominantly Calamoecia lucasi, but with small numbers of Boeckella delicata) were allowed to feed for nine days on natural phytoplankton assemblages suspended in the lake within 1160 litre polyethylene enclosures. The copepods reduced the total phytoplankton biomass of the dominant species in all experiments but were most effective in summer (the time of highest grazer biomass) followed by spring and autumn. In response to grazing pressure the density of individual algal species showed either no change or a decline. There were no taxa which increased in density in the presence of the copepods. The calanoid copepods suppressed the smallest phytoplankton species (especially those with GALD (Greatest Axial Linear Dimension) < µm) and there appeared to be no selection of algae on the basis of biovolume. Algal taxa which showed strong declines in abundance in the presence of the copepods include Cyclotella stelligera, Coelastrum spp., Trachelomonas spp., Cryptomonas spp., and Mallomonas akrokomos. Calanoid copepods are considered important grazers of phytoplankton biomass in this lake. The study supports the view that high phytoplankton:zooplankton biomass ratios and large average algal sizes characteristic of New Zealand lake plankton may, at least partly, be caused by year round grazing pressure on small algae shifting the competitive balance in favour of larger algal species.     

The influence of fluctuating light intensities on species composition and diversity of natural phytoplankton communities

The influence of fluctuating light intensities on phytoplankton composition and diversity was investigated for 49 days under semi-continuous culture conditions with sufficient nutrient supply, using phytoplankton assemblages from Lake Biwa, Japan. Light conditions were either periodically changed from high intensity (100 µmol photons m^-2 s^-1) to low intensity (20 µmol photons m^-2 s^-1) at intervals of 1, 3, 6 and 12 days, or fixed to constant intensities (permanent high and low light levels). All treatments additionally experienced a day:night cycle of 16:8 h. Phytoplankton abundance increased and reached a saturation level on day 19 of the treatment with permanent high light, but increased continuously until the end of the experiment (day 49) in the treatment with permanent low light intensity. In treatments with periodically changing light intensities, the phytoplankton abundance reached saturation levels between these dates. Under phytoplankton abundance saturation, chlorophytes predominated in the treatment with permanent high light, while either cyanophytes or diatoms were abundant under permanent low light intensity. Treatments with changing light supply had chlorophyte- and cyanobacteria-dominated replicates as well as replicates with balanced proportions of both. Furthermore, species diversity, measured by the Shannon index, was low in cultures under permanent light intensity, while slow fluctuating light at the scale of 3 -12 days resulted in an increased diversity index. These results indicate that species composition and diversity of the phytoplankton were affected by the periodically changing light regime in the order of days, and suggest that temporal changes in weather conditions are a major impediment to competitive exclusion of phytoplankton species in nature.     

Stirring influences the phytoplankton species composition within enclosed columns of coastal sea water

A brief daily stirring of two enclosed water columns led to the maintenance of larger phytoplankton cells than in two unstirred columns. Small herbivorous copepods, Noctiluca sp., and the ctenophore Pleurobrachia sp. were also maintained over the 6 weeks of the experiment. Grazing pressure appeared to decrease the average size of the phytoplankton cells during periods of low standing stocks.Mechanical stirring is a useful technique for manipulating the phytoplankton species composition and cell size in enclosed water columns where natural mixing processes are greatly reduced. N : P utilization ratios were lower and more similar to the ‘Redfield’ ratio of 15.5 in stirred (16–19) than in unstirred enclosures (25–27). Fluctuations in phytoplankton, small copepods, and Pleurobrachia standing stocks, suggest that stirring may hasten progress to equilibrium among the components of the food web in the enclosures.     

