Showing papers on "Productivity (ecology) published in 2000"

Ecosystem size determines food-chain length in lakes

Abstract: Food-chain length is an important characteristic of ecological communities: it influences community structure, ecosystem functions and contaminant concentrations in top predators. Since Elton first noted that food-chain length was variable among natural systems, ecologists have considered many explanatory hypotheses, but few are supported by empirical evidence. Here we test three hypotheses that predict food-chain length to be determined by productivity alone (productivity hypothesis), ecosystem size alone (ecosystem-size hypothesis) or a combination of productivity and ecosystem size (productive-space hypothesis). The productivity and productive-space hypotheses propose that food-chain length should increase with increasing resource availability; however, the productivity hypothesis does not include ecosystem size as a determinant of resource availability. The ecosystem-size hypothesis is based on the relationship between ecosystem size and species diversity, habitat availability and habitat heterogeneity. We find that food-chain length increases with ecosystem size, but that the length of the food chain is not related to productivity. Our results support the hypothesis that ecosystem size, and not resource availability, determines food-chain length in these natural ecosystems.

The relationship in lake communities between primary productivity and species richness

Abstract: An understanding of the relationship between species richness and productivity is crucial to understanding biodiversity in lakes. We investigated the relationship between the primary productivity of lake ecosystems and the number of species for lacustrine phytoplankton, rotifers, cladocerans, copepods, macrophytes, and fish. Our study includes two parts: (1) a survey of 33 well-studied lakes for which data on six major taxonomic groups were available; and (2) a comparison of the effects of short- and long-term whole-lake nutrient addition on primary productivity and planktonic species richness. In the survey, species richness of all six taxa showed a significant quadratic response to increased annual primary productivity ( 14 C estimate, g C-m -2 -yr -1 ) when lake area is taken into account. However, the richness-productivity relationship for phytoplankton and fish was strongly dependent on lake area. The relationship for phytoplankton, rotifers, cladocerans, copepods, and macrophytes was significantly unimodal. Species richness generally peaked at levels of primary productivity in the range of 30-300 g C-m -2 -yr -1 . For the average lake size, the highest biodiversity tended to occur in lakes with relatively low primary productivity, such as those found in the Northern Temperate Lakes Long-Term Ecological Research (LTER) site in the upper Midwest (United States) and in the Experimental Lakes Area of Ontario (Canada). Based on short-term (3 yr) and long-term (21-24 yr) experiments, we tested whether individual lakes respond to whole-lake enrichment experiments in the manner suggested by analyses of survey data. Experimental addition of nutrients produced varied and unpredictable responses in species richness, probably due to transient dynamics and time lags. Responses to nutrient addition were taxon and lake specific. Phytoplankton showed a variety of relationships between species richness and pelagic primary productivity (PPR), depending on the history of enrichment and recovery. No significant effect of primary productivity on rotifer richness occurred in any of the experimental lakes, whereas richness of crustacean zooplankton was negatively correlated with primary productivity in both the short- and long-term experiments.

Fertilization effects on species density and primary productivity in herbaceous plant communities

Abstract: Fertilization experiments in plant communities are often interpreted in the context of a hump-shaped relationship between species richness and productivity. We analyze results of fertilization experiments from seven terrestrial plant communities representing a productivity gradient (arctic and alpine tundra, two old-field habitats, desert, short- and tall-grass prairie) to determine if the response of species richness to experimentally increased productivity is consistent with the hump-shaped curve. In this analysis, we compared ratios of the mean response in nitrogen-fertilized plots to the mean in control plots for aboveground net primary productivity (ANPP) and species density (D; number of species per plot of fixed unit area). In general, ANPP increased and plant species density decreased following nitrogen addition, although considerable variation characterized the magnitude of response. We also analyzed a subset of the data limited to the longest running studies at each site (]4 yr), and found that adding 9 to 1 3gNm 2 yr 1 (the consistent amount used at all sites) increased ANPP in all communities by approximately 50% over control levels and reduced species density by approximately 30%. The magnitude of response of ANPP and species density to fertilization was independent of initial community productivity. There was as much variation in the magnitude of response among communities within sites as among sites, suggesting community-specific mechanisms of response. Based on these results, we argue that even long-term fertilization experiments are not good predictors of the relationship between species richness and productivity because they are relatively small-scale perturbations whereas the pattern of species richness over natural productivity gradients is influenced by long-term ecological and evolutionary processes.

