The 1994 experimental opening of the Bonnet Carre Spillway to divert Mississippi River water into Lake Pontchartrain, Louisiana

A diversion of Mississippi River water into Lake Pontchartrain, Louisiana, USA by way of the Bonnet Carre Spillway has been proposed as a restoration technique to help offset regional wetland loss. An experimental diversion of Mississippi River water into Lake Pontchartrain was carried out in April 1994 to monitor the fate of nutrients and sediments in the spillway and Lake Pontchartrain. Approximately 6.4×10⁸ m³ of Mississippi River water was diverted into Lake Pontchartrain over 42 days. As water passed through the Bonnet Carre Spillway, there were reductions in total suspended sediment concentrations of 82–83%, nitrite+nitrate (NO_x) of 28–42%, in total nitrogen (TN) of 26–30%, and in total phosphorus (TP) of 50–59%. 3.9±1.1 cm of accretion was measured in the spillway. Nutrient concentrations at the freshwater plume edge in Lake Pontchartrain compared to the Mississippi River were lower for NO_x (44–81%), TN (37–57%), and TP (40–70%), and generally higher for organic nitrogen (−7–57%). The Si:N ratio generally increased and the N:P ratio decreased from the river to the plume edge. Nutrient stoichiometric ratios indicate water at the plume edge was not silicate limited, suggesting conditions favoring diatomic phytoplankton.     

Transport and fate of dissolved organic carbon in the Lake Pontchartrain esutary,Louisiana, U.S.A.

Cycling dynamics of dissolved organic carbon (DOC) were examinedin Lake Pontchartrain estuary, Louisiana, in relation to changesin freshwater inputs. DOC concentrations ranged from 5.3 to 8.5mg C L-1 reaching their highest during high river inflow.The percentage of DOC represented by HMW DOC (or colloidal material)was greatest (ca. 11%) at stations where freshwaterdischarge from rivers and surrounding wetlands was most significant.Moreover, the lignin-phenol content of this material (ranged from 0.09 to 0.33 and from 0.11 to 0.39)confirmed that a significant fraction of colloidal organic carbon wasderived from terrestrial sources. Riverine and benthic fluxes representedthe dominant sources of DOC to the estuary. On an annual basis, riverineand benthic DOC concentrations were estimated to be 2.8 ×10 10 g C yr-1 and 8.8 × 10 10 g C yr-1, respectively, while the totalDOC pool in the estuary was 3.8 × 10 10 gC. Annual average concentrations of dissolved inorganic carbon (DIC)(1298 M) and pCO2 (5774 atm)were comparable to those found in other freshwater systems that reachedCO2 saturation levels. Net losses of DOC in the LakePontchartrain estuary appeared to be primarily controlled by heterotrophicconsumption (conversion of CO2) – whichmay have been amplified by the long residence time (approximately 120days) of DOC in this system.     

Nitrogen limitation and particulate elemental ratios of seston in hypersaline Mono Lake, California, U.S.A.

Particulate elemental ratios (C:N, N:P and C:Chl a) of seston in hypersaline (70–90 g kg^–1) Mono Lake, California, were examined over an 11-year period (1990–2000) which included the onset and persistence of a 5-year period of persistent chemical stratification. Following the onset of meromixis in mid-1995, phytoplankton and dissolved inorganic nitrogen were substantially reduced with the absence of a winter period of holomixis. C:N, N:P and C:Chl a ratios ranged from 5 to 18 mol mol^–1, 2 to 19 mol mol^–1 and 25 to 150 g g^–1, respectively, and had regular seasonal patterns. Deviations from those expected of nutrient-replete phytoplankton indicated strong nutrient limitation in the summer and roughly balanced growth during the winter prior to the onset of meromixis. Following the onset of meromixis, winter ratios were also indicative of modest nutrient limitation. A 3-year trend in C:N and N:P ratios toward more balanced growth beginning in 1998 suggest the impacts of meromixis weakened due to increased upward fluxes of ammonium associated with weakening stratification and entrainment of ammonium-rich monimolimnetic water. A series of nutrient enrichment experiments with natural assemblages of Mono Lake phytoplankton conducted during the onset of a previous episode of meromixis (1982–1986) confirm the nitrogen will limit phytoplankton before phosphorus or other micronutrients. Particulate ratios of a summer natural assemblage of phytoplankton collected under nitrogen-depleted conditions measured initially, following enrichment, and then after return to a nitrogen-depleted condition followed those expected based on Redfield ratios and laboratory studies.     

Leaf litter decomposition in an ephemeral karst lake (Chaney Lake,Kentucky, U.S.A.)

Litter decomposition in temporary aquatic environments has not been experimentally studied as much as it has in perennial systems. However, litter is likely a critical resource for organisms inhabiting ephemeral aquatic habitats. In this study, we used litterbags under different conditions of submergence and water physical and chemical properties/characteristics to study mass and nutrient losses of terrestrial materials in an ephemeral karst lake in south-central Kentucky (USA). In the first experiment, which was designed to compare decomposition rates in submerged and dry sites, total mass and carbon declined more rapidly in the litter at fully submerged sites than in dry sites. In the second experiment, which was designed to compare decomposition rates in two different submerged environments, total mass and carbon showed similar decomposition trends between the two submerged areas with different seasonal temperature patterns. Nitrogen patterns were variable but in general nitrogen levels increased in the litter in both experiments over a period of several months. These results are similar to those found in some perennially inundated systems and indicate that litter decomposition dynamics in this temporary lake can be greatly affected by lake hydrology. Year-to-year variations in hydrology may thus have strong impacts on nutrient and energy release within this system, which may affect the organisms within this karst lake and in other areas of the karst ecosystem that are ecologically connected to it.     

