Nutrient-enriched Sites Research Articles

Ecological aspects of ntcA gene expression and its use as an indicator of the nitrogen status of marine Synechococcus spp.

Nitrogen nutrition in cyanobacteria is regulated by NtcA, a transcriptional activator that is subject to negative control by ammonium. Using Synechococcus sp. strain WH7803 as a model organism, we show that ntcA expression was induced when cells were exposed to nitrogen stress but not when they were subjected to phosphorus or iron deprivation. Transcript levels accumulated in cells grown on a variety of inorganic and organic nitrogen sources, with the sole exception of ammonium. ntcA transcription was induced when ammonium levels dropped below 1 microM and reached maximal levels within 2 h. Furthermore, the addition of more than 1 microM ammonium led to a rapid decline in ntcA mRNA. The negative effect of ammonium was prevented by the addition of L-methionine-D,L-sulfoximine (MSX) and azaserine, inhibitors of ammonium assimilation. Thus, basal ntcA transcript levels are indicative of ammonium utilization. Conversely, the highest ntcA transcript levels were found in cells lacking a nitrogen source capable of supporting growth. Therefore, maximal ntcA expression would indicate nitrogen deprivation. This state of nitrogen deprivation was induced by a 1-h incubation with MSX. The rapid response of ntcA gene expression to the addition of ammonium and MSX was used to design a protocol for assessing relative ntcA transcript levels in field populations of cyanobacteria, from which their nitrogen status can be inferred. ntcA was basally expressed in Synechococcus at a nutrient-enriched site at the northern tip of the Gulf of Aqaba, Red Sea. Therefore, these cyanobacteria were not nitrogen stressed, and their nitrogen requirements were met by regenerated nitrogen in the form of ammonium.

The green tide threat in the UK — a brief overview with particular reference to Langstone Harbour, south coast of England and the Ythan Estuary, east coast of Scotland

Summary World-wide, our coastal waters have been subject to an increased nutrient input since the latter part of the nineteenth century. This has led to the eutrophication or ‘nutrient pollution’ of many coastal sites, including Langstone Harbour and the Ythan Estuary here in the UK. Eutrophication at these and, indeed, at other nutrient enriched sites is evident by the appearance of large blooms of fast-growing opportunistic macroalgae. Blooms of macroscopic species of green algae (Chlorophyta: Ulvophyceae) are particularly common and the phenomenon is often referred to as the occurrence of green tides. Green tides may have a dramatic environmental impact, causing much damage to the local ecosystem. Numerous strategies have hence been employed in order to combat the problem, but to date there has been limited success. For this reason, current research in the UK is aimed at increasing our knowledge of green tide algae in terms of their ecophysiology, whilst further investigation of the nutrient pathways and fluxes within specific ecosystems has been deemed necessary. It is anticipated that this ‘backto basics’ approach will ultimately contribute to the development of new, successful eutrophication management strategies.

Keywords: ornithochory; fleshy fruit; succession; species losses; cultural biotope.

Abandonment of traditional agricultural practices in fruit orchards on hillsides in Central Germany results in successive changes in vegetation. We examined three hypotheses relating to these changes: (1) thickets of fleshy-fruited plants develop around planted trees as a result of ornithochory and local soil and site amelioration, (2) woody plants have long-term effects on soil fertility, and (3) thicket development reduces the plant-species diversity of semi-dry grassland between the trees. Field observations and nursery experiments supported the first and second hypotheses. Increased soil fertility, a seed rain of fleshy-fruited shrubs, and shrub establishment occurred mainly around planted fruit trees. Soils from old tree-planting sites remained fertile after the trees had died and disappeared. The third hypothesis was rejected because no decrease in species' richness or diversity occurred during the initial stages of thicket formation. Nevertheless, herbaceous plant species characteristic of the Festuco-Brometea community were absent from the vegetation and seed bank of shaded, nutrient-enriched sites. Conservation of semi-dry grasslands following orchard abandonment will therefore require active control of woody plants.

Reconstructing Historical Changes in Everglades Plant Community Composition Using Pollen Distributions in Peat

