Kaj Sand-Jensen

Patterns in decomposition rates among photosynthetic organisms : the importance of detritus C:N:P content

The strength and generality of the relationship between decomposition rates and detritus carbon, nitrogen, and phosphorus concentrations was assessed by comparing published reports of decomposition rates of detritus of photosynthetic organisms, from unicellular algae to trees. The results obtained demonstrated the existence of a general positive, linear relationship between plant decomposition rates and nitrogen and phosphorus concentrations. Differences in the carbon, nitrogen, and phosphorus concentrations of plant detritus accounted for 89% of the variance in plant decomposition rates of detritus orginating from photosynthetic organisms ranging from unicellular microalgae to trees. The results also demonstrate that moist plant material decomposes substantially faster than dry material with similar nutrient concentrations. Consideration of lignin, instead of carbon, concentrations did not improve the relationships obtained. These results reflect the coupling of phosphorus and nitrogen in the basic biochemical processes of both plants and their microbial decomposers, and stress the importance of this coupling for carbon and nutrient flow in ecosystems.

Interactions among phytoplankton, periphyton, and macrophytes in temperate freshwaters and estuaries

Abstract The physico-chemical environment and the resource needs of phytoplankton, periphyton and macrophytes are markedly different. In this paper we compare the characteristics of the different phototrophs with respect to water movements and diffusive boundary layers, nutrient demands, carbon and oxygen dynamics, and light climate and light requirements. We discuss how these characteristics affect growth dynamics, biomass limitation and biotic interactions of phototrophs in natural habitats, and, finally, we discuss how plant community dominance can be predicted from ecosystem size, depth and nutrient loading. Phytoplankters live in a stirred environment with thin diffusive boundary layers, low nutrient availability and a highly variable light climate. They grow fast and have high nutrient requirements but their biomass is often nutrient limited. Diffusive boundary layers of benthic microalgae and rooted macrophytes are thicker and reduce the exchange of gases and nutrients. Sessile organisms live in a more predictable light climate but may experience severe self-shading and generally grow more slowly than phytoplankters. The nutrient requirements of rooted macrophytes are lower than those of microalgae because of low growth rates, high internal C:N:P ratios and the existence of nutrient conserving mechanisms, and nutrient limitation is less important because the plants exploit the rich nutrient pools of the sediment. The phototrophs compete for light, nutrients and inorganic carbon, and the balance among phototrophs changes with size, depth and nutrient richness of the ecosystem. Phytoplankters dominate in deep lakes and oceanic waters and may also, together with periphyton, dominate in nutrient-rich shallow waters because of shading effects on macrophytes and benthic microalgae. However, shallow lakes and estuaries with low nutrient availability in the water column are dominated by benthic phototrophs because of their lower nutrient requirements and contact to sediment nutrient pools.

Effect of epiphytes on eelgrass photosynthesis

The effect of epiphytes on eelgrass photosynthesis was measured at varying light intensities and HCO3− concentrations by means of the 14C-technique. Eelgrass was collected in Vellerup Vig, Denmark during October and November 1975. The epiphytes, mainly diatoms of the species Cocconeis scutellum Ehr., formed a crust several layers thick on the older leaves. The epiphytes reduced the photosynthetic rate of the leaves by acting both as a barrier to carbon uptake and by reducing light intensity. At optimal light intensity, the reduction was about 45% at 0.2 meq. HCO3−1−1 and it gradually decreased to nearly zero at 2.55 meq.1−1. At varying light intensity and a HCO3− concentration of 1.7 meq.1−1, corresponding to Vellerup Vig water, both effects of the epiphytes were seen. Above 7.2 mW cm−2, they caused a constant reduction of photosynthesis due to carbon deficiency. Below 7.2 mW cm−2, the reduction increased linearly to about 58% at 0.44 mW cm−2 corresponding to the increasing importance of shading from the epiphytes. Influence of epiphytic populations on photosynthesis and survival of aquatic macrophytes is discussed. It is suggested that macrophytes can limit the epiphytic stands by excreting algal antibiotics or by keeping a high replacement rate of photosynthetic tissues as illustrated by eelgrass in Vellerup Vig.

Biomass, net production and growth dynamics in an eelgrass (Zostera marina L.) population in Vellerup Vig, Denmark

Abstract The biomass of an eelgrass population in Vellerup Vig, Denmark showed a unimodal seasonal pattern, March to October 1974, with the peak in August. Biomass of leaves and flowering turions was quadrupled, biomass of rhizomes doubled from March to August. The maximum total biomass was 443 g dry wt/m2. The leaf production was determined by a leaf marking technique, which also made it possible to estimate the rhizome production. In the period 9 April to 16 October 1974, the leaf production was 856 g dry wt/m2 and the rhizome production 241 g dry wt/m2 , which made a total of 1097 g dry wt/m2. The dominance of leaf production, though leaf and rhizome biomass were of the same magnitude, arose from a higher turnover rate of leaves (1.8 % per day) than of rhizomes (0.7 % per day). On the average a new leaf was produced on each turion every 14 days. The lifetime of the leaves was about 56 days. Total radiation and not temperature seemed to control leaf production. The maximum leaf production rate of 7.9 g ...

