A key challenge in ecology is to understand how nutrients and light affect the biodiversity and community structure of phytoplankton and plant communities. According to resource competition models, ratios of limiting nutrients are major determinants of species composition. At high nutrient levels, however, species interactions may shift to competition for light, which might make nutrient ratios less relevant. The "nutrient-load hypothesis" merges these two perspectives, by extending the classic model of competition for two nutrients to include competition for light. Here, we test five key predictions of the nutrient-load hypothesis using multispecies competition experiments. A marine phytoplankton community sampled from the North Sea was inoculated in laboratory chemostats provided with different nitrogen (N) and phosphorus (P) loads to induce either single resource limitation or co-limitation of N, P, and light. Four of the five predictions were validated by the experiments. In particular, different resource limitations favored the dominance of different species. Increasing nutrient loads caused changes in phytoplankton species composition, even if the N:P ratio of the nutrient loads remained constant, by shifting the species interactions from competition for nutrients to competition for light. In all treatments, small species became dominant whereas larger species were competitively excluded, supporting the common view that small cell size provides a competitive advantage under resource-limited conditions. Contrary to expectation, all treatments led to coexistence of diatoms, cyanobacteria and green algae, resulting in a higher diversity of species than predicted by theory. Because the coexisting species comprised three phyla with different photosynthetic pigments, we speculate that niche differentiation in the light spectrum might play a role. Our results show that mechanistic resource competition models that integrate nutrient-based and light-based approaches provide an important step forward to understand and predict how changing nutrient loads affect community composition.

The influence of the Mekong River (South China Sea) on N 2 fixation and phytoplankton distribution was investigated during the lowest-and highest-discharge seasons (April 2007 and September 2008, respectively). The river plays an essential role in providing nutrients (nitrate, phosphate, silicate) for the adjacent sea and creates different salinity and nutrient gradients over different seasons. River water (salinity 0), mesohaline waters (salinity 14-32), a transition zone with salinities between 32 and 33.5, and marine waters (salinity above 33.5) were sampled at different spatial resolutions in both cruises. High N 2 fixation rates were measured during both seasons, with rates of up to 5.05 nmol N L 21 h 21 in surface waters under nitrogen-replete conditions, increasing to 22.77 nmol N L 21 h 21 in nitrogen-limited waters. Asymbiotic diatoms were found only close to the river mouth, and symbiotic diatoms, which potentially hosted diazotrophs, were most abundant in waters where N 2 fixation rates were highest, nitrate concentrations were at the detection limit, and phosphate and silicate were still available. Filamentous cyanobacteria like Trichodesmium were present only in marine waters with salinities above 33.5. Overall, N 2 fixation accounts for 1-47% of the nitrogen demand of primary production.

An extensive study on morphological parameters of ceramic sponge structures of different material, porosity, and pores per linear inch (ppi) number was performed. Volume image analyses of magnetic resonance imaging (MRI) and X-ray tomography (CT) measurements were carried out to determine experimentally the geometric specific surface area of the structures. Furthermore, face and strut diameters, porosities, and densities were assessed through conventional methods including optical microscopy, mercury porosimetry, and helium pycnometry. The resulting data were used to evaluate morphological models representing the irregular strut network by packings of regular polyhedra. On the basis of an adapted Weaire-Phelan model, a novel correlation was developed that enables the calculation of the specific surface area of sponges with high accuracy. The input parameters for this correlation are easily accessible by standard microscopy and porosimetry measurements.

Air pollution by nitrogen oxides represents a serious environmental problem in urban areas where numerous sources of these pollutants are concentrated. One approach to reduce the concentration of these air pollutants is their light-induced oxidation in the presence of molecular oxygen and a photocatalytically active building material which uses titanium dioxide as the photocatalyst. Herein, results of an investigation concerning the influence of the photon flux and the pollutant concentration on the rate of the photocatalytic oxidation of nitrogen(II) oxide in the presence of molecular oxygen and UV(A) irradiated titanium dioxide powder are presented. A Langmuir-Hinshelwood-type rate law for the photocatalytic NO oxidation inside the photoreactor comprising four kinetic parameters is derived being suitable to describe the influence of the pollutant concentration and the photon flux on the rate of the photocatalytic oxidation of nitrogen(II) oxide.