Showing papers on "Productivity (ecology) published in 1994"

Influences of Trees on Savanna Productivity: Tests of Shade, Nutrients, and Tree-Grass Competition

Abstract: To determine why herbaceous productivity in tropical and subtropical savannas is often significantly higher under crowns of isolated trees than in adjacent grass—lands, experimental plots were established in three concentric zones, crown, tree—root, and grassland, surrounding isolated trees of Acacia tortilis in low—rainfall and high—rainfall savannas in Tsavo National Park, Kenya. Plots were fertilized (to determine the importance of nutrient enrichment by trees), shaded (to determine the importance of crown shade), fertilized and shaded (to identify fertilizer x shade interactions), or trenched (tree roots entering plots were severed to determine the importance of belowground competition between overstory trees and understory herbaceous plants). In addition, vertical root distributions of trees and herbaceous species were determined, and root systems of A. tortilis saplings were excavated. At both sites fertilization significantly increased herbaceous productivity in tree—root and grassland zones, but not in canopy zones; artificial shade had no effect on productivity at the low—rainfall site but increased productivity in the tree—root zone at the high—rainfall site; and severing tree roots had no effect on herbaceous productivity at the low—rainfall site, but increased productivity in the crown and tree—root zones at the high—rainfall site. Roots of herbaceous and woody species co—occurred within the same soil horizons, but tree roots extended farther into grasslands at the low—rainfall site than at the high—rainfall site. These studies suggest that savanna trees completed more intensely with understory plants at wetters sites, where their roots terminated in or near crown zones, than at drier sites, where their roots extended farther into open grassland. Nutrients added by trees to crown zones in the form of tree litter and animal droppings increased understory productivity by fertilizing nutrient—limited soils. Shade contributed more to regrowth after severe defoliation than to growth under more normal conditions.

A line in the sea

Abstract: THE ocean has considerable spatial and temporal heterogeneity in biomass and productivity owing in part to the effects of ocean circulation and mixing1,2. Water mass boundaries (fronts) in coastal waters are well-known sites of enhanced biological activity3,4. Comparatively little is known of open-ocean fronts, and one of the few biological studies of an oceanic front showed phytoplankton biomass at only slightly higher densities than in surrounding waters5. Here we present photographs and measurements from satellites, aircraft, ships and the Space Shuttle Atlantis which show dramatic biological responses to circulation and mixing processes associated with an open-ocean front. Breaking waves (whitecaps) caused by water turbulence and mixing, and very dark green water caused by extremely high concentrations (>20 mg of chlorophyll a per m3) of buoyant diatoms (Rhizosolenia sp.) made a distinct line in the sea visible for hundreds of kilometres. The line traced the northern edge of a westward-progagating (50 km per day) tropical instability wave (1,000-km wavelength) delineating the boundary between cold, upwelled waters and warmer waters to the north. High phytoplankton biomass and primary production associated with the extensive diatom patches may explain anecdotal observations of high animal abundance along this frontal boundary.

Biomass, Productivity, Leaf Longevity, and Forest Structure in the Central Himalaya

Abstract: Patterns of leaf characteristics, forest structure, tree species diversity, bio- mass, and productivity across a gradient of 3300 m and 15.70C in mean annual temperature in Kumaun, in the Indian central Himalaya, were summarized and compared to values from other similar forests. Throughout the elevational gradient, the annual rainfall was high (100-300 cm), but not correlated with elevation. Evergreen species with a 1-yr leaf life-span dominated most of the elevational transect; above 1800 m, species with deciduous and multiyear evergreen leaves were also well represented. Although variability among sites within forest types was high, a number of consistent patterns were apparent. Forests of Pinus roxburghii and those at high elevations were most consistently different from other forest types. Leaf life-span was not strongly correlated with leaf mass, specific leaf mass, or leaf production efficiency (net primary productivity per unit leaf mass), contrary to relationships presented in the literature. Tree species richness and basal area were lower than for most similar types in Nepal. Biomass and productivity of the forests in Kumaun were relatively high, compared to mean values for similar forest types elsewhere. Measured values for most variables describing these forests (but not all) fell within the ranges for the variables in similar forests worldwide. The maximal values for forest biomass remained high, 500-600 Mg/ha, up to 2600 m elevation, but declined sharply in birch forest (_ 170 Mg/ha) above 3100 m. Net primary productivity (NPP) varied little (15-20 Mg* ha- I.yr- 1) below 2700 m, despite a 10WC gradient in mean annual temperature and marked changes in basal area, tree density, growth form, and leaf char- acters. The level of productivity appeared not to be limited by rainfall, forest structure, leaf type, or temperature above an annual mean of 1 PC. Leaf mass (LM) varied consid- erably among forest types, being 3.7-8.6 Mg/ha for deciduous species, 5.7-8.9 Mg/ha for P. roxburghii, and 10.0-28.2 Mg/ha for evergreen broad-leaved species. Leaf mass duration (leaf mass x months of the year with leaves present) was related directly to NPP and inversely to leaf production efficiency (NPP/LM). These data add substantially to the data base for forest properties, especially for broad-leaved evergreen forests.