ABSTRACT During the past century, the historical hydrology and nutrient regimes of the Everglades, a 700,000 hectare freshwater wetland in southern Florida, have been altered by agricultural and urban development. Changes in plant community composition (e.g. encroachment of cattail (Typha domingensis Pers.) into sawgrass (Cladium jamaicense Crsntz) and slough communities) have been observed in areas receiving nutrient and water loadings from agricultural activities. The distribution of 210Pb and pollen in depth increments of peat was measured at a nutrient-enriched site and an unenriched site in Water Conservation Area 2A (WCA 2A) to describe how nutrient enrichment and water management might have altered the historical plant species composition of the Everglades. Species composition of the plant communities has changed during the past 20 to 50 years at both enriched and unenriched sites. At the unenriched location, there has been a general shift in community composition since 1950 as slough species (arrowhead (Sagittaria spp.), water lily (Nymphaea spp.)) have decreased and terrestrial plants (pigweed (Amaranthaceae), ragweed (Ambrosia spp.), and Eupatorium spp.) have increased. The nutrient-enriched site also has seen an increase in terrestrial plant species since about 1950. In addition, since 1970, the sawgrass and slough communities at the enriched site have been replaced by a cattail-dominated community. The increase in terrestrial vegetation at both sites since 1950 could be due to drainage activities (construction of canals and levees) along the eastern perimeter of the Everglades and in the Everglades Agricultural Area (EAA) during the 1950s. The increase in cattail and concurrent decline in slough species at the nutrient-enriched site since 1970 probably is the result of the massive inputs of water (459,000,000 m3yr−1) and nutrients (1814 MT Nyr−1, 60 MT Pyr−1) from the EAA since WCA 2A was impounded in 1961.

Colonization by lichens and the development of lichen-dominated communities in the maritime Antarctic

Three long-term studies of lichen growth and colonization have been undertaken at Signy Island, South Orkney Islands, in the maritime Antarctic. Small individual thalli of several crustose species and uncolonized plots on 12 fresh rock surfaces were photographically monitored at intervals of 3–4 years over a period of up to 20 years. The development of Ochrolechia frigida colonies on a regenerating moss bank, recently uncovered by a receding glacier, was similarly monitored. The results indicate that many lichens growing in sites enriched by nitrogenous compounds derived from populations of sea birds, have relatively rapid colonization and growth rates. Mean percentage increase in thallus area can be as high as 15–32% per annum in some nitrophilous saxicolous species (e.g. Acarospora macrocyclos, Xanthoria elegans and species of Buellia and Caloplaca), but as low as 0·4–4% in nitrophobous species ( Lecanora physciella, Lecidea sp., Rhizocarpon geographicum). Umbilicaria antarctica and Usnea antarctica also yielded data indicating high growth rates, with colonist plants reaching several centimetres after 20 years. Colonization by mixed assemblages of lichens of new rock surfaces can attain 40→90% cover after 20 years in nutrient-enriched sites, and even 20–25% in non-biotically influenced sites. Colonization by or increase in extant O. frigida on the regenerating moribund moss bank was also quite rapid. It is suggested that the ~exceptionally large thalli of several lichen species and the locally extensive dense lichen fellfield communities in the maritime Antarctic may be much younger than previously supposed.

Colonization by Lichens and the Development of Lichen-Dominated Communities in the Maritime Antarctic

Three long-term studies of lichen growth and colonization have been undertaken at Signy Island, South Orkney Islands, in the maritime Antarctic. Small individual thalli of several crustose species and uncolonized plots on 12 fresh rock surfaces were photographically monitored at intervals of 3–4 years over a period of up to 20 years. The development of Ochrolechia frigida colonies on a regenerating moss bank, recently uncovered by a receding glacier, was similarly monitored. The results indicate that many lichens growing in sites enriched by nitrogenous compounds derived from populations of sea birds, have relatively rapid colonization and growth rates. Mean percentage increase in thallus area can be as high as 15–32% per annum in some nitrophilous saxicolous species (e.g. Acarospora macrocyclos, Xanthoria elegans and species of Buellia and Caloplaca), but as low as, 0·4–4% in nitrophobous species (Lecanora physciella, Lecidea sp., Rhizocarpon geographicum). Umbilicaria antarctica and Usnea Antarctica also yielded data indicating high growth rates, with colonist plants reaching several centimetres after 20 years. Colonization by mixed assemblages of lichens of new rock surfaces can attain 40->90% cover after 20 years in nutrient-enriched sites, and even 20–25% in non-biotically influenced sites. Colonization by or increase in extant O. frigida on the regenerating moribund moss bank was also quite rapid. It is suggested that the exceptionally large thalli of several lichen species and the locally extensive dense lichen fellfield communities in the maritime Antarctic may be much younger than previously supposed.

Colonization by lichens and the development of lichen-dominated communities in the maritime Antarctic

Three long-term studies of lichen growth and colonization have been undertaken at Signy Island, South Orkney Islands, in the maritime Antarctic. Small individual thalli of several crustose species and uncolonized plots on 12 fresh rock surfaces were photographically monitored at intervals of 3–4 years over a period of up to 20 years. The development of Ochrolechia frigida colonies on a regenerating moss bank, recently uncovered by a receding glacier, was similarly monitored. The results indicate that many lichens growing in sites enriched by nitrogenous compounds derived from populations of sea birds, have relatively rapid colonization and growth rates. Mean percentage increase in thallus area can be as high as 15–32% per annum in some nitrophilous saxicolous species (e.g. Acarospora macrocyclos, Xanthoria elegans and species of Buellia and Caloplaca), but as low as 0.4–4% in nitrophobous species ( Lecanora physciella, Lecidea sp., Rhizocarpon geographicum). Umbilicaria antarctica and Usnea antarctica also yielded data indicating high growth rates, with colonist plants reaching several centimetres after 20 years. Colonization by mixed assemblages of lichens of new rock surfaces can attain 40–>90% cover after 20 years in nutrient-enriched sites, and even 20–25% in non-biotically influenced sites. Colonization by or increase in extant O. frigida on the regenerating moribund moss bank was also quite rapid. It is suggested that the exceptionally large thalli of several lichen species and the locally extensive dense lichen fellfield communities in the maritime Antarctic may be much younger than previously supposed.

