Inorganic Nutrient Pools Research Articles

A review of the feedbacks between bivalve grazing and ecosystem processes

This paper gives an overview of interactions betweenbivalve grazing and ecosystem processes, that mayaffect the carrying capacity of ecosystems for bivalvesuspension feeders. These interactions consist of anumber of positive and negative feedbacks.Bivalve grazing can result in local food depletion,which may negatively influence bivalve growth. On alarger scale, it may induce a top-down control ofphytoplankton biomasss, and structural shifts inphytoplankton composition. In the case of harmfulalgal blooms, phytoplankton may negatively affectbivalve grazing rates.The processing of large amounts of particulate mattermay change nutrient cycling on the scale of estuaries,and can result in changes in the inorganic nutrientpool available for phytoplankton, through regenerationand reduced storage of nutrients in algal biomass.This can reduce nutrient limitation of thephytoplankton and stimulate algal growth rates.Observations from mesocosm studies suggest that apositive feedback from bivalve grazing onphytoplankton growth may also change the physiologicalstate of the algae and improve food quality.

Effects of Carbohydrate Amendments on Nutrient Partitioning, Plant and Microbial Performance of a Grassland-Shrub Ecosystem

Nutrient partitioning, microbial and plant nutrient assimilation and biomass production were investigated after addition of carbon of low and high decomposability (sawdust and sugar, respectively) to the soil of a non-agricultural mixed grassland-shrub ecosystem at intervals during one growing season. The aim was to investigate how the energy source available for soil microorganisms, and the quality of the source, interacted with ecosystem nutrient circulation and plant performance. By the end of the season, sugar, but not sawdust amendment, had reduced the pools of soil inorganic N and P strongly whereas the pools of N and P in the microbial biomass had increased. Nutrient uptake and biomass production of herbs, but not of shrubs, declined to about one third of that in non-sugar amended soils, and tissue N and P concentrations declined. Total P in circulation between the soil inorganic, the microbial and the plant biomass pools was unchanged, whereas N declined as a result of a comparatively larger decrease in plant biomass N and soil inorganic N than the simultaneous gain in microbial biomass N. The results show, firstly, that microbial nutrient immobilization and competition with plants for inorganic soil nutrients can affect plant nutrient uptake and reduce biomass production strongly if microbial activity is stimulated by an increase of the available carbon in the soil. Secondly, the lack of increase of P or, for N, the decline of the amounts in circulation between the biological pools and the soil inorganic pool indicates that ecosystem gross mineralization was not stimulated when extra labile carbon became available. Hence, the increased microbial demand for nutrients without any stimulation of nutrient mineralization depleted the soil inorganic nutrient pool, led to reduced plant nutrient uptake and biomass production, and to a higher proportion of ecosystem nutrients allocated to the microbial biomass.

Spatial and Temporal Patterns of Substrate Physicochemical Parameters in Different-aged Barrier Island Marshes

Physicochemical substrate parameters were measured monthly from June to October 1990 in two young and two old barrier island marshes off the eastern shore of Virginia. Porewater salinity, substrate redox potential (EH), organic matter content, and sediment grain size were measured at three depths at high marsh and low marsh stations of all four sites. Porewater ammonium and phosphate, and extractable ammonium were also measured at one old and one young marsh site. Higher sand content, lower organic content, and lower elevation of the young sites relative to the old sites was representative of differences in development time between the different-aged sites. Porewater salinity peaked in mid-summer at higher levels at the old sites relative to the young sites and was higher at the high marsh stations compared to the low marsh stations of both old sites and one young site. Substrate EH was higher at the young sites, presumably as a function of better drainage related to a coarser-grain size and a steeper topographic slope relative to the old sites. Nutrient pools increased throughout the growing season in both the young and old sites, but peaked at higher levels in the old marsh below 10 cm which was consistent with the finer-grain size and higher organic content at depth of the latter site. Differences in physical substrate characteristics, EH, and nutrient pools, found in this study, generally agree with other age comparison studies which are usually between young, created marshes and older, natural marshes. Created marshes have been shown to develop inorganic nutrient pools comparable to established, natural marshes in 2-15 years. The results of this study, representing few comparisons of substrate nutrient concentration in natural, different-aged marshes, demonstrated that nutrient pools are comparable at 10 cm between 10-13-year-old marshes and marshes whose age is most likely in the hundreds of years. Nutrient pools at 30 and 50 cm of young marshes take longer than 10-13 years to reach levels comparable to older marshes at the same depths.