Nutrient additions by waterfowl to lakes and reservoirs: predicting their effects on productivity and water quality

Abstract: Lakes and reservoirs provide water for human needs and habitat for aquatic birds. Managers of such waters may ask whether nutrients added by waterfowl degrade water quality. For lakes and reservoirs where primary productivity is limited by phosphorus (P), we developed a procedure that integrates annual P loads from waterfowl and other external sources, applies a nutrient load-response model, and determines whether waterfowl that used the lake or reservoir degraded water quality. Annual P loading by waterfowl can be derived from a figure in this report, using the days per year that each kind spent on any lake or reservoir. In our example, over 6500 Canada geese (Branta canadensis) and 4200 ducks (mostly mallards, Anas platyrhynchos) added 4462 kg of carbon (C), 280 kg of nitrogen (N), and 88 kg of P y-1 to Wintergreen Lake in southwestern Michigan, mostly during their migration. These amounts were 69% of all C, 27% of all N, and 70% of all P that entered the lake from external sources. Loads from all external sources totaled 840 mg P m-2 y-1. Application of a nutrient load-response model to this concentration, the hydraulic load (0.25 m y-1), and the water residence time (9.7 y) of Wintergreen Lake yielded an average annual concentration of total P in the lake of 818 mg m-3 that classified the lake as hypertrophic. This trophic classification agreed with independent measures of primary productivity,chlorophyll-a, total P, total N, and Secchi disk transparency made in Wintergreen Lake. Our procedure showed that waterfowl caused low water quality in Wintergreen Lake.

Physical and biological consequences of a climate event in the central North Pacific

Abstract: Climatic changes over the North Pacific which began in the mid 1970s, peaked in the early 1980s, and ended by the late 1980s, appear to have altered productivity at various trophic levels in the marine ecosystem in the central North Pacific. The climatic change resulted in increased mixed layer depth and the frequency of deep mixing events, particularly during January-March. A number of biological time series for species ranging from primary to apex levels in the North-western Hawaiian Islands, show corresponding declines in productivity of 30–50% from the early 1980s to the present. We hypothesize that during the early 1980s, increased mixing due to the climate event resulted in greater nutrient input into the euphotic zone and ultimately increased ecosystem productivity. Productivity over a range of trophic levels declined when the climate event ended.

Comparative and experimental approaches to top-down and bottom-up regulation of bacteria

Abstract: The regulation of bacterial community biomass and productivity by resources and predators is a central concern in the study of microbial food webs. Resource or bottom-up regulation refers to the limitation of bacteria by carbon and nutrients derived from allocthonous inputs, primary production, and heterotrophic production. Predatory or top-down regulation refers to the limitation of bacteria below levels supportable by resources alone. Large scale comparative studies demonstrate strong correlations between bacterial productivity and biomass, suggesting significant resource regulation. Comparisons of the abundances of heterotrophic flagellates and bacteria, however, imply that in some cases there may be top-down regulation of bacteria in eutrophic environments. Experimental studies in lakes support the importance of resource regulation and reveal little top-down control from protozoans. Increases in bacterial abundance and production with nutrient enrichment were limited in enclosure experiments with high abundances of the cladoceran, Daphnia. Regulation of bacteria by Daphnia may occur in many lakes seasonally and prevail in some lakes throughout the year where these animals sustain dense populations. In most situations, however, bacteria appear to be limited primarily by resources.