Novel interactions between phytoplankton and microzooplankton: their influence on the coupling between growth and grazing rates in the sea

Understanding the processes that regulate phytoplankton biomass and growth rate remains one of the central issues for biological oceanography. While the role of resources in phytoplankton regulation (`bottom up' control) has been explored extensively, the role of grazing (`top down' control) is less well understood. This paper seeks to apply the approach pioneered by Frost and others, i.e. exploring consequences of individual grazer behavior for whole ecosystems, to questions about microzooplankton–phytoplankton interactions. Given the diversity and paucity of phytoplankton prey in much of the sea, there should be strong pressure for microzooplankton, the primary grazers of most phytoplankton, to evolve strategies that maximize prey encounter and utilization while allowing for survival in times of scarcity. These strategies include higher grazing rates on faster-growing phytoplankton cells, the direct use of light for enhancement of protist digestion rates, nutritional plasticity, rapid population growth combined with formation of resting stages, and defenses against predatory zooplankton. Most of these phenomena should increase community-level coupling (i.e. the degree of instantaneous and time-dependent similarity) between rates of phytoplankton growth and microzooplankton grazing, tending to stabilize planktonic ecosystems. Conversely, phytoplankton, whose mortality in the sea is overwhelmingly due to microzooplankton grazing, should experience strong pressure to evolve grazing resistence. Strategies may include chemical, morphological, and `nutrient deficit' defenses. Successful deployment of these defenses should lead to uncoupling between rates of phytoplankton growth and microzooplankton grazing, promoting instability in ecosystem structure. Understanding the comparative ecosystem dynamics of various ocean regions will require an appreciation of how protist grazer behavior and physiology influence the coupling between rates of phytoplankton growth and microzooplankton grazing.     

Effects of grazing,competition, disturbance and fire on species composition and diversity in grassland communities

Abstract. The effects of grazing, physical disturbance, interspecific competition, and fire on populations of individual species and on the structure and diversity of grassland communities were investigated in short-, mid-, and tall-grasslands in the Serengeti National Park, Tanzania. The treatments included deep and shallow artificial disturbances, early dry-season burning, and removal of dominant grass species from 2m² plots located inside and outside of large-mammal-proof exclosures. Species-cover data were collected for five years and analyzed by analysis of variance and diversity indices. In all three communities, protection from grazing and physical disturbance had greater impacts on species cover and diversity than removal of dominant species or fire. More species were significantly affected (positively and negatively) by disturbance and protection from grazing (50 - 100%) than by species removals and fire (< 41%). In most cases, the cover of tall, perennial species increased following protection from grazing while the cover of annual and short, perennial species decreased. Over the five years of the study, vegetatively reproducing species benefited more than sexually reproducing species from protection from grazing, and tall rhizomatous species benefited more than tall stoloniferous species. Disturbance caused annual and short perennial species to increase and tall perennials to decrease in cover. Following species removals, tall species were either unaffected or they increased in cover while some short species increased and others decreased in cover. Species diversity in two of the three communities decreased when the communities were protected from grazing but increased when disturbed, when dominant species were removed, and when burned.     

The influence of salinity in the domoic acid effect on estuarine phytoplankton communities

Toxic species of the diatom genus Pseudo-nitzschia, observed worldwide from coastal waters to the open ocean, produce the neurotoxin domoic acid (DA). DA is an important environmental and economic hazard due to shellfish contamination with subsequent effects on higher trophic levels. Previous research has demonstrated that, among other environmental factors, salinity influences the abundance and toxicity of Pseudo-nitzschia. In this study, the environmental factors driving the growth of Pseudo-nitzschia and the production of dissolved DA (dDA) in North Inlet estuary were examined. The effect of salinity on the growth inhibition of phytoplankton induced by the initial presence as well as by an addition of dDA was also assessed. Initially, the diatom abundance was negatively correlated with the abundance of Pseudo-nitzschia and with the concentration of dDA. With the addition of a concentrated solution of dDA, the percent inhibition of cryptophytes and diatoms was significantly correlated with salinity and suggested a higher sensitivity to dDA at extreme salinities. These results emphasize the importance of salinity in assessing the properties of DA and potentially of other phycotoxins on phytoplankton.     