Photosynthetic carbon assimilation in aquatic macrophytes

Abstract In submerged aquatic macrophytes the relationship between net photosynthesis and carbon concentration in the water often follows a less gradual pattern than anticipated assuming simple Michaelis-Menten kinetics. This indicates that other factors than the activity of the carboxylation enzymes are important in the regulation of photosynthesis in these plants. At low external concentrations of inorganic carbon, photosynthesis is restricted by the slow rate of diffusion from the bulk medium to the site of carboxylation, whereas the maximum photosynthetic capacity appears to be set by the enzyme activity or the turnover of intermediates in the carbon reduction cycle, including adenosine triphosphate (ATP) and reducing agents. The potential for active transport of inorganic carbon across the plasmalemma may also be of importance in some instances. Different strategies, which could be seen as adaptations to ameliorate the carbon constraints, have evolved in submerged macrophytes. These could be separated into physiological and exploitation strategies. Among the former, the most widespread is use of HCO3− in photosynthesis. The rationale for this strategy is the rather high concentration of HCO3− in most freshwaters and in seawater. Bicarbonate use has been found in approximately 50% of the species tested. Other physiological adaptations involve use of C4 acids in photosynthesis based on some kind of C4 or crassulacean acid metabolism (CAM). These strategies, however, appear to be of only minor importance, C4-like photosynthesis is found in a few freshwater and marine species and CAM is found mainly among the isoetids and in a few other species. The exploitation strategies all involve morphological features which allow the plants to avoid or reduce carbon limitation by exploiting alternative carbon sources in addition to those in the water. The most widespread is the development of floating or aerial leaves, which allow access to the atmospheric CO2 pool, where CO2 is more readily available owing to the higher diffusion rate and current velocity in air relative to water. Use of sediment-CO2, another exploitation strategy, has been recognized in several species, especially from the isoetid group. These species all have an extensive lacunal system with longitudinal channels from the roots to the leaves, providing an efficient transport route for sediment-CO2. In these species, sediment-derived CO2 can be responsible for more than 90% of the total carbon uptake. Sediment-CO2 concentrations are nearly always higher than the concentration in the water column; nevertheless, significant use of sediment-CO2 is not found in elodeid species. It has been suggested that this is due to the long diffusion path, low shoot/root ratio and a high, inherent rate of photosynthesis in these species. The carbon extraction capacity, which reflects the extent to which the plants use HCO3− or C4 acids in photosynthesis, is not a constant attribute among submerged macrophytes. However, some general trends are present. The extraction capacity is high most macroalgae, intermediate in elodeid freshwater species and low in isoetids and in elodeid species with the potential of exploiting CO2 sources other than those in water. Accordingly, light-saturated photosynthesis in marine macroalgae is often carbon saturated at natural inorganic carbon concentrations, but rarely so among freshwater macrophytes. In addition to these apparently genetic differences in carbon extraction capacity among the plant groups, environmental conditions also influence the carbon uptake characteristics. When carbon is not limiting, i.e. at high CO2 and low light, high extraction capacity and carbon affinity are suppressed. This response seems appropriate as the plants thereby reduce investment in carbon acquisition systems when not needed and instead allocate energy to increase assimilation of more limiting resources. Thus, it is apparent that submerged macrophytes possess a homeostatic capacity that reduces the effects of imbalance among resources needed. However, it is evident that aquatic macrophytes, in particular freshwater species, often have an excess capacity for carbon assimilation not realized at ambient conditions of inorganic carbon. The significance of this overcapacity is unclear, but it stresses the importance of inorganic carbon as a potential limiting factor for photosynthesis and possibly growth of submerged aquatic macrophytes.

CO 2 increases oceanic primary production

The regulation of oceanic primary production of biomass is important in the global carbon cycle because it constitutes 40% of total primary production on Earth. Here we present results from short-term experiments in the nutrient-poor central Atlantic Ocean. We find a small but significant stimulation of primary production (15-19%) in response to elevated CO2 concentrations that simulate the CO2 rise in surface waters that will occur over the next 100-200 years.

Fine-scale patterns of water velocity within macrophyte patches in streams

Rooted macrophytes in temperate lowland streams are often distributed in monospecific patches which control flow, carbon fluxes, and the abundance of invertebrates and fish. Small high-resolution hot-wire probes provided detailed measures of flow velocities within and around macrophyte patches of four plant species of contrasting morphologies in Danish streams. Flow velocity declined rapidly at the surface of the plant patches and species with large leaf area on bushy shoots (e.g. Callitriche cophocarpa and Elodea canadensis) reduced the flow more than species with streamlined, strap-formed leaves (e.g. Sparganium emersum). Variable flow-resistance resulted in flow velocities at 2 cm above the sediment which were 11-fold lower inside C. cophocarpa patches than upstream of the patches, whereas no significant differences in near-bed velocities were found inside and outside the more open patches of S. emersum. The reduced velocity within flow-resistant patches remains sufficiently fast (i.e. >1 cm s -1 ) to prevent carbon depletion and oxygen accumulation and should be optimal to photosynthesis and plant growth. The deflected flow is accelerated around the patches and contributes to form a mosaic of highly variable plant cover, flow and substrate conditions. These relations have important implications for flow resistance, areal expansion of patches and spatial variability of sediment and invertebrate composition in streams.