Structure, biomass distribution, and energetics of the pelagic ecosystem in the Barents Sea: A synopsis

Abstract: Biomass distribution and energetics of trophic levels in the pelagic ecosystem of the Barents Sea are presented as averages over several years for the whole Barents Sea using data from the research programme Pro Mare in 1984–1989 and mathematical ecosystem models Average biomasses range from more than 3 tonnes carbon km−2 (zooplankton) to 01 kg C km−2 (polar bears) and P/B ratios from 300 (bacteria) to 0035 (minke whales) However, the Barents Sea ecosystem is in a far from steady state with, for instance, capelin stocks ranging from 30–700 kg C km−2 between years and cod stocks from 150–700 kg C km−2 As a general rule, the various fish stocks grow adequately, albeit at different rates, in “warm” years characterized by large influxes of Atlantic water and high zooplankton productivity The skewed populations distribution which arises in “warm” years may lead to grave imbalances in “cold” years and even to the “collapses” of stocks, such as of capelin in the eighties The food requirements of average-sized stocks of cod, seabirds and marine mammals correspond to more than twice the average productivity of capelin Thus other species of pelagic fish (herring, polar cod) and zooplankton obviously play major roles as prey for these animals

Planktonic Trophic Transfer in an Estuary: Seasonal, Diel, and Community Structure Effects

Abstract: The high tertiary production of estuaries is largely supported by phytoplank- ton primary production. An important question thus concerns how much phytoplankton production enters the food web through planktonic grazing and what physical, chemical, or biological factors influence this trophic transfer. We conducted a 2-yr, diel investigation of planktonic trophic transfer and the factors influencing it in the Neuse River Estuary, North Carolina. Zooplankton community grazing rates were generally lowest in winter and highest spring through late summer, ranging from 0.1 to 310 mL L-l I h-1. There were few significant diel differences in community grazing rates. The overall daytime mean (? 1 SE) rate was 3.30 ? 0.62 mL L-l h-I while the night mean rate was 3.07 ? 0.60 mL L-l h-1. Post- naupliar copepods were usually more abundant at night than day, but tintinnids were often more abundant by day, while total zooplankton, copepod nauplii, and rotifers displayed no significant diel abundance differences. Community grazing was positively correlated with primary productivity and the abun- dance of total phytoplankton, centric diatoms, dinoflagellates, and the small centric diatom Thalassiosira. Community grazing was also positively correlated with upstream river flow and negatively correlated with salinity. However, there were no significant correlations with water temperature, nutrient concentrations, or grazer abundance variables. On an annual basis, the zooplankton community grazed - 38-45% of daily phytoplank- ton production. Planktonic trophic transfer was coincidentally greatest in late spring through summer, during the period when anadromous fish larvae migrating from the open ocean reach their estuarine primary nursery areas. The fish arrive when planktonic trophic cou- pling is strongest and depart in fall, when planktonic trophic transfer, zooplankton abun- dance, and phytoplankton productivity all decrease.

A preliminary mass balance model of primary productivity and dissolved oxygen in the Mississippi River Plume/Inner Gulf Shelf Region

Abstract: A deterministic, mass balance model for phytoplankton, nutrients, and dissolved oxygen was applied to the Mississippi River Plume/Inner Gulf Shelf (MRP/IGS) region The model was calibrated to a comprehensive set of field data collected during July 1990 at over 200 sampling stations in the northern Gulf of Mexico The spatial domain of the model is represented by a three-dimensional, 21-segment water-column grid extending from the Mississippi River Delta west to the Louisiana-Texas border, and from the shoreline seaward to the 30–60 m bathymetric contours Diagnostic analyses and numerical experiments were conducted with the calibrated model to better understand the environmental processes controlling primary productivity and dissolved oxygen dynamics in the MRP/IGS region Underwater light attenuation appears relatively more important than nutrient limitation in controlling rates of primary productivity Chemical-biological processes appear relatively more important than advective-dispersive transport processes in controlling bottom-water dissolved oxygen dynamics Oxidation of carbonaceous material in the water column, phytoplankton respiration, and sediment oxygen demand all appear to contribute significantly to total oxygen depletion rates in bottom waters The estimated contribution of sediment oxygen demand to total oxygen-depletion rates in bottom waters ranges from 22% to 30% Primary productivity appears to be an important source of dissolved oxygen to bottom waters in the region of the Atchafalaya River discharge and further west along the Louisiana Inner Shelf Dissolved oxygen concentrations appear very sensitive to changes in underwater light attenuation due to strong coupling between dissolved oxygen and primary productivity in bottom waters The Louisiana Inner Shelf in the area of the Atchafalaya River discharge and further west to the Texas border appears to be characterized by significantly different light attenuation-depth-primary productivity relationships than the area immediately west of the Mississippi Delta Nutrient remineralization in the water column appears to contribute significantly to maintaining chlorophyll concentrations on the Louisiana Inner Shelf