The distribution and abundance of krill faecal material and oval pellets in the Scotia and Weddell Seas (Antarctica) and their role in particle flux

Summary The abundance and depth distribution of zooplankton faeces in spring to early summer were investigated along meridional transects (47°W and 49°W) that extended from the Scotia Sea (57°S) across the Weddell-Scotia Confluence and into the Weddell Gyre (62°S). The sea ice edge retreated from 59°30S to 61°S during the study. Faeces were sampled with nets, Niskin bottles and sediment traps and subsequently analysed by light and electron (SEM) microscopy. Krill faecal strings and oval faecal pellets of unknown origin were by far the most important zooplankton faeces and highest concentrations were always found in the Confluence often close to the ice border. Krill faeces were usually more abundant in the uppermost layer (0–50m) where they contributed an average of 130 g dry weight m^–3. There was an exponential decrease with depth, with a minimum of 0.6 g dry weight m^–3 in the 500–1000 m stratum. Oval pellets were more evenly distributed in the upper 1000 m of the water column, with an average of 9 g dry weight m ^–3, although there was a small peak (20 g dry weight m^–3) in the subsurface layer (50–150 m depth). Consecutive collections (day-night) of krill faeces using drifting sediment traps showed that only the larger strings sank from 50 to 150 m depth. Peritrophic membranes appeared to deteriorate during sinking. Diatoms (in particular Nitzschia and Thalassiosira spp.) contributed by far the bulk of material in krill and oval faeces. In samples collected near or under the pack ice, remains of crustaceans in both krill- and oval faeces were also found.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

城市公共绿地常见木本植物组成对鸟类群落的影响

快速城市化背景下, 城市公共绿地已经成为重要的鸟类栖息地, 其中的木本植物群落构成对鸟类群落结构有显著影响, 研究木本植物配置与鸟类多样性的关系对提升城市公共绿地作为鸟类栖息地的生态服务功能有重要的理论和应用价值。我们于2009–2010年间在上海市滨江森林公园就木本植物和鸟类群落的相互关系展开研究。样线法结合样点调查共记录到鸟类10目25科64种5,368只(次), 鸟类多度全年变化显著, 峰值分别出现在4月和11月。丰富度全年也呈现双峰型变化, 峰值出现在4月和12月。全年鸟类多样性(Simpson指数)差异显著, 10月最高, 8月最低。样方调查共记录到77种木本植物, 其中有14种(乔木9种、灌木5种)出现率超过5个样方, 定义为常见种, 其果期主要集中在9月到翌年2月, 其上常见林业致病害虫的发生盛期集中在5–11月。主成分分析显示, 常见木本植物上观察到的鸟类可划分为8个鸟类集团, Spearman秩相关检验显示秋冬季常见木本植物果期与植食性、肉食性、食虫性、杂食性等多个鸟类集团的多度均呈显著正相关。说明在秋冬季鸟类迁徙高峰期不同鸟类集团均能获得丰富的食物资源。就常见木本植物对鸟类群落的重要值进行排序, 结果显示, 大叶杨(Populus lasiocarpa)、枫杨(Pterocarya stenoptera)、香樟(Cinnamomum camphora)等乔木对鸟类重要值较高, 而灌木层对鸟类的重要性整体偏低, 说明滨江森林公园的灌木层作为鸟类栖息地的功能建设尚需加强。根据以上研究结果, 我们提出了提高上海城市公园鸟类多样性的植被配置建议。     

Sponge communities of the Antarctic Peninsula: influence of environmental variables on species composition and richness