The influence of arbuscular mycorrhizae on the relationship between plant diversity and productivity

Abstract: Ecological theory predicts a positive and asymptotic relationship between plant diversity and ecosystem productivity based on the ability of more diverse plant communities to use limiting resources more fully. This is supported by recent empirical evidence. Additionally, in natural ecosystems, plant productivity is often a function of the presence and composition of mycorrhizal associations. Yet, the effect of mycorrhizal fungi on the relationship between plant diversity and productivity has not been investigated. We predict that in the presence of AMF, productivity will saturate at lower levels of species richness because AMF increase the ability of plant species to utilize nutrient resources. In this study we manipulated old-field plant species richness in the presence and absence of two species of AMF. We found that in the absence of AMF, the relationship between plant species richness and productivity is positive and linear. However, in the presence of AMF, the relationship is positive but asymptotic, even though the maximum plant biomass was significantly different between the two AMF treatments. This is consistent with the hypothesis that AMF increase the redundancy of plant species in the productivity of plant communities, and indicates that these symbionts must be considered in future investigations of plant biodiversity and ecosystem function.

No Consistent Effect of Plant Diversity on Productivity

Abstract: Hector et al . ([1][1]) reported on BIODEPTH, a major international experiment on the response of plant productivity to variation in the number of plant species. They found “an overall log-linear reduction of average aboveground biomass with loss of species,” leading to what the accompanying

Life History and Production of Stream Insects

Abstract: ▪ Abstract Studies of the production of stream insects are now numerous, and general factors controlling the secondary production of stream communities are becoming evident. In this review we focus on how life-history attributes influence the production dynamics of stream insects and other macroinvertebrates. Annual production of macroinvertebrate communities in streams world-wide ranges from approximately 100 to 103 g dry mass m−2. High levels are reported for communities dominated by filter feeders in temperate streams. Filter feeding enables the accrual and support of high biomass, which drives the very highest production. Frequently disturbed communities in warm-temperate streams are also highly productive. Biomass accrual by macroinvertebrates is limited in these streams, and production is driven by rapid growth rates rather than high biomass. The lowest production, reported for macroinvertebrate communities of cool-temperate and arctic streams, is due to the constraints of low seasonal temperatures ...

The seasonal cycle of phytoplankton biomass and primary productivity in the Ross Sea, Antarctica

Abstract: Phytoplankton standing stocks and carbon assimilation were measured during four cruises to the southern Ross Sea, Antarctica during 1996 and 1997 in order to assess the details of the seasonal cycle of biomass and productivity. The seasonal composite showed that phytoplankton biomass increased rapidly during the austral spring, and integrated chlorophyll reached a maximum during the summer (January 15) and decreased thereafter. Particulate matter ratios (carbon:nitrogen, carbon:chlorophyll) also showed distinct seasonal trends with summer minima. Carbon assimilation increased rapidly in the spring, and reached a maximum of 231 mmol C m−2 d−1, ca. four weeks earlier than the maximum observed biomass (during early December). It decreased rapidly thereafter, and in austral autumn when ice formed, it approached zero. The time of maximum growth rate coincided with the maximum in C-assimilation, and at 0.66 d−1 equaled predictions based on laboratory cultures. Growth rates over the entire growing season, however, were generally much less. Deck-board incubations suggested that photoinhibition occurred at the greatest photon flux densities, but in situ incubations revealed no such surface inhibition. We suggest that due to the nature of the irradiance field in the Antarctic, assemblages maintained in on-deck incubators received more light than those in situ, which resulted in photoinhibition. This in turn resulted in a 17% underestimate in on-deck productivity relative to in situ determinations. The phytoplankton bloom appeared to be initiated when vertical stability was imparted in austral spring, coincident with greater daily photon flux densities. Conversely, decreased productivity likely resulted from trace metal limitation, whereas biomass declines likely resulted from enhanced loss rates, such as aggregate formation and enhanced vertical flux of larger particles. The seasonal progression of productivity and biomass in the southern Ross Sea was similar to other areas in the ocean that experience blooms, and the cycling of carbon in this region is extensive, despite the fact that the growing season extends no more than five months.