Multiple Factors Limit Seasonal Variation in Periphyton in a Forest Stream

Abstract: This study was conducted to characterize periphyton biomass, productivity, and algal species composition, and to examine how these parameters changed seasonally over the course of two years in Walker Branch, a temperate forest stream. Seasonal variations in several factors potentially controlling periphyton (irradiance, streamwater nutrient concentrations, temperature, and the incidence of severe storms) were large. However, there was no consistent seasonal variation in herbivore (snail) density, which was high (> 1000 animals/m 2 ). Regression analysis was conducted to determine the influence of potential controlling factors on periphyton biomass and productivity. Seasonal changes in periphyton biomass were small and only weakly related to irradiance (-), snail density (-), and temperature (+). Biomass was lowest in early spring and highest in summer in 1989; but during the following year, biomass was similar year-round. Seasonal variation in primary productivity was also relatively small, but w...

Variations in the invertebrate abyssal megafauna in the North Atlantic Ocean

Abstract: Epibenthic sledge and otter trawl samples of invertebrate megabenthos were collected from the abyss of the northeastern Atlantic Ocean. Stations were selected to have contrasting overlying water column structure. Strong seasonal pulses of phytodetritus on the sea floor have been recorded on the Porcupine Abyssal Plain (PAP) but not on the Madeira Abyssal Plain (MAP). Sledge-and trawl-derived abundances of 13.73 and 8.13 individuals 103 m−2 were found at PAP, with corresponding wet biomass values of 169.4 and 189.2 g 103 m−2. Data from sledge hauls taken at MAP gave an abundance of 7.48 individuals and 5.15g 103 m−2. Sledge and trawl abundances at Great Meteor East (GME) were 2.91 and 2.19 individuals 103 m−2 and 4.31 and 11.30 g 103 m−2. Holothurians dominated the invertebrate biomass at PAP while Astoroidea and Decapoda Natantia were important taxa at MAP and GME. Otter trawl samples demonstrated fish to be the major component of total megadaunal biomass and suggest that small fixed-frame trawls give gross underestimates of fish abundance and biomass. Size spectra based on abundance and biomass data indicated the megafauna to be a functional group at PAP stationl only. Here peak invertebrate biomass occurred in the 40–80 g wet wt size class. In contrast, no large invertebrates (>20 g wet wt) were found at MAP of GME. Concurrent photographic records indicate higher faunal densities than do net catches. Faunal differences between stations were examined, and a weak relationship between surface productivity and megafaunal abundance was established. Data are inadequate to detect a similar relationship with biomass. Biomass values at PAP of 16–39 times those of MAP and GME appear to be explained by the deposition of phytodetritus at PAP but not at MAP or, probably, at GME. Among invertebrates, detritivores form the dominant feeding guild at all localities, but carnivores are important at MAP and GME. Phytodetritus appears to support the abundant surface-grazing holothurians at PAP.

Resource edibility and trophic exploitation in an old-field food web

Abstract: I tested a food web model that predicts how environmental productivity (nutrient supply) and top carnivores should mediate interactions among herbivores, edible plants, and plants that are resistant to herbivory because they possess anti-herbivore defenses. Feeding trials with the dominant grasshopper herbivore at the study site confirmed that certain plant species were resistant to herbivory because of protection by pubescent leaves and stems. Experimental food webs with various numbers of trophic levels composed of edible and resistant plants, grasshoppers, and hunting spiders were assembled in enclosure cages. I randomly assigned half of the cages to a nutrient-enrichment treatment and half remained as a control. Nutrient supply directly enhanced primary productivity and plant and herbivore biomass. Experimentally changing spider abundance caused a classic "trophic cascade" in which herbivore biomass increased and edible plant biomass decreased. Resistant plant biomass increased. These results matched predictions of the model with one exception. A trophic cascade was not observed under enriched conditions. The study nevertheless shows that a simple model attempting to explain heterogeneous interactions in food webs may give considerable insight into the dynamics of natural systems.