Sponge communities on the Antarctic continental shelf currently represent one of the most extensive sponge grounds in the world, and all sponge classes are known to occur in the Southern Ocean. Main objectives of this study conducted at the tip of the Antarctic Peninsula were (1) to identify all sampled sponges and (2) to investigate whether the species composition and species richness of Southern Ocean sponge communities in the area of the Antarctic Peninsula are significantly influenced by environmental variables. The studied material originated from 25 AGT catches and was sampled during the expedition ANT-XXIX/3 of RV Polarstern. Samples were collected in three large-scale areas in the vicinity of the Antarctic Peninsula: Bransfield Strait, Drake Passage and Weddell Sea. The following six environmental variables were measured from bottom water samples (except for sea-ice cover): depth (m), light transmission (%), oxygen (µmol/kg), salinity, sea-ice cover (%) and temperature (°C). Two hundred and sixty-three sponge samples were analyzed, and 81 species of 33 genera from all Porifera classes (Calcarea, Demospongiae, Hexactinellida and Homoscleromorpha) were identified. Total numbers of sponge species per sample station ranged from 1 to 29. A detrended correspondence analysis and a backward-stepwise model selection were performed to check whether species composition and richness were significantly influenced by environmental variables. The analyses revealed that none of the measured environmental variables significantly influenced species composition but that species richness was significantly influenced by (1) temperature and (2) the combination of temperature and depth. Results of this study are of crucial importance for development, performance and assessment of future protection strategies in case of ongoing climatic changes at the Antarctic Peninsula.     

The phytoplankton species composition and the seasonal periodicity in Lake Vechten from 1956 to 1979

The species composition and seasonal periodicity of the phytoplankton in Lake Vechten (The Netherlands) have been studied over a 20 year period. The results show a more or less regular pattern of algal succession during the year. Phytoplankton growth starts early in the year leading to a maximum in early spring, dominated by Bacillariophyceae and some Chlorophyceae. In summer a second maximum develops mainly consisting of a chrysophycean and a dinophycean species, accompanied by several Chlorophyceae. In September the summer species are gradually succeeded by diatoms. By the end of the summer stratification in October algal numbers decrease rapidly. The winter community consists of several species of Cryptophyceae and Chrysophyceae.When the data of 1975–1979 are compared with those from earlier records, distinct changes can be observed. These changes are discussed with reference to the trophic status of the lake. The vertical distribution of some species is described.     

The overwintering strategy of Antarctic krill under the pack-ice of the Weddell Sea

Summary Antarctic krill (Euphausia superba Dana) occurs in enormous swarms in Antarctic waters during the ice-free summer months. The winter whereabouts of this stock were hitherto unknown. Evidence collected during the Winter Weddell Sea Project 1986 (WWSP'86, G. Hempel 1988) covering a large area of the eastern and southern Weddell Sea indicates that the seasonal sea ice cover sustains the bulk of the krill population. Results presented here, show that known aspects of krill morphology and behavior are actually adaptations to the ice habitat, suggesting that the dominance of krill in the Antarctic marine ecosystem is a result of its capacity to grow and reproduce in the water column in summer, and find both food and shelter in the ice cover during the rest of the year. This conclusion has far-reaching implications for our understanding of Southern Ocean biology and ecology.     

Gastropod grazing of epiphytic lichen-dominated communities depends on tree species

Tree species differ in longevity, canopy structure, and bark texture, chemistry and water storage. Tree species-specific traits play a role in shaping epiphytic vegetation and likely influence the community assembly of organisms feeding on epiphytes. Lichenivorous gastropods, species with calcium-rich shells in particular, need calcium and likely occur more abundantly in and around tree species with high available calcium. We quantified gastropod grazing on the epiphytic lichens Lobaria pulmonaria and Lobaria scrobiculata transplanted to blocks of adjacent trunks of Acer platanoides, Quercus robur and Tilia cordata. We tested the hypothesis that tree species known to have more available Ca, exhibit more grazing damage on transplanted lichens than trees with lower Ca-availability. The grazing pressure was 1.6–1.8 times higher for lichen transplants on Acer and Tilia known to produce litter with easily soluble Ca than on Quercus, which binds Ca as oxalate. Trees with a high grazing pressure on transplants had greater natural abundance of Lobaria virens than of L. pulmonaria. Gastropods preferred L. scrobiculata to L. pulmonaria, evidenced by more observed grazing marks and greater measured biomass loss. We attribute this difference to the lower concentration of carbon-based secondary compounds in L. scrobiculata. However, the strength of the preference varied between the three tree species receiving lichen transplants and was strongest on A. platanoides, while gastropods on T. cordata grazed equal amounts of each transplanted lichen. In conclusion, tree species influenced grazing patterns of gastropods on epiphytic lichens. In addition to bark pH and other factors, we have shown that tree species-specific differences in grazing pressure play a role in shaping the epiphytic macrolichen community.     

