Changes In Ecosystem Structure Research Articles

Physico-Chemical Disturbances Associated with Spatial and Temporal Variation in a Mediterranean River

We examine environmental survey data to reveal partial aspects of the longitudinal structure and function of a Mediterranean river. The study area is the Ter River which experiences various disturbances (spates, pollution, reservoirs) that result in discrete patches along the river. In this paper, we use physico-chemical data to measure and interpret the dynamics of these patches through time and space. Principal Components Analysis (PCA) of the physico-chemical variables revealed longitudinal chemical structure (FI) and temporal variation in chemical composition due to discharge fluctuations (FII). We found that the chemistry of headwater regions is more constant than lower reaches since this river is heavily perturbed by human activities downstream. The chemistry of lower reaches is more time dependent. Temporal and spatial variation are not independent because both processes transform external energy into changes in ecosystem structure.

The first biomanipulation conference: a synthesis

At the First Biomanipulation Conference held in Amsterdam (8–11 August 1989), studies presented considered mainly trophic interactions in lakes, enclosures and laboratory systems. Studies on the interactions between phytoplankton and zooplankton emphasized the edibility of the phytoplankton in relation to the zooplankton size structure and the trophic state. Most lake experiments involved 50–100% reduction in fish standing stock or alternatively heavy stocking with piscivorous fish. The most dramatic effects of biomanipulation were found in shallow, eutrophic lakes which exhibited radical changes in ecosystem structure because of changes in light climate and consequently, luxuriant development of macrophytes. There was still much controversy about the top-down effects in relation to trophic state, especially those concerning the role of fish and zooplankton in the development and succession of phytoplankton. Many experiments showed indirect effects within the food web, emphasizing the importance of feedbacks and the complexity of the food web rather than the simplicity of the food chain. The stabilizing effects of refugia for zooplankton and fish on the ecosystem were stressed. Shallow lakes responded generally more rapidly to biomanipulation and this was most successfully accomplished when TP concentration was<50µg l−1, even though in a few cases at 10–20 fold higher TP concentrations (mostly PO4-P lakes) the results achieved could be maintained for two or more summers. For a guaranteed success of the measures an almost complete removal of fish appeared to be indispensible; moreover in many cases removal of benthivorous fish appeared to be even more important than that of planktivorous fish.

Experimental Perturbations of Whole Lakes as Tests of Hypotheses concerning Ecosystem Structure and Function

Production, respiration, nutrient concentrations and changes in biological communities were monitored for several years in two experimentally eutrophied and two experimentally acidified lakes in the Experimental Lakes Area, Canada. Results were examined to detect changes in ecosystem structure and function that might be sensitive indicators of ecosystem stress. Phytoplankton production in eutrophied lakes was higher than in reference lakes. It decreased quickly to background values when fertilization was terminated. Winter respiration was unchanged, P/R ratios were constant under constant N/P fertilization, but decreased for three years after the N/P ratio in fertilizer was decreased from 14:1 to 5:1 by weight. Phytoplankton production was not affected by acidification to a pH of 5.0. Winter respiration declined at pH values of 5.1 and below, causing P/R to be highest at the lowest pH values. Respiration returned to normal when the pH of the lake was increased to 5.4. Gross periphyton production and respiration both increased with acidification, but P/R ratios declined at pH values below 6.2. Nitrification in both acidified lakes ceased at pH values of 5.4 to 5.7. It recovered when pH was allowed to increase, but with a time lag of one year. No other nutrient cycles appeared to be disrupted. Both eutrophication and acidification caused declines in species diversity among several taxonomic groups. Among phytoplankton, both stresses caused an increase in size of organisms in summer. Among higher taxa, there was not a consistent tendency for either small or large organisms to be favored. Periphyton metabolism was the most sensitive indicator of acidification, followed by taxonomic changes in several groups. Ecosystem-level production and respiration were the most resistant properties to acid stress. The earliest serious changes in ecosystems and food webs occurred when acidification eliminated acid-sensitive organisms that were also the sole occupants of key ecological niches. Such situations occur frequently in northern lakes.

Delineating a wetland boundary using vegetative, soil and hydrologic characteristics: A Florida cypress dome example

Vegetation indices have historically and conveniently been used to delineate wetland boundaries. This delineation method is often successful where abrupt or obvious changes in ecosystem structure occur. More obscure vegetation patterns however, may require a multiple parameter approach in order to delineate such a boundary. This study was designed to integrate soil and hydrologic parameters together with vegetation to enhance the poorly defined wetland boundary in north Florida. Edaphic factors found to be useful indicators of hydric conditions included percent organic matter in the A horizon and degree of mottling in the argillic horizon. Tree buttressing and lichen line formation also served as reliable indicators of inundation. The wetland boundary delineated by the above mentioned soil factors closely corresponded with the vegetation boundary determined both visually and by vegetation analysis.

The ecology of Lake Nakuru (Kenya) : III. Abiotic factors and primary production.

Abiotic factors, standing crop and photosynthetic production were studied in the equatorial alkaline-saline closed-basin Lake Nakuru (cond. 10,000-160,000 μS). Meteorological conditions and abiotic factors offer suppositions for a high primary productivity: mean solar radiation is 450-550 kerg·cm-2·s-1, with little seasonal variation, regular winds circulate the lake every day and nutrient concentrations are usually high (>100 μg P-PO4·l-1). Oxygen concentrations near sediments were <1 gO2·m-3 for at least 6 h·d-1 in 1972/73, resulting in a release of ∼45 mg P-PO4·m-2·d-1. Attenuation coefficients vary from 3.6-16.5 according to algal densities and mean depth from 0-400 cm. Algal biomass was ∼200 g·m-3 (d.w.) in 1972/73, due to a lasting Spirulina platensis bloom (98.5% of algal biomass). In 1974 algal biomass suddenly dropped to 50 g·m-3 (d.w.). Spirulina and several consumer organisms almost vanished, but coccoid cyanobacteria, Anabaenopsis and diatoms increased. Several causes for this change in ecosystem structure are discussed. The use of the light/dark bottle method to measure photosynthetic production in eutrophic alkaline lakes is discussed and relevant experiments were done. Oxygen tensions of 2-35 gO2·m-3 do not influence primary production rates. Net photosynthetic rates (mgO2·m-3·h-1; photosynthetic quotient=1.18) reached 12-17.7 in 1972/73 and 2-3 in 1974, but vertically integrated rates were only 1-1.4 in 1972/73 and 0.8 in 1974, and daily net photosynthetic rates (gO2·m-3·24 h-1) 3.5 in 1972/73 and 1 in 1974. 50% of areal rates were produced within the 10 most productive cm of the depth profile. The disproportion between high algal standing crops and relatively low production rates is due to self-shading of the algae, reducing the euphotic zone to 35 cm in 1972/73 and 77 cm in 1974. Efficiency of light utilization is 0.4-2%, varying with time of day and phytoplankton density. In situ efficiencies show an inverse relationship to light intensities. Photosynthetic rates of L. Nakuru remain within the range of other African lakes (0.1-3 gO2·m-2·h-1). The relation of O2 produced/Chl a of the euphotic zone is 50% lower then in tropical African freshwater lakes and conforms to lakes of temperate regions.