Photosynthetic Carbon Sources of Stream Macrophytes

Rates of photosynthesis of four submerged stream macrophyteswere examined under varying pH and composition of inorganiccarbon species. Callitriche stagnatis and Sparganium simplexused only CO₂ for photosynthesis. Potamogeton crispus and P.pectinatus used HCO₃ in addition to CO₂, but with much lowerefficiency. The photosynthetic rates at air equilibrium anda total inorganic carbon concentration of 5.0 mM were 2–3times lower than maximum rates at CO2 saturation for the HCO₃users and 10–14 times lower for the CO₂ users. The CO₂compensation point of entire plants of Callitriche (2.5 µM)and Sparganium (6.0µM) was well below the equilibriumconcentration (15 µM). and the low saturation points (250–500µM) also pointed to efficient use of CO2. Callitricheand Sparganium compete successfully with HCO₃^– users inhardwater streams, which have a higher exchange and generationcapacity of CO2 than stagnant and more soft waters. Rates ofphotosynthesis of Potamogeton crispus and P. pectinatus decreasedat high pH. Depending on the two alternative hypotheses forHCO₃use, this decline can be explained by CO₃^––inhibition of HCO₃^– uptake or by increasing capacity tobuffer H+efflux from the plant. Habitats subject to high pH,e. g. small ponds with dense vegetation, may have a strong selectionfor efficient mechanisms of HCO₃ use. Key words: Photosynthesis, Macrophytes, Carbon-source     

Variability of invertebrate herbivory on the submerged macrophyte Potamogeton perfoliatus

1. Invertebrate herbivory was studied in twenty-eight populations of the submerged macrophyte Potamogeton perfoliatus in Danish streams and lakes in mid-June. All populations but one experienced invertebrate herbivory and loss ranged from 0 to 11.9% of leaf area among populations. Loss generally increased with leaf age towards the base of the plants, and young apical leaves were rarely damaged. 2. Herbivory loss was significantly higher in streams (mean 5.0%) than in lakes (mean 2.2%), but varied greatly among populations within the same stream or lake and was not correlated to physico-chemical site characteristics, size or density of plant population, or leaf N and P content. High levels of invertebrate herbivory were therefore not associated with certain types of streams or lakes. 3. High herbivore biomass relative to abundance of plants was conducive to high loss. In streams, the biomass of the trichopteran Anabolia nervosa accounted for 50% of the variability in loss. No single species appeared to be equally important in lakes, although loss was correlated to the biomass of the chrysomelid beetle Macroplea appendiculata. Obligate herbivores, such as lepidopteran larvae, apparently exerted little damage on P. perfoliatus, and leaf mining and channelization from specialist feeders were negligible. It is concluded that shredders acting as facultative herbivores were the most important invertebrate herbivores on P. perfoliatus in Danish freshwaters.     

Patch dynamics of the stream macrophyte, Callitriche cophocarpa

1. We examined changes in position and growth of Callitriche cophocarpa patches in a shallow section of a small Danish stream during the main expansion of plant cover between April and June. Mean upstream growth of patches was only 7.5% of downstream growth. The mean growth rate was 1,02cm day^?1 in length and 0.55 cm day^?1 in width and the growth rates were not significantly dependent on patch size. For patches with an area (A) above 100cm², the growth rates in patch area (dA/dt) was proportional to patch circumference and thus √Aand the relative growth rate (dA/dt A^?1) was inversely proportional to √A.The smallest patches (<100cm²), however, expanded less than expected because of their combined tendency to grow more slowly in patch length and width than larger patches. 2. The expansion of plant cover will be much more rapid in many medium-sized patches compared to few large patches of a similar combined area, because internal selflimitation by light and space is partially relieved in smaller patches. We anticipate that recruitment and mortality of new-formed patches are critical steps preceding areal expansion. Quantification of these processes are needed fully to evaluate patch dynamics and space occupation.     