Fluctuations in sawgrass and cattail densities in Everglades Water Conservation Area 2A under varying nutrient, hydrologic and fire regimes

A 6 year (1986–1991) study in Water Conservation Area 2A (WCA2A), a remnant portion of the Florida Everglades, was conducted in which multiple plots within six study sites across a surface water nutrient gradient were monitored for water quality, depth, and sawgrass ( Cladium jamaicense Crantz) and cattail ( Typha domingensis Pers.) densities. A negative logarithmic relationship was demonstrated between total P concentrations and distance from the nutrient source (levee L-39). Dense cattail stands illustrated by remote sensing in 1987 coincided spatially with high sediment total P concentrations measured in 1990. Cattail density appeared to increase more rapidly than did sawgrass densities during wet years. Cattail decline was greater at nutrient enriched sites than at the low nutrient site during dry years. Drought stimulated sawgrass expansion at nutrient enriched sites, but not at the low nutrient site. Cattail increased at both nutrient enriched and low nutrient sites following fire, yet fire and nutrient enrichment together appeared to stimulate greater increases in cattail density. Multi- and univariate ANOVA revealed significant nutrient effects on plant density during the last 3 years of the study ( P<0.05). Stepwise regression analysis suggested that total P loading into WCA2A best explained plant density fluctuations at three of the four sites closest to the levee, while hydrology best explained plant density fluctuations at the site most distant from the levee. Although the single low nutrient site provided an inadequate basis for understanding sawgrass and cattail dynamics under low nutrient conditions, the results of this study support two hypotheses of causal factors leading to the spread of cattail in WCA2A: (1) nutrient enrichment stimulates cattail growth; (2) a prolonged hydroperiod is conducive to cattail proliferation.

Protozoan communities of the Flint River-Lake Blackshear ecosystem (Georgia, USA)

Freshwater protozoa are poorly characterized in river ecosystems. We report here the richness of the protozoan biotas in relation to environmental gradients from an ecosystematic survey of a large, coastal plain river. Communities were collected from natural and artificial substrates concurrent with water chemistry analysis at 11 sites along the Flint River and Lake Blackshear impoundment. Community similarity, the distribution of collected taxa in functional feeding groups, and the relation of communities to environmental gradients were evaluated. Two principal compenents determined from water chemistry data showed important downstream gradients of decreasing water hardness and increasing nutrient levels. Taxonomic richness was high; 200 to 450 taxa were collected depending on season and collecting technique. Artificial substrates provided the richest collections. Bactivorous species were the vast majority of all taxa collected. Community composition showed an orderly transition from upstream to downstream, and photosynthetic forms were enhanced at nutrient enriched sites. Communities were strongly influenced by increasing nutrient levels. Protozoan community analysis showed that microbial community composition reflects human influences on river ecosystems. Since microbial species exploit detrital resources and respond sensitively to human influences, they can provide important information regarding ecosystem conditions.

ALGAL BIOASSAY AND GROSS PRODUCTIVITY EXPERIMENTS USING SEWAGE EFFLUENT IN A MICHIGAN WETLAND'

ABSTRACT: One component of the filamentous algal community of a northern fen ecosystem in central Michigan was studied under conditions of nutrient enrichment by secondarily treated sewage effluent during one growing season. The productivity of Cladophora spp. measured by continuous flow bioassay was 2.6 g dry weight m day at the site of effluent addition compared to 0.085 g m day at the control site. Under conditions of nutrient enrichment, uptake by bioassay Cladophora spp. averaged 12 mg m−2day−1for phosphorus and 55 mg m−2day−1for nitrogen, compared to 0.01 mg m−2 day−1and 0.16 mg m−2day−1for phosphorus and nitrogen, respectively, in the control area. At the end of the growing season approximately 4.3 g N m−2 and 0.96 g P m−2were immobilized in Cladophora algal biomass. Algal growth temporarily immobilized 3.0 percent of the nitrogen and 1.0 percent of the phosphorus added as sewage effluent. Gross productivity of surface water in the fen averaged 1.5 g O2m−2day−1at the nutrient enriched site, compared to 0.5 g O2 m−2day−1at the control area. Gross productivity, community respiration and reaeration constant values in the fen were similar to data collected by other researchers in shallow water aquatic systems, but only at the fertilized sites.