Stable isotopes' as trophic tracers: combining field sampling and manipulative labelling of food resources for macrobenthos

Abstract: We combined 3 different approaches to determine the relative importance of micro- phytobenthos production as food for intertidal macrobenthic animals: (1) the natural abundance of stable-isotope ratios of carbon and nitrogen, (2) an in situ deliberate tracer addition of 13 C-bicarbonate, which was transferred through the benthic food chain after its incorporation by benthic algae, and (3) a dual labelling experiment in a flume, where pelagic and benthic algae were labelled with 15 N and 13 C, respectively. The results of the 3 approaches confirmed the high importance of microphytobenthos as a food source for (surface) deposit feeders. Despite the clearly demonstrated resuspension of benthic algae at high current velocities, suspension feeders appeared to depend almost exclusively on pelagic algae (and possibly detrital carbon) as a food source. Based on the results of the experiments, we determined an approximate degree of dependence on microphytobenthos for different species of intertidal macrobenthos. The macrobenthic biomass at 5 study locations, when weighted by these coefficients, correlated very well with measured productivity of the microphytobenthos.

Rationalizing elemental ratios in unicellular algae

Abstract: Keywords: Gaia hypothesis, iron, nitrogen fixation, oceanic productivity, Redfield ratio

Basin-scale variability of phytoplankton biomass, production and growth in the Atlantic Ocean

Abstract: The latitudinal distributions of phytoplankton biomass, composition and production in the Atlantic Ocean were determined along a 10,000-km transect from 50°N to 50°S in October 1995, May 1996 and October 1996. Highest levels of euphotic layer-integrated chlorophyll a (Chl a) concentration (75–125 mg Chl m−2) were found in North Atlantic temperate waters and in the upwelling region off NW Africa, whereas typical Chl a concentrations in oligotrophic waters ranged from 20 to 40 mg Chl m−2. The estimated concentration of surface phytoplankton carbon (C) biomass was 5–15 mg C m−2 in the oligotrophic regions and increased over 40 mg C m−2 in richer areas. The deep chlorophyll maximum did not seem to constitute a biomass or productivity maximum, but resulted mainly from an increase in the Chl a to C ratio and represented a relatively small contribution to total integrated productivity. Primary production rates varied from 50 mg C m−2 d−1 at the central gyres to 500–1000 mg C m−2 d−1 in upwelling and higher latitude regions, where faster growth rates (μ) of phytoplankton (>0.5 d−1) were also measured. In oligotrophic waters, microalgal growth was consistently slow [surface μ averaged 0.21±0.02 d−1 (mean±SE)], representing <20% of maximum expected growth. These results argue against the view that the subtropical gyres are characterized by high phytoplankton turnover rates. The latitudinal variations in μ were inversely correlated to the changes in the depth of the nitracline and positively correlated to those of the integrated nitrate concentration, supporting the case for the role of nutrients in controlling the large-scale distribution of phytoplankton growth rates. We observed a large degree of temporal variability in the phytoplankton dynamics in the oligotrophic regions: productivity and growth rates varied in excess of 8-fold, whereas microalgal biomass remained relatively constant. The observed spatial and temporal variability in the biomass specific rate of photosynthesis is at least three times larger than currently assumed in most satellite-based models of global productivity.

Seasonally shifting limitation of stream periphyton: response of algal populations and assemblage biomass and productivity to variation in light, nutrients, and herbivores

Abstract: We determined whether there were seasonal changes in the relative importance of consumers and resources in controlling stream periphyton. Our analysis included effects on algal populations and asse...

Dead delta's former productivity: Two trillion shells at the mouth of the Colorado River

Abstract: The diversion of the Colorado River by dams and irrigation projects, started in the 1930s, triggered the collapse of the Colorado delta ecosystem. Paleontological, ecological, geochronological, stable isotope, field, and satellite image data provide estimates of the delta's benthic productivity during the 1 k.y. directly preceding the artificial shutdown of the river. At least 2 × 1012 shells of bivalve mollusks make up the current beaches and islands of the delta. The 125 individual valves dated using 14C-calibrated amino acid racemization indicate that these shells range in age from A.D. 950 to 1950. Seasonal intrashell cycles in δ180 values indicate that average-sized bivalves lived at least 3 yr. The most conservative calculation based on these numbers indicates that during the time of natural river flow, an average standing population of ∼6 × 109 bivalve mollusks (population density ∼50/m2) thrived on the delta. In contrast, the present abundance of shelly benthic macroinvertebrates is ∼94% lower (3/m2 in 1999–2000). The dramatic decrease in abundance testifies to the severe loss of benthic productivity resulting from diversion of the river's flow and the inadequacy of its partial resumption (1981 to present). An integration of paleontological records with geomorphological, geochemical, and geochronological data can provide quantitative insights into human impact on coastal ecosystems.