A PORTABLE DIGITAL SOUND EMITTER FOR INTERACTIVE PLAYBACK OF ANIMAL VOCALISATIONS

ABSTRACT

A new portable digital sound emitter (DSE) for normal and interactive playback of sound signals in the field and in the laboratory is described together with two examples of applications of the DSE in interactive field experiments. The DSE may be loaded with a broad spectrum of digitised sounds, e.g. artificial signals or natural animal vocalisations, and it is controlled via the keyboard of a portable PC running a dedicated program. This program, which can be tailored to support a vast number of different demands, enables the experimenter to start and stop the analog output from the DSE at any time and to choose freely between the available sounds and playback modes. The use of the DSE for interactive playback experiments therefore is only limited by the ability of the operator to perceive the vocalisations of the participating animal and to operate the keyboard of the PC.     

Wolf Movement Patterns: a Key to Estimation of Kill Rate?

Abstract: To estimate wolf (Canis lupus) kill rates from fine-scale movement patterns, we followed adult wolves in 3 territories of the Scandinavian wolf population using Global Positioning Systems (GPS) during the winters of 2001–2003. The resulting 6 datasets of 62–84 study days gave a total of 8,747 hourly GPS positions. We visited clusters of positions in the field on average 8.8 days after positioning and found moose (Alces alces) killed by wolves during the study period on 74 (8%) of the 953 clusters. The number of positions and visits to a cluster, their interaction, and the proportion of afternoon positions were significant fixed effects in mixed logistic-regression models predicting the probability of a cluster containing a wolf-killed moose. The models, however, displayed a poor goodness-of-fit and were not a suitable tool for estimating kill rates from positioning data alone. They might be used to reduce fieldwork by excluding unlikely clusters, although the reduction was not substantial. We discuss proximate factors (i.e., human disturbance and access to prey) as well as ultimate factors (i.e., social organization, intra-guild dominance, and litter size) as potential causes of the observed high temporal and spatial variation in prey-handling. For similar future kill-rate studies, we recommend increasing field efforts and shortening positioning intervals.     

Multistage,Long-Range Natal Dispersal by a Global Positioning System-Collared Scandinavian Wolf

Abstract: We document a new record dispersal for wolves worldwide. The natal straight-line dispersal distance of a Global Positioning System-collared female wolf from the Scandinavian population was 1,092 km from southeast Norway to northeast Finland, with a multistage actual travel distance of >10,000 km. Natural gene flow to the isolated, inbred Scandinavian wolf population may occur if survival of dispersers is improved.     

Oxygen stress and reduced growth of Lobelia dortmanna in sandy lake sediments subject to organic enrichment

1. Lobelia dortmanna is a common representative of the small isoetid plants dominating the vegetation in nutrient‐poor lakes in Europe and North America. Because of large permeable root surfaces and continuous air lacunae Lobelia exchanges the majority of O₂ and CO₂ during photosynthesis across the roots. This leads to profound diel pulses of O₂ and CO₂ in sandy sediments with low microbial O₂ consumption rates. The ready radial root loss of O₂ may, however, make Lobelia very susceptible to more reducing sediments. Therefore, we grew Lobelia for 6 months on natural and organically enriched sandy sediments to test how: (i) root oxygenation influenced degradation of organic matter and depth profiles of N and C; (ii) Lobelia and microbial O₂ consumption rates influenced pool size and depth penetration of O₂ in the sediments; and (iii) sediment enrichment influenced growth and mineral nutrition of Lobelia. 2. Naturally low‐organic sediments (0.32% DW) accumulated organic C and N during the experiment as a result of growth of Lobelia and surface micro‐algae. In contrast, surface layers of enriched sediments (0.58, 0.87 and 2.46% DW) lost organic C and N because of enhanced mineralisation rates because of oxygen availability. In deeper layers of enriched sediments no significant differences in organic C and N pools were found between plant‐covered and plant‐free sediments probably because faster organic degradation because of root oxygenation was balanced by release of organic matter from the plants and because short roots with dense Fe‐Mn coatings in the most enriched sediments constrained O₂ release. 3. Depth‐integrated O₂ pools were much higher in light than darkness, higher in plant‐covered than plant‐free sediments and higher in sandy than in organically enriched sediments. All sediments had a primary O₂ maximum 1–2 mm below the sediment surface in light because of photosynthesis of micro‐algae. Plant‐covered sediments of low organic content (0.32 and 0.58% DW) also had a secondary deep maximum (2–4 cm) because of higher O₂ release from Lobelia roots than microbial O₂ consumption. Nitrification occurred here resulting in depletion of NH and accumulation of NO. In low organic sediments, oxygen pools increased with higher plant biomass both in light and darkness. The deep O₂ and NO₃ maxima disappeared in high organic sediments of greater O₂ consumption rates and smaller O₂ release rates. 4. Lobelia was stressed by increasing O₂ consumption rate of the sediments. Plant weight and leaf number declined twofold and maximum root length declined fourfold suggesting severe problems maintaining sufficient axial O₂ transport to the root tips because of rapid radial O₂ loss. Despite markedly higher nutrient concentrations in the enriched sediments, leaf‐N declined twofold and leaf‐P declined fourfold to growth‐limiting levels. These responses can be explained by constrains on mycorrhisal activity, root metabolism and vascular transport because of O₂ depletion. Management efforts to stop the decline and ensure the recovery of the isoetid vegetation should therefore focus on improving water quality as well as sediment suitability for growth.     