Alkalinity and trophic state regulate aquatic plant distribution in Danish lakes

Abstract Main distribution patterns of submerged macrophytes in a large number of Danish lakes were determined and relationships to environmental variables evaluated by different multivariate analysis techniques. The lakes varied greatly in location, size, depth, alkalinity and trophic status. There were distinct differences in the distribution of species and growth forms among the lakes. The lakes separated into five groups of characteristic species compositions. Alkalinity was the main factor responsible for the species distribution. Lakes of high alkalinity were dominated by vascular plants of the elodeid growth form, lakes of intermediate alkalinity contained a variety of elodeids and vascular plants of the isoetid growth form, while lakes of low alkalinity and low pH had several isoetids and bryophytes, but very few elodeids. Alkalinity is a close descriptor of the bicarbonate concentration, which is an important source of inorganic carbon in the photosynthesis of many elodeids. The species distribution was related to their ability to use bicarbonate and extract inorganic carbon, implying that the observed distribution has an eco-physiological foundation, though a substantial variation suggests an influence of phenotypic plasticity and local environmental heterogeneity. Trophic state also influenced the distribution of species, with very eutrophic lakes having only a few robust elodeid species able to compensate for turbid conditions, while small elodeids and slow-growing isoetid species were absent. The distance separating the lakes did not influence similarity in species composition among them.

Environmental variables and their effect on photosynthesis of aquatic plant communities

Environmental variables affect both photosynthetic pigments and enzymes and instantaneous photosynthetic rates of aquatic plants. Environmental conditions have been better described for pelagic phytoplankton than for littoral communities of macrophytes and attached microalgae which, furthermore, live in a structurally more complex and dynamic environment. Phytoplankton in well-mixed surface waters is circulated in a light gradient over a long distance, and this makes precise light adaptation difficult. Light is rapidly attenuated with depth in dense communities of macrophytes and attached algae, but the light climate is nevertheless more predictable and light adaptation simpler because of the fixed vertical position of a macrophyte leaf or an attached microalga. Dense communities of macrophytes and attached algae have high rates of photosynthesis in light and respiration in the dark per unit volume of the medium they live in. Because of reduced exchange of dissolved metabolites with the more uniform pelagial waters, they experience marked variations in dissolved inorganic carbon (DIC), CO2 and O2 with depth and light and darkness. The simultaneous depletion of DIC and CO2 and build-up of O2 which occur in the light within the diffusive boundary layers that surround leaf surfaces and attached algal communities call for particularly efficient mechanisms to assimilate inorganic carbon.

Is total primary production in shallow coastal marine waters stimulated by nitrogen loading

The global nitrogen cycle has been extensively modified by human activity to the extent that more N is fixed annually by human-driven than by natural processes (Vitousek 1994). This alteration influences production and species composition of terrestrial ecosystems (Tilman 1987) and contributes to acidification and forest dieback (Schulze 1989). The influence of nitrogen on eutrophication of coastal marine ecosystems is even stronger with profound effects on plant communities and food webs (Nixon et al. 1986). The total primary production of marine plants in coastal areas (as organic carbon or dry matter produced annually per unit of surface area within the ecosystem) is generally assumed to increase with increasing loading of nutrients from land (e.g. Boynton et al. 1982, Nixon et al. 1986, Paerl 1993). The effects of widespread eutrophication, such as oxygen deficiency and mass mortality of benthic invertebrates and fish, have alarmed the public and are considered to emerge from the enhanced oxygen consumption required to mineralize the increasing amounts of organic matter produced within the ecosystem. Much of our current analysis and understanding of energy flow and carbon and oxygen dynamics in the coastal marine environment presuppose that anthropogenic loading and total primary production are directly positively related. In the following we will argue, however, that the total primary production of shallow coastal areas, i.e. the combined production of microand macroscopic plants living in the water and at the bottom, does not change systematically by nutrient enrichment. Our analysis is based on a comparison of published rates of total primary production and nitrogen loading from land using the same approach as Boynton et al. (1982) and Nixon et al. (1986) to analyse the dependence of phytoplankton production on nitrogen loading. The data originate from different temperate coastal ecosystems with very different morphometry and hydraulics. The quality of the data (in terms of suitability of methods and number of measurements) also varies considerably among the systems but does not systematically influence the patterns reported. We have tried to ensure a fair representation of systems which are most likely to respond to enhanced nitrogen loading by including relatively deep (mean depth down to 40 m) and clearly phytoplankton dominated ecosystems and by omitting data we suspect overestimate benthic primary production. Thereby, we obtain a balanced analysis of our hypothesis.