Nutrient-limited productivity of calcareous versus fleshy macroalgae in a eutrophic, carbonate-rich tropical marine environment

Abstract: The results of a study of nutrient enrichment with nitrogen (N) and phosphorus (P) on productivity and calcification of fleshy and calcareous algae are reported in this study. Plants were collected from a nearshore eutrophic site in the Florida Keys (USA) and experimentally pulsed during the night with combinations of N and P. After several days of pulsing (7–10 days), net productivity, calcification, and alkaline phosphatase activity (APA), were measured. Productivity of fleshy algae were frequently enhanced by N, P, and N+P, during both summer and winter. Phosphorus limited the productivity of Hydroclathrus clathratus during winter and Ulva spp. during summer, whereas nitrogen limited the productivity of Laurencia intricata during both seasons. During summer, Dictyota cervicornis productivity was not enhanced by any nutrient enrichment. Nitrogen limited the productivity of the three calcareous species Penicillus capitatus, Penicillus dumetosus and Halimeda opuntia during winter and that of H. opuntia during summer. Neither N nor P enrichment increased calcification of calcareous species, and P enrichment greatly inhibited calcification of P. dumetosus during winter. Nutrient enrichment enhanced the productivity of the fleshy species to a greater extent than that of calcareous algae. The seawater DIN:SRP molar ratio was low at our eutrophic study site (molar ratio average of 3:1 during winter and 9:1 during summer) compared to more oligotrophic sites in the Florida Keys, suggesting that in carbonate-rich environments, eutrophication shifts nutrient regulation of productivity from P to N. APA activities of fleshy macroalage were higher than calcareous algae, and rates of all macro algae were 2- to 7-fold higher in summer compared to winter. Productivity was also about 3-fold higher in fleshy compared to calcareous species and about 2-fold higher in summer compared to winter. These results suggest that nutrient enrichment enhances productivity of fleshy algae to a greater extent than that of calcareous algae. Thus, overgrowth of calcareous algae by more opportunistic fleshy forms could reduce carbonate accretion in tropical coastlines experiencing increased eutrophication.

Why does the relationship between sinking flux and planktonic primary production differ between lakes and oceans

Abstract: The fraction of primary production lost to sinking (the export ratio) increases with productivity in the ocean and decreases slightly with productivity in lakes. To explore why this distinction exists, we compared marine and freshwater regressions relating chlorophyll concentrations in the euphotic zone to each of the three variables that define the export ratio: primary productivity, carbon sinking fluxes, and euphotic zone depth. Chlorophyll was found to predict these three variables well (r2 = 0.54–0.90) in both lakes and the ocean. The differences between the marine and freshwater export ratio—productivity relationships stem primarily from a discrepancy in the primary productivity—Chl relationships. On average, a >10-fold difference in Chl-specific productivity exists between the most oligotrophic lakes (Chl = 0.2 mg−3) and oceanic regions with similar Chl concentrations. This difference disappears at higher concentrations of Chl because primary productivity: Chl ratios increase with productivity in lakes. In addition, carbon sinking rates average 2–3-fold higher in the oceans than in lakes with similar concentrations of Chl. The trends of marine and freshwater export ratio-production can be qualitatively reproduced with Chl-based predictions of euphotic zone primary productivity, depth, and carbon sinking losses from regressions. Marine and freshwater ecosystems may differ systematically in the efficiency of nutrient recycling processes in the water column and in the nature of settling material.

Phytoplankton ecology of North Carolina estuaries

Abstract: Numerous phytoplankton-oriented ecological studies have been conducted since 1965 in the extensive North Carolina estuarine system. Throughout a range of geomorphological estuarine types, a basic underlying pattern of phytoplankton productivity and abundance following water temperature seasonal fluctuations was observed. Overlying this solar-driven pattern was a secondary forcing mechanism consisting of a complex interaction between meteorology and hydrology, resulting in periodic winter or early spring algal blooms and productivity pulses in the lower riverine estuaries. Wet winters caused abundant nitrate to reach the lower estuaries and stimulate the blooms, whereas dry winters resulted in low winter phytoplankton abundance and primary production. Dinoflagellates (Heterocapsa triquetra, Prorocentrum minimum, Gymnodinium spp.) and various cryptomonads dominated these cool-weather estuarine blooms. Sounds were less productive than the riverine estuaries, and were dominated by diatoms such asSkeletonema costatum, Thalassiosira spp.,Melosira spp., andNitzschia spp., as were the most saline portions of riverine estuaries. Nutrient-limitation studies found that nitrogen was the principal limiting nutrient in these estuarine systems over a range of trophic states, with phosphorus occasionally co-limiting. Freshwater and oligohaline portions of large coastal plain rivers were often subject to summer blue-green algal blooms. Formation of these blooms on a year-to-year basis was also determined by meteorology and hydrology: wet winters or springs and consequent nutrient loading, coupled with low summer flow conditions and regeneration of nutrients from the sediments. Dry winters or springs resulted in less available nutrients for subsequent summer regeneration, and high flow conditions in summer flushed out the blooms. In recent years, there has been a dramatic increase in reported fish kills attributed to toxic dinoflagellate blooms, particularly in nutrient-enriched estuarine areas. This issue has become a major coastal ecological and economic concern.