Productivity, herbivory, and species traits rather than diversity influence invasibility of experimental phytoplankton communities

Biological invasions are a major threat to natural biodiversity; hence, understanding the mechanisms underlying invasibility (i.e., the susceptibility of a community to invasions by new species) is crucial. Invasibility of a resident community may be affected by a complex but hitherto hardly understood interplay of (1) productivity of the habitat, (2) diversity, (3) herbivory, and (4) the characteristics of both invasive and resident species. Using experimental phytoplankton microcosms, we investigated the effect of nutrient supply and species diversity on the invasibility of resident communities for two functionally different invaders in the presence or absence of an herbivore. With increasing nutrient supply, increased herbivore abundance indicated enhanced phytoplankton biomass production, and the invasion success of both invaders showed a unimodal pattern. At low nutrient supply (i.e., low influence of herbivory), the invasibility depended mainly on the competitive abilities of the invaders, whereas at high nutrient supply, the susceptibility to herbivory dominated. This resulted in different optimum nutrient levels for invasion success of the two species due to their individual functional traits. To test the effect of diversity on invasibility, a species richness gradient was generated by random selection from a resident species pool at an intermediate nutrient level. Invasibility was not affected by species richness; instead, it was driven by the functional traits of the resident and/or invasive species mediated by herbivore density. Overall, herbivory was the driving factor for invasibility of phytoplankton communities, which implies that other factors affecting the intensity of herbivory (e.g., productivity or edibility of primary producers) indirectly influence invasions.     

Acoustic observations on krill spring-summer migration and patchiness in the northern Weddell Sea

Summary Continuous acoustic observations with a 30 kHz and a 150 kHz echo-sounder were made from November 1988 to January 1989 on one repeatedly sampled transect, running along 49°W from the open waters of the Scotia Sea into the Weddell Sea pack-ice. Swarm signals occurring on the echorecords were mainly found in the upper 100 m of the watercolumn, in size varying vertically from 1 to 70 m, and horizontally from less than 5 to over 3,000 m. Catches with a RMT 1+8 indicated that the observed swarm signals were most probably caused by krill, Euphausia superba. From late November to early January the swarms migrated northwards away from the ice-edge and towards greater depths, while simultaneously growing in size. The average number of swarms observed per 10 nautical miles along the transects remained fairly constant throughout the cruise, but the average swarmsize and total aggregation size increased during the period studied. The echo-data give evidence of the spring-summer migration of major parts of the krill stock from under the ice-cover towards open waters.Data presented here were collected during the European Polarstern Study (EPOS), funded by the European Science Foundation     

Green krill, the indicator of micro- and nano-size phytoplankton availability to krill

Size-fractionated chlorophyll-a concentrations of surface seawater were measured for pico-, nano-, and micro-size fractions (<2 μm, 2–10 μm, and >10 μm respectively) during commercial krill fishery operations in the waters north of the South Shetland Islands. The proportion of green krill (individuals discoloured due to active feeding on phytoplankton) had significant regressions with chlorophyll-a concentrations in micro- and nano-size fractions. Between these two fractions, chlorophyll-a concentration in the micro-size fraction showed the higher partial regression coefficient. This result shows the importance of phytoplankton larger than nano-phytoplankton, especially micro-phytoplankton, in terms of a phytoplanktonic food source for Antarctic krill in the natural environment. Accepted: 6 February 1999     