Patterns of diversity in plant and soil microbial communities along a productivity gradient in a Michigan old-field.

Abstract: The relationship between plant diversity and productivity has received much attention in ecology, but the relationship of these factors to soil microbial communities has been little explored. The carbon resources that support soil microbial communities are primarily derived from plants, so it is likely that the soil microbial community should respond to changes in plant diversity or productivity, particularly if the plant community affects the quality or quantity of available carbon. We investigated the relationship of plant diversity and productivity to the composition of the soil microbial community along a topographic gradient in a mid-successional old-field in southwestern Michigan. Soil moisture, soil inorganic N, and plant biomass increased from the top to the base of the slope, while light at ground level decreased along this same gradient. We characterized the changes in resource levels along this gradient using an index of productivity that incorporated light levels, soil N, soil moisture, and plant biomass. Average plant species richness declined with this productivity index and there were associated compositional changes in the plant community along the gradient. The plant community shifted from predominantly low-growing perennial forbs at low productivities to perennial grasses at higher productivities. Although there was variation in the structure of the soil microbial community [as indicated by fatty acid methyl ester (FAME) profiles], changes in the composition of the soil microbial community were not correlated with plant productivity or diversity. However, microbial activity [as indicated by Biolog average well color development and substrate-induced respiration (SIR)] was positively correlated with plant productivity. The similarity between patterns of plant biomass and soil microbial activity suggests that either plant productivity is driving microbial productivity or that limiting resources for each of these two communities co-vary.

Organic matter sources and rehabilitation of the Sacramento–San Joaquin Delta (California, USA)

Abstract: 1. The Sacramento–San Joaquin River Delta, a complex mosaic of tidal freshwater habitats in California, is the focus of a major ecosystem rehabilitation effort because of significant long-term changes in critical ecosystem functions. One of these functions is the production, transport and transformation of organic matter that constitutes the primary food supply, which may be sub-optimal at trophic levels supporting fish recruitment. A long historical data set is used to define the most important organic matter sources, the factors underlying their variability, and the implications of ecosystem rehabilitation actions for these sources. 2. Tributary-borne loading is the largest organic carbon source on an average annual Delta-wide basis; phytoplankton production and agricultural drainage are secondary; wastewater treatment plant discharge, tidal marsh drainage and possibly aquatic macrophyte production are tertiary; and benthic microalgal production, urban run-off and other sources are negligible. 3. Allochthonous dissolved organic carbon must be converted to particulate form—with losses due to hydraulic flushing and to heterotroph growth inefficiency—before it becomes available to the metazoan food web. When these losses are accounted for, phytoplankton production plays a much larger role than is evident from a simple accounting of bulk organic carbon sources, especially in seasons critical for larval development and recruitment success. Phytoplankton-derived organic matter is also an important component of particulate loading to the Delta. 4. The Delta is a net producer of organic matter in critically dry years but, because of water diversion from the Delta, transport of organic matter from the Delta to important, downstream nursery areas in San Francisco Bay is always less than transport into the Delta from upstream sources. 5. Of proposed rehabilitation measures, increased use of floodplains probably offers the biggest increase in organic matter sources. 6. An isolated diversion facility—channelling water from the Sacramento River around the Delta to the water projects—would result in substantial loading increases during winter and autumn, but little change in spring and summer when food availability probably matters most to developing organisms. 7. Flow and fish barriers in the channel could have significant effects, especially on phytoplankton sources and in dry years, by eliminating ‘short-circuits’ in the transport of organic matter to diversion points. 8. Finally, productivity of intentionally flooded islands probably would exceed that of adjacent channels because of lower turbidity and shallower mean depth, although vascular plants rather than phytoplankton could dominate if depths were too shallow. Copyright © 2000 John Wiley & Sons, Ltd.