Distribution, productivity and reproductive phenology of emergent macrophytes in relation to water regimes at wetlands of south-western Australia

Abstract: Variability in the response of Baumea articulata and Typha orientalis to water regime was assessed at eight urban wetland lakes on the Swan Coastal Plain, south-western Australia. Distribution, productivity and reproduction along water regime gradients were assessed, and the importance of wetland nutrient status in altering plant response to water regime was also considered. The overall range in distribution observed across the lakes was similar for both species, but there was significant between-lake variation in distribution relative to mean water depth. Neither species occupied its full 'potential' distribution range, relative to water regime, at any one wetland. Standing biomass and ramet and inflorescence densities varied along the water regime gradient, with peak values generally occurring at intermediate water depths. There was a shift in phenology (ramet emergence, new leaf growth, flowering and seed production) with increasing mean water depth and nutrient status. Seasonal values of aboveground productivity changed along the water regime gradient for both species. Comparison of productivity at sampling points of different nutrlent status but similar water regime showed a significantly greater growth response in T. orientalis compared with B. articulata. The significance of these results to urban wetland management is discussed.

Deep-Sea Benthic Foraminifers: Food and Bottom Water Masses

Abstract: Benthic foraminifers are an abundant, nearly cosmopolitan and easily preserved component of the deep-sea meiofauna. The benthic foraminiferal faunas are very strongly structured on regional scales. In the western North and South Atlantic the distribution of benthic assemblages follows the distribution of named “water masses,” leading to the expectation that fossil benthic foraminifers from sediment cores allow a reconstruction of past deep-water circulation patterns. However, in many other areas the faunal composition is strongly correlated with the productivity of the overlying surface waters. Such results demonstrate that the benthic meiofauna responds to the integral of the deep environment and it is the limiting or overwhelming influence that determines the success or failure of individual species and thus the composition of the fauna. Analysis of available data clearly indicates that benthic foraminifers are unequivocal indicators of productivity in areas where productivity is high. In areas of low or very uniform productivity the composition of the benthic fauna clearly carries the imprint of deep water mass structure as the dominant feature. Faunal sequences from late Quaternary cores from the Atlantic and Pacific oceans show very strong differentiations. Not only is the glacial/postglacial transition shown, but also much superimposed fine structure. Faunal composition is as sensitive to deep-water environmental change as are geochemical indicators. The faunal composition allows at least some appreciation of the cause for the major changes, but a quantitative differentiation between productivity or water mass changes is not yet possible. If, however, an independent assessment of productivity change is available, then the benthic foraminiferal faunas are exquisite indicators of changes in deep water circulation.

Primary and bacterial production in lakes: are they coupled over depth?

Abstract: The coupling of prlmary and bacterial production over depth was examined in three lakes which differed greatly in vertical patterns of primary productivity. We measured bacterial production, chlorophyll and light, and estimated primary production in Paul Lake (Gogebic County, Michigan) and Crystal and Trout lakes (Vilas County, Wisconsin) during the summer strat~fication period (May-September 1991). Bacterial productivity was measured using the ('Hlleucine incorporation method and primary productivity estimated from measured photosynthesis-irradiance relationships. Three distinct vertical patterns were observed. In Paul Lake, bacterial production was highest at the interface between the aerobic and anaerobic layers, well below the depth of maximum primary production. In Crystal Lake, bacterial production was uniform with depth, although primary b 4-' gaductivity was highest in the hypolimnion. In the largest lake, Trout Lake, primary and bacterial production tended to co-vary with maximum rates of both processes occurring in the metalimnion. Overall, bacterial productivity was poorly related to contemporaneous primary production in the three lakes, suggesting that other factors, such as nutrient recycling, phytoplankton loss rates and allocthonous loading, determine patterns in the depth distribution of bacterial productivity.