Lipid composition of halophilic species of Dunaliella from the dead sea

The lipid patterns of six halophilic Dunaliella species from the Dead Sea (C₉, D_11a, D_11b, D₁₃, F_20a and D. viridis) were found to be generally similar to those of halotolerant Dunaliella species previously examined, except for the presence in the halphilic Dunaliella of small to trace amounts of one or more (up to five) unidentified glycolipids. The lipids of two of the halophilic algae, species C₉ and D_11b, were studied in detail and were found to be similar in composition. Glycolipids were the major group (55.0 and 53.2 mol % for C₉ and D_11b, respectively), followed by neutral lipids (mainly triacyglycerols: 21.3 mol %; 24.6 mol %), whereas phospholipids were a much smaller fraction (6.5 mol %; 5.8 mol %). Monogalactosyldiacylglycerol was the largest component (22.0 mol %; 24.3 mol %) but digalactosyldiacylglycerol (18.7 mol %; 14.9 mol %) and sulfoquinovosyldiacylglycerol (14.3 mol %; 14.0 mol %) were also present in high concentrations. All phospholipids were present at low concentrations: phosphatidylglycerol (4.4 mol %; 3.1 mol %); phosphatidylethanolamine (1.1 mol %; 0.7 mol %); phosphatidylcholine (0.9 mol %; 1.9 mol %); and phosphatidylinositol (traces). Diacylglyceryl-O-(N,N,N-trimethyl)homoserine was present in C₉ and D_11b (3.3 mol %; 9.3 mol %) and in all the other species examined. Fatty acid composition of the individual lipid components of C₉ and D_11b showed characteristic differences between glycolipids and phospholipids, in a similar pattern for both algae. The major fatty acids detected in all species examined were -linolenic, linoleic, palmitic, oleic and a polyunsaturated sixteen carbon acid.     

The effect of different zooplankton grazing patterns resulting from diel vertical migration on phytoplankton growth and composition: a laboratory experiment

Diel vertical migration (DVM) of herbivorous zooplankton is a widespread behavioural phenomenon in freshwater ecosystems. So far only little attention has been paid to the impact of DVM on the phytoplankton community in the epilimnion. Some theoretical models predict that algal population growth in the epilimnion should depend on the herbivores migration and grazing patterns: even if migrating zooplankton consume the same total amount of algae per day in the epilimnion as non-migrating zooplankton, nocturnal grazing should result in enhanced algal growth and favour algal species with high intrinsic growth rates over species with lower intrinsic growth rates. To test these hypotheses we performed experiments in which several algal species were confronted with different feeding regimes of Daphnia. In the experiments algal growth did not only depend on the absolute time of grazing but was comparatively higher when grazing took place only during the night, even when the grazing pressure was the same. Furthermore, algal species with higher intrinsic growth rates had higher advantages when being grazed upon only discontinuously during the night than algal species with a smaller intrinsic growth rate. The grazing pattern itself was an important factor for relative algal performance.     

Predators and mutualists influence the exclusion of aphid species from natural communities

We investigated why two species of aphids (Aphis jacobaeae and Brachycaudus cardui) were very rare in a study site despite their abundance in the surrounding area. The study site contained many common species of aphid and we tested the hypothesis that the community of aphid predators in the field excluded the missing species. Colonies of the two aphid species were artificially initiated in the experimental site and allocated to one of four treatments: control; ground predator exclusion; total predator exclusion, and provision of mutualist ants. Two measures of colony performance were analysed: longevity and cumulative aphid density. Colonies decline naturally in late summer but control colonies disappeared very quickly while colonies protected from all predators survived the longest. The performance of colonies protected from just ground predators was intermediate. We failed to persuade ants to tend A. jacobaeae. Colonies of B. cardui attended by ants performed better than controls and those with ground predators excluded, but not as well as those with all predators excluded. We conclude that the absence of the two species of aphid in the study site is influenced by the resident predator community, and by the availability of mutualists. Received: 27 April 1998 / Accepted: 30 November 1998