Seasonal pattern in nutrient limitation and grazing control of the phytoplankton community in a non-stratified lake

1. The relative importance of zooplankton grazing and nutrient limitation in regulating the phytoplankton community in the non-stratified Lake Kvie, Denmark, were measured nine times during the growing season.
2. Natural phytoplankton assemblage bioassays showed increasing importance of nutrient limitation during summer. Growth rates at ambient nutrient concentrations were continually below 0.12 per day, while co-enrichment with nitrogen (N) and phosphorus (P) to above concentration-saturated conditions enhanced growth rates from May to the end of July.
3. Stoichiometric ratios of important elements in seston (C : N, C : P, N : P), in lake water (TN : TP), in external loading (TN : TP) and in internal loading (DIN : DIP) were measured to determine whether N or P could be the limiting nutrient. TN : TP molar ratio of both lake water, benthic fluxes and external loading suggested P limitation throughout the growing season. However, seston molar ratios suggested moderate P-deficiency only during mid-summer.
4. Abundance and community structure of the zooplankton varied considerably through the season and proved to be important in determining the responses of algal assemblages to grazing. High abundance of cladocerans and rotifers resulted in significant grazing impact, while cyclopoid copepods had no significant effect on the phytoplankton biomass.
5. Regeneration of ammonium and phosphate by zooplankton were periodically important for phytoplankton growth. A comparison of nutrient regeneration by zooplankton with nutrient inputs from sediment and external sources indicated that zooplankton may contribute significantly in supplying N and P for the growth of phytoplankton.     

Changes in trace metal species and other components of the rhizosphere during growth of radish

Changes in the properties of soil solution in the rhizosphere of developing radish plants were investigated. Variations in these properties were expected to affect the distribution and speciation of metals in the soil and soil solution. Applications of essential nutrients were linked to plant transpiration rates and prevented excess addition of nutrient ions, so that subtle changes in soil solution composition would not be obscured. Soil solution pH, the concentration of dissolved organic carbon (DOC) and the concentrations of major and trace elements in solution were found to vary over time. Strict control of fertilizer additions led to the maintenance of a relatively low ionic strength in the soil solution, and under such conditions trace metal solubility appeared to be highy influenced by the concentration of DOC. A chemical speciation analysis was performed which showed that, while dissolved Cd and Zn were largely uncomplexed in unplanted soil, Cd and Zn in the rhizosphere existed mainly as complexed forms. It is hypothesized that this is partly a result of Ca-metal-ligand equilibrium in solution, with higher Ca concentrations in unplanted soil leading to more of the Cd and Zn in solution existing in the uncomplexed state. Changes in the concentrations of uncomplexed Cd and Zn with time gave the best correlations with changes in plant uptake of these metals over time, supporting the hypothesis that plants mainly absorb the free metal ion from soil solution.     

Developmental instability, hybridization and heterozygosity in stick insects of the genus Bacillus (Insecta; Phasmatodea) with different modes of reproduction

Several genetic factors are assumed to influence developmental instability (DI). One is the level of heterozygosity, with higher levels often being associated with decreased DI; another is genetic incompatibility in hybrids, which in several cases has been shown to increase DI. The genus Bacillus includes species which have both amphigonic heterozygous reproducing populations and homozygous parthenogenetic reproducing populations (B. rossius rossius and B. r. redtenbacheri). Furthermore, Bacillus includes hybrid parthenogenetic species, which have very high levels of almost fixed heterozygosities (B. atticus, B. whitei, B. lynceorum). We investigated the phenotypic variance (σ²p) and the impact of hybridization and level of heterozygosity on DI in females from these populations and species of Bacillus. DI was estimated as fluctuating asymmetry (FA) for three bilateral traits: the labial palpus, the maxillary palpus and the antenna. For the labial palpus and maxillary palpus we found, in general, a lower level of DI in the amphigonic females compared with parthenogenetic counterparts from the same species and with parthenogenetic females from the three hybrid species. A higher DI of the antenna was found in the hybrid species when compared with both parthenogenetic and amphigonic populations of the nonhybrid species, suggesting that the genes controlling antenna development are located on the sex chromosomes. The development of the investigated bilateral characters in the hybrid species seemed to be affected more by factors relating to genetic incompatibilities as a consequence of hybridization than by the stabilizing force of increased heterozygosity. Only few differences in σ²p were observed, supporting the possibility that the observed differences in DI are related mainly to internal genetic factors. © 2006 The Linnean Society of London, Biological Journal of the Linnean Society, 2006, 87 , 249–259.     