Impact of eutrophication on the life cycle, population dynamics and production of Ampithoe valida (Amphipoda) along an estuarine spatial gradient (Mondego estuary, Portugal)

Abstract: The life cycle, population dynamics and production of Ampithoe valida was studied from an intertidal mudflat in central Portugal, close to the northern limit of the species' distributional range in the eastern Atlantic Ocean. Sampling was carried out in eutrophicated areas, where macroalgae blooms of Enteromorpha spp. occur usually from January to early summer, and also in non-eutrophicated areas, with Zostera noltii meadows. A. valida showed a contagious distribution and the population density clearly changed during the study period along the eutrophication gradient. No migratory patterns were detected between the estuary and the sea, but migrations inside the estuary might have occurred. Females were morphologically recognisable at smaller sizes than males. Females reached sexual maturity before males, but males may live slightly longer than females. Females are iteroparous, producing 2, perhaps 3, broods. A 2-generation life cycle involving a short-lived (7 mo), fast-growing summer generation and a longer-lived (9 mo), slower-growing generation that overwinters is hypothesised. Ovigerous females were present year-round. Eggs, depending on the season, increase differently in volume during marsupial development. No correlations were found between fecundity (number of eggs) and the size of females. Along the eutrophication gradient no differences were found regarding the biology of the species. Besides these features, differences were observed between eutrophicated and non-eutrophicated areas with regard to productivity. Growth production (P) of A. valida in the most eutrophicated area was 0.098 g m -2 18 mo -1 and 0.64 g m -2 18 mo -1 in the Z. noltii meadows. P/B and E/B ratios (where E is the elimination production and B is the average population biomass) ranged from 1.42 and 3.06 in the most eutrophicated area to 5.98 and 12.41 in the Z. noltii beds. To a certain extent, the increase of macroalgae biomass may favour A. valida populations, but extensive blooms affecting the whole area of distribution of this species will determine its disappearance.

Effects of nutrient availability and other elevational changes on bromeliad populations and their invertebrate communities in a humid tropical forest in Puerto Rico

Abstract: Nutrient inputs into tank bromeliads were studied in relation to growth and productivity, and the abundance, diversity and biomass of their animal inhabitants, in three forest types along an elevational gradient. Concentrations of phosphorus, potassium and calcium in canopy-derived debris, and nitrogen and phosphorus in phytotelm water, declined with increasing elevation. Dwarf forest bromeliads contained the smallest amounts of debris/plant and lowest concentrations of nutrients in plant tissue. Their leaf turnover rate and productivity were highest and, because of high plant density, they comprised 12.8% of forest net primary productivity (0.47 t ha−1 y−1), and contained 3.3 t ha−1 of water. Annual nutrient budgets indicated that these microcosms were nutrient-abundant and accumulated < 5% of most nutrients passing through them. Exceptions were K and P in the dwarf forest, where accumulation was c. 25% of inputs. Animal and bromeliad biomass/plant peaked in the intermediate elevation forest, and were positively correlated with the debris content/bromeliad across all forest types. Animal species richness showed a significant mid-elevational peak, whereas abundance was independent of species richness and debris quantities, and declined with elevation as forest net primary productivity declined. The unimodal pattern of species richness was not correlated with nutrient concentrations, and relationships among faunal abundance, species richness, nutrient inputs and environment are too complex to warrant simple generalizations about nutrient resources and diversity, even in apparently simple microhabitats.

Bacterial activity in sediments of the deep Arabian Sea in relation to vertical flux

Abstract: In the Arabian Sea, productivity in the surface waters and particle flux to the deep sea are controlled by monsoonal winds. The flux maxima during the South-West (June–September) and the North-East Monsoon (December–March) are some of the highest particle fluxes recorded with deep-sea sediment traps in the open ocean. Benthic microbial biomass and activities in surface sediments were measured for the first time in March 1995 subsequent to the NE-monsoon and in October 1995 subsequent to the SW-monsoon. These measurements were repeated in April/May 1997 and February/March 1998, at a total of six stations from 1920 to 4420 m water depth. This paper presents a summary on the regional and temporal variability of microbial biomass, production, enzyme activity, degradation of 14 C -labeled Synechococcus material as well as sulfate reduction in the northern, western, eastern, central and southern Arabian deep sea. We found a substantial regional variation in microbial biomass and activity, with highest values in the western Arabian Sea (station WAST), decreasing approximately threefold to the south (station SAST). Benthic microbial biomass and activity during the NE-monsoon was as high or higher than subsequent to the SW-monsoon, indicating a very rapid turnover of POC in the surface sediments. This variation in the biomass and activity of the microbial assemblages in the Arabian deep sea can largely be explained by the regional and temporal variation in POC flux. Compared to other abyssal regions, the substantially higher benthic microbial biomasses and activities in the Arabian Sea reflect the extremely high productivity of this tropical basin.