What controls levels of dissolved phosphate and ammonium in surface waters?

Dissolved inorganic nutrient pools are small relative to particulate pools, and dissolved pools turnover rapidly. It has been observed that pools change little from day to day on the sampling scales usually employed. A simple model is presented where uptake and regeneration rates balance to cause a local steady state concentration for dissolved inorganic nutrients. Enrichment and dilution perturbation experiments with lake water support the idea of steady state nutrient concentrations. Although inorganic nutrient concentrations are often controlled by biota, the absolute concentrations present tell little about the activity of that biota.

Comparison of bacterial dynamics in tidal creeks of the lower Delmarva Peninsula, Virginia, USA

Bacterial biornass, abundance, and product~vity were greater in 3 tidal marsh creeks on the Chesapeake Bay side of the lower Delmarva Peninsula than in nearby creeks of the seaside coastal lagoon complex (biomass: 462 and 71 ng C ml' ; abundance: 12 X 10' and 3.8 X 106 cells ml-l ; productivity: 46 and 7.3 ng C rnl-' h-'; bayside and seaside respectively). Bacterial cell-size distributions were also significantly different between the seaside and bayside creeks, with a larger proportion of smaller cells dominating samples from the seaside creeks. Bayside and seaside concentrat~ons of total suspended solids (TSS) and dissolved organic carbon (DOC) were similar (approximately 49 mg 1.' TSS and 3.5 mg I ' DOC) The amount of organic matter (OM) and chlorophyll a was higher in the bayside creeks, while inorganic N and P concentrations were higher in the seaside creeks (OM. 9 0 and 3.4 mg 1 l ; chlorophyll a. 6 0 and 4.1 g 1.'; PO, 0.2 and 1.2 FM; NH,' 0.6 and 1.2 ubI; bayside and seaside respectively). The high inorganic nutrient pools combined with the low levels of bacterial productivity suggest that bacterial production is not l~nli ted by N or P in the seaside creeks and that the amount of carbon moving through the bacterial loop is much less than on the bayside. In fact, DOC turnover times were much longer for the seaside (22 d ) than for the bayside (6 d ) . Reasons for the observed differences in bacterial dynamics for the bay and seasides are not known specifically, but may be related to differences in the source of the DOC (marsh grass vs phytoplankton), grazlng on the bacterial cells, or bacterial community structure

The Influence of Nutrient Availability on the Ecosystem Behaviour of a Salt Water Lake

The influence of the availability of silicon, nitrogen and phosphorus on the behaviour of the ecosystem in the salt Lake Grevelingen is evaluated. Quantification of the nutrientcycling is essentially based on a dynamic calculation of detritus formation resulting from mortality of phytoplankton and microphytobenthos, including mineralization as function of temperature and substrate concentration. Also changes of the dissolved inorganic nutrientpools resulting from primary production, mineralization and net loading are dynamically ca1culated. Silicon and nitrogen are reversibly fixed in bottomdetritus during the summer halfyear, which implies that primary production depends on the balance between accumulation of detritus in the bottom and mineralization. In contrast the dynamics of phosphorus depend only to a minor extent on primary production and net loading. The net yearly load of nitrogen equals the amount of nitrogen present in the waterphase during the winterperiod. However winter concentrations do not increase and nitrogen remains a limiting factor for primary production. 3-4 g N/m2/year is apparently removed from the lake by denitrification. The environmental factors enabling denitrification are important for consolidation of the ecosystem behaviour. From a management point of view stratification which inhibits denitrification, should be avoided.