A multidisciplinary approach to understanding the recent and historical occurrence of the freshwater plant, Littorella uniflora

1. The vulnerability of softwater, oligotrophic lakes to eutrophication has caused the disappearance of many, if not most, of the unique isoetid plant communities. We tested whether the presence or disappearance of the isoetid Littorella uniflora (L.) could be predicted from environmental parameters, soil types and land use in the catchment area, and atmospheric nitrogen deposition. 2. We found that the topographic catchment area of a lake was an irrelevant unit to study effects of soil type and land use. Instead, using a GIS‐generated buffer zone around the lakes it proved feasible to classify 472 lakes into historical (if L. uniflora had disappeared) or recent (if L. uniflora was still present) Littorella lakes, based on soil type and land use. Our analysis showed that aeolian sand deposits and heath in the buffer zone favoured the presence of L. uniflora, whereas moraine clay and agriculture were strongly linked to the disappearance of L. uniflora. 3. However, in order to understand fully the presence or disappearance of L. uniflora, environmental data were needed in addition to soil types, land use and nitrogen deposition, and the use of discriminant analysis allowed us to classify 96% of the investigated lakes correctly into recent or historical sites. Alkalinity, total phosphorus, total nitrogen, aeolian sand deposits and heath were the most important parameters explaining the presence or disappearance of L. uniflora. Our analysis also indicated that eutrophication, rather than acidification, has likely caused the disappearance of L. uniflora from 218 of the 472 lakes investigated. 4. Our findings have widespread implications for the conservation or restoration of isoetid habitats and we recommend applying a wide buffer zone around lakes, with restrictions on farming and traditional forestry activities. In addition, our buffering concept may prove a useful tool for aquatic ecologists to investigate relationships between catchment features and organisms (plants, insects and amphibians) with aquatic as well as terrestrial life forms.     

SENSORY PROFILES OF THE MOST COMMON SALMON PRODUCTS ON THE DANISH MARKET

The sensory profiles of the most common chilled and frozen salmon products available to consumers on the Danish market were studied. A sensory profiling was made on 12 salmon products varying in salmon species, origin, storage method and time. Samples stored in ice between 7 and 16 days, frozen for 1 month or stored in modified atmosphere for 5 days all had sensory profiles dominated by sea/seaweed odor, juicy and oily texture, fresh fish oil, and sweet and mushroom flavor. Marked differences in the sensory profiles of the frozen samples were found to correlate to differences in storage time. Frozen storage for 6 months resulted in firm texture, discolored appearance and rancid flavor. The samples stored in modified atmosphere for 7 days had a sensory profile with marked rancid and sour odor.     

Patterns of ant community organization in mesic southeastern Australia

study of the ants of 11 sites at Wilson's Promontory, Victoria, found that patterns of ant community organization varied markedly between habitats, being controlled by climate and vegetation structure. Community organization showed continuous change from drier sites (especially woodlands) where broadly-adapted species of Rhytidoponera were particularly abundant, but many other taxa were also well-represented, to wet forests where cool-climate specialists (Prolasius and certain Chelaner) and cryptic species (mostly Solenopsis) cotnprised almost all ants. These patterns appear to be typical of those in mesic regions of Australia, and contrast with those in the arid zone where ant faunas are dominated by highly active and aggressive species of Iridomyrmex which apparently play a pivotal role in community organization. The results suggest that ant communities in mesic regions are less tightly structured than those in the arid zone, and that interspecific competition is a less important factor influencing community organization. The findings of this study are briefly discussed in relation to the potential use of ants as bio-indicators.