Climate‐induced variations in productivity and planktonic ecosystem structure from the Younger Dryas to Holocene in the Cariaco Basin, Venezuela

Abstract: A high-resolution molecular organic geochemical study of sediments in the anoxic Cariaco Basin indicates significant changes in primary productivity and planktonic community structure associated with the transition from the Younger Dryas to the Holocene. Variations in climate conditions over the past 12 14C kyr have induced large-scale changes in upwelling intensity, which directly affected levels of primary productivity as reflected in accumulation rates of bulk productivity proxies. Concentrations and accumulation rates of sterol and alkenone biomarkers have been used to identify how productivity changes affected the structure of the planktonic ecosystem. A shift in the dominant primary producer from diatoms (Younger Dryas) to coccolithophores (Holocene) is identified. If productivity and ecosystem variations like those identified in the tropical upwelling zone of the Cariaco Basin region, occur throughout the tropical oceans, they have the potential to affect global climate through perturbations in the biogeochemical cycle of carbon.

Intense winter heterotrophic production stimulated by benthic resuspension

Abstract: A major episodic sediment resuspension event (25-yr high), which was triggered by atmospheric and water column instability in an El Niň;o year, temporarily altered the dynamics of autotrophy and heterotrophy in Lake Michigan. Resuspended sediments, rich in organic and inorganic nutrients, especially phosphorus, stimulated heterotrophic production despite low water temperatures (2oC or less). During the resuspension event, southern basin winter heterotrophic bacterial productivity was high (64% of summertime productivity), especially along the margins of the lake. The mean bacterial cell size increased in regions where productivity was highest during the resuspension event. Although resuspended sediments stimulated bacterial secondary productivity, they simultaneously decreased water-column light availability and autotroph biomass. Consequently, heterotrophic bacteria were temporally decoupled from the commonly recognized source of organic matter for bacterial production—photoautotrophy. Variation in the magnitude and frequency of these resuspension events likely influences variation in interannual productivity in this system. Such previously underappreciated, intermittent, and ephemeral benthic-pelagic exchange events may significantly influence plankton dynamics and biogeochemical cycling in coastal marine and freshwater ecosystems.

Primary productivity in the equatorial Pacific during the 1997–1998 El Niño

Abstract: Shipboard biological and physical measurements made during 1996, 1997, and 1998 in the equatorial Pacific are used to quantify the effect of the 1997–1998 El Nino on the phytoplankton community. This El Nino was by some measures the strongest ever observed and resulted in extremely low phytoplankton biomass and productivity throughout this normally moderately productive region. At the height of the event in late 1997 to early 1998, in the central Pacific, nitrate was absent throughout the entire euphotic zone (∼100 m), resulting in chlorophyll concentrations (0.05 μgL−1) that were among the lowest ever observed in the region and rates of primary production (∼0.41 g C m−2 d−1) that were approximately half the climatological mean. These conditions persisted until May 1998 when the trade winds resumed and upwelling, with its associated supply of nutrients, was restored along the equatorial Pacific. The phytoplankton community quickly recovered, and by June 1998, nitrate, chlorophyll, and primary productivity levels were comparable to, or in excess of, their respective climatological means.

Resource-niche complementarity and autotrophic compensation determines ecosystem-level responses to increased cladoceran species richness.