A marine calanoid copepod (Temora longicornis) in a semi-stagnant coastal lagoon (Lake Grevelingen,S.W.-Netherlands)

The marine calanoid copepodTemora longicornis was introduced in saline Lake Grevelingen (S.W.-Netherlands), when the lake was continuously flushed with North Sea water in 1979, to enhance the reduced salinity. Development of the copepod proceeded suboptimally as a consequence of an unfavourable environment in the lake: 1. The phytoplankton assemblage, mainly consisting of crytomonad flagellates, less suitable for growth and reproduction of the adult stages. 2. An acute food shortage occurring at a crucial moment in the development. 3. A. pronounced salinity-and oxygen stratification in June, preventing further development ofTemora.Communication nr. 474 of the Delta Institute for Hydrobiological Research, Yerseke.     

The effect of permafrost on stream biogeochemistry: A case study of two streams in the Alaskan (U.S.A.) taiga

Understanding interactions between permanently frozen soils and stream chemistry is important in predicting the effects of management, natural disturbance and changing permafrost distribution on stream ecosystems and nutrient budgets in subarctic watersheds. Chemical measurements of groundwater, soil water and stream water were made in two watersheds in the taiga of interior Alaska. One watershed (HiP) had extensive permafrost and the other (LoP) had limited permafrost. Soil water collected within the rooting zone (0.3--0.5 m) in both watersheds was high in dissolved organic carbon (DOC), dissolved organic nitrogen (DON) and dissolved inorganic nitrogen (DIN) but low in dissolved minerals (dominantly Ca, Mg and Na) and conductivity. The reverse was true for groundwater from springs and wells. Permafrost in the HiP basin prevented deep percolation of water and generated stormflows rich in DOC. The presence of permafrost in HiP resulted in higher fluxes of DOC, DON and DIN into stream water from upland soils.     

Phytoplankton ecology in an unusually stable environment (Montezuma Well,Arizona, U.S.A.)

Relatively minor annual amplitudes of change in certain major nutrients, and especially pH and water temperature were measured in the spring-fed system of Montezuma Well, Arizona during a four year study. phytoplankton diversity was low but for the most part, composition was spatially and temporally constant; total seasonal phytoplankton density was significantly correlated with regional incident light. Phytoplankton species composition changed briefly during and for a short period following the summer monsoon. Ultraplankton (<5 µm diam.) numerically comprised nearly 80% of the phytoplankton community throughout most of the year. The limited residence time of water in the Well may have provided a competitive advantage for cells with high surface area:volume ratios and correspondingly rapid division rates. Nannochloris bacillaris Naum. and Coccomyxa minor Skuja were perennial dominants. Diatom populations did not increase with annual increases in vernal solar radiation. Low pH, high dissolved CO₂, and limited residence time for metabolic inhibitors are considered to be largely responsible for the reduced blue-green populations in the Well. The only flagellated photosynthetic group present in Montezuma Well was the Cryptophyta. Desmid populations were minimal, even though pH was consistently below circumneutral (6.5) and free CO₂ concentrations high. The role of grazing by an amphipod, Hyalella montezuma, on annual phytoplankton abundance is examined.     

Trophic ecology of striped bass, Morone saxatilis, in a freshwater reservoir (Lake Texoma, U.S.A.)

Many vertebrate predators consume a wide variety of prey types, depending upon availability and vulnerability. In contrast, striped bass, Morone saxatilis , that have been introduced to Lake Texoma (Oklahoma-Texas, U.S.A.) use a very limited array of fish (mostly clupeids of the genus Dorosoma ) as prey. As a large, mobile predator, M. saxatilis should be capable of capturing and consuming numerous other species of fish that are available in the reservoir. However, examination of 1845 stomachs year-around over 5 years showed that the only marked 'switching' among prey was from Dorosoma to a diet including a high percentage of insects during spring–early summer, ignoring most other fish taxa that could have served as food. Even under essentially starvation conditions in late summer of years with scarce Dorosoma, M. saxatilis in Lake Texoma did not switch to other available fish as prey. Patterns of predation by M. saxatilis are trenchantly different from place to place: very narrow prey selectivity even under starvation conditions has been reported once previously for the species in a freshwater reservoir, but in its native marine and estuarine environment and in some other reservoirs the species is more catholic in its use of prey. Why this large predator shows fidelity in some environments to particular prey, even to the extent of starvation, remains an enigma.     

Nitrogen retention in rivers: model development and application to watersheds in the northeastern U.S.A.

A regression model (RivR-N) was developed that predicts the proportion of N removed from streams and reservoirs as an inverse function of the water displacement time of the water body (ratio of water body depth to water time of travel). When appliedto 16 drainage networks in the eastern U.S.,the RivR-N model predicted that 37% to 76%of N input to these rivers is removed duringtransport through the river networks.Approximately half of that is removed in1st through 4th order streams whichaccount for 90% of the total stream length. The other half is removed in 5th orderand higher rivers which account for only about10% of the total stream length. Most Nremoved in these higher orders is predicted tooriginate from watershed loading to small andintermediate sized streams. The proportion ofN removed from all streams