Modeling seagrass communities in tropical and subtropical bays and estuaries: a mathematical model synthesis of current hypotheses

Abstract: A preliminary simulation model was generated to predict changes in the biomass of five components of the autotrophic seagrass community that dominates tropical and subtropical bays and estuaries. Changes in productivity and biomass are based on relationships among three species of seagrass (Thalassia testudinum, Halodule wrightii, and Syringodium filiforme), epiphytes attached to seagrass, macroalgae, and several environmental factors, including light, temperature, salinity, sediment nutrients, and water-column nutrient concentrations. These relationships were derived from the published literature and include both experimental data and current alternative hypotheses. The model predicts that Thala~'sia is the community dominant under "normal" bay or estuarine conditions in tropical and subtropical regions, including high solar insolation, interrnediate levels of seasonal variability in temperature and salinity, and low water-column and intermediate-to-high sediment nutrient concentrations. Increasing the supply of nutrients to the water column stimulates the productivity of epiphytes on seagrass, resulting in decreased light to seagrass blades and less Thalassia productivity. Thalassia and epiphyte biomass undergo seasonal changes in abundance; however, epiphyte biomass lags Thalassia by about 40 days. Halodule dominates when sediment nutrients are high and when there are environmental extremes of temperature and salinity, Syringodium is the community dominant in areas with more oceanic influence, characterized by less variability in salinity and temperature and lower water-column and sediment nutrients, This model is still in an early developmental stage. Preliminary sensitivity analyses identified important factors for community productivity and composition. The most important model parameters for seagrass include the productivity/biomass relationships, differential tolerances to extreme salinities, and the PII curves (especially for Thalassia). All of the relationships between environmental factors and epiphytes are important, and these are the least certain derivations. We need to conduct a thorough sensitivity analysis, validate the model with field data, and generate more inforrnation on the algal components of the community. This simple community model will eventually be expanded to simulate seagrass dynamics across a spatial

The effect of temporal undersampling on primary production estimates

Abstract: Annual primary production estimates for specific oceanic regions have typically been made using a variety of measures of productivity spaced, at best, several weeks apart Primary productivity in the oceans is known to be extremely episodic. It is hypothesized here that primary production data with a temporal resolution of several weeks have a high potential for error due to undersampling. In the present analysis, time series of gross primary productivity were calculated using time series of photosynthetically available radiation and chlorophyll a concentration as input to an optical production model. The input data are of minute scale resolution and were gathered during a number of moored experiments. These took place over the past 5 years at several oceanic sites. The minute scale productivity time series were integrated to form time series of daily estimates of gross production. These range in duration from 40 to 260 days. The time series exhibit several regimes characteristic of oceanic primary productivity, such as phytoplankton blooms, productivity pulses associated with advected water masses, steady state growth, and development of a subsurface productivity maximum. The presence of these features makes our time series ideal for investigating (1) the sensitivity of annual production estimates to the timing of the sample set and (2) the error introduced by undersampling inherent in coarser sampling methods. It was found that distinct pulses of productivity generate the greatest error and that high variability leads to large errors, even for well-resolved sampling intervals. The maximum percent error due to undersampling was found to be 85%. Additionally, up to a fourfold range between the maximum and minimum estimates of average daily production was found over all sampling intervals. Finally, the maximum expected range (300 g C m−2 yr1) and the expected standard deviation (±42 g C m−2 yr1) for annual water column production were determined at a Sargasso Sea site for which long-term productivity time series were available at four depths within the euphotic zone.

Impact of herbaceous competition and drainage conditions on the early productivity of willows under short-rotation intensive culture

Abstract: The productivity of trees under short-rotation intensive culture is influenced by herbaceous competition and drainage conditions. During the first year of establishment of Salixdiscolor Muhl. and Salixviminalis L., four weed-suppression treatments were applied to two sites showing different drainage conditions, one well drained and the other is poorly drained. On the well-drained site, the productivity of the trees increased in all the plots under treatment, whereas on the poorly drained site, only the use of a plastic mulch increased the biomass production. The mean productivity on the well-drained site was always superior to the productivity measured on the poorly drained site except when the vegetation was controlled by the use of a plastic mulch. The analyses related to nutrition revealed that the trees on the well-drained site absorbed more nutrients than those on the poorly drained site. Salixdiscolor showed a higher uptake of potassium and S. viminalis, a higher concentration of calcium. Our result...