Abstract: This study examines the relationship between cladoceran species richness and ecosystem functioning. I conducted an experiment in which four cladocerans, Daphnia. magna, D. longispina, D. pulex and Chydorus sphaericus, were cultured in microcosms using different species combinations and levels of species richness. The results demonstrate that even within this closely related group of organisms the effects on ecosystem-level variables, such as total algae and zooplankton biomass, per capita productivity, and nutrient concentrations, as well as phytoplankton community structure, were highly variable between different combinations of these species. Since only four species where involved in this study, species-specific effects dominated the general relationship between species richness and ecosystem functioning. Particular combinations of species resulted in effects that indicated more efficient grazing. These effects, which were most pronounced in combinations including both D. magna and C. sphaericus, were manifested as an indirect effect as the prey community shifted towards grazing-resistant species. As a result, the productivity of the prey community decreased, because phytoplankton species with lower per capita productivity became more dominant. I suggest that the primary mechanism that caused this significant effect was complementarity in prey-size use of D. magna and C. sphaericus. In terms of prey-size range, D. pulex and D. longispina were redundant when D. magna was present and were quickly out-competed by the latter despite higher per capita filtering efficiency. The results show that different mechanisms are important for different combinations of species. Furthermore, the ability of the prey community to respond to changes of consumer species composition is an important factor in experiments in which consumer species richness is experimentally manipulated.

Flood disturbance, algal productivity, and interannual variation in food chain length

Abstract: The length of a river food chain changed from year to year, shifting with the hydrologic regime. During drought years, grazers suppressed algae across a nutrient gradient, while predators were functionally unimportant. Following flood disturbance, predators suppressed grazers, releasing algae. These results suggest that hydrologic regime, rather than productivity, determines the functional length of this river food chain. Within years, algae and grazer biomass responded to an experimental productivity gradient in patterns predicted by simple trophic models that assume efficient energy transfer. Understanding differences among species within trophic levels, however, was crucial in delineating the controlling interactions.

Nutrient Limitation of Heterotrophic Bacteria in Florida Bay

Abstract: We examined heterotrophic bacterial nutrient limitation at four sites in Florida Bay, U S in summer 1994 and winter 1995 Bacterial growth and biomass production in this system were most limited by inorganic phosphorus (P) in the eastern and southern regions of the bay Nutrient additions stimulated productivity and biomass accumulation mostly in summer The magnitude of growth responses (thymidine incorporation) to nutrient additions was nearly an order of magnitude less in winter than summer Biomass-normalized alkaline phosphatase activity in the northeast and south-central region was 5–20 times greater than in the northwest and north-central regions, suggesting that P is most limiting to planktonic growth in those areas Chlorophyll levels were higher in the northwest and north-central regions and P-uptake into particles >1 μm, primarily phytoplankton, was also higher in these regions Consistent with these observations, others have observed that P is advected into the bay primarily in the northwestern region Abundant seagrasses in Florida Bay may promote heterotrophic bacterial production relative to phytoplankton production by releasing dissolved organic carbon that makes bacteria more competitive for limiting quantities of inorganic phosphate, especially in the eastern bay where turbidity is low, P is most limiting, and light levels reaching the benthic plants are high

Control of phytoplankton production and biomass in a river-dominated estuary : Apalachicola Bay, Florida, USA

Abstract: Physical, chemical, and biological processes interact in complex patterns through time to control estuarine phytoplankton productivity and biomass. Hydrodynamic model results, together with biological, nutrient, and physical data acquired from Apalachicola Bay from May 1993 through May 1996, were used to elucidate factors that control phytoplankton productivity and chlorophyll (chl a). The estuary receives freshwater, which originates in the Apalachicola, Chattahoochee, and Flint River watersheds, with maximum river flow occurring in late winter and early spring. Maximum chl a (mean ± 1 SE: 5.38 ± 0.40 μg chl a l -1 ) values occurred during winter, while primary productivity maxima occurred in late spring and summer months when temperature and photosynthetically active radiation (PAR) reached their annual maxima. Approximately 75% of annual primary production (255 ± 78 g C m -2 yr -1 ) occurred from May through November of each year. During this period, however, river dissolved inorganic nitrogen and soluble reactive phosphorus input accounted for 40% of the annual inputs (30.9 ± 5.1 g N m -2 yr -1 and 0.60 ± 0.15 g P m -2 yr -1 , respectively). Approximately 25 % of annual primary production occurred from December to April, concurrent with low PAR values and low water temperature. Low chl a concentrations in summer months were concurrent with high phytoplankton productivity, high zooplankton abundance, low river flow, and low nutrient input to the estuary. In 2 of 3 years, export from Apalachicola Bay provided a significant control for phytoplankton biomass magnitude during winter. However, on an annual basis grazing accounted for 80% of the chl a loss from the estuary.