Seasonal changes in temperature and nutrient control of photosynthesis, respiration and growth of natural phytoplankton communities

1. To investigate the influence of elevated temperatures and nutrients on photosynthesis, respiration and growth of natural phytoplankton assemblages, water was collected from a eutrophic lake in spring, summer, autumn, winter and the following spring and exposed to ambient temperature and ambient +2, +4 and +6 °C for 2 weeks with and without addition of extra inorganic nutrients. 2. Rates of photosynthesis, respiration and growth generally increased with temperature, but this effect was strongly enhanced by high nutrient availability, and therefore was most evident for nutrient amended cultures in seasons of low ambient nutrient availability. 3. Temperature stimulation of growth and metabolism was higher at low than high ambient temperature showing that long‐term temperature acclimation of the phytoplankton community before the experiments was of great importance for the measured rates. 4. Although we found distinct responses to relatively small temperature increases, the interaction between nutrient availability, time of the year and, thus, ambient temperature was responsible for most of the observed variability in phytoplankton growth, photosynthesis and respiration. 5. Although an increase in global temperature will influence production and degradation of organic material in lakes, the documented importance of ambient temperatures and nutrient conditions suggests that effects will be most pronounced during winter and early spring, while the remaining part of the growth season will be practically unaffected by increasing temperatures.     

Rapid oxygen exchange across the leaves of Littorella uniflora provides tolerance to sediment anoxia

1. Littorella uniflora and Lobelia dortmanna are prominent small rosette species in nutrient‐poor, soft‐water lakes because of efficient root exchange of CO₂ and O₂. We hypothesise that higher gas exchange across the leaves of L. uniflora than of L. dortmanna ensures O₂ uptake from water and underlies its greater tolerance to sediment anoxia following organic enrichment. 2. We studied plant response to varying sediment O₂ demand and biogeochemistry by measuring photosynthesis, gas exchange across leaves and O₂ dynamics in plants during long‐term laboratory and field studies. Frequent non‐destructive sampling of sediment pore water was used to track changes in sediment biogeochemistry. 3. Addition of organic matter triggered O₂ depletion and accumulation of , Fe²⁺ and CO₂ in sediments. Gas exchange across leaf surfaces was 13–16 times higher for L. uniflora than for L. dortmanna. Oxygen in the leaf lacunae of L. uniflora remained above 10 kPa late at night on anoxic sediments despite organic enrichment. Leaf content of N and P of L. uniflora remained sufficient to keep up photosynthesis despite prolonged sediment anoxia, whereas nutrient content was too low for long‐term survival of L. dortmanna. 4. High gas exchange across L. uniflora leaves improves its performance and survival on anoxic sediments compared with L. dortmanna. Lobelia dortmanna uses the same gas‐tight leaves in air and water, which makes it highly susceptible to sediment anoxia but more cost‐effective in ultra‐oligotrophic environments because of slow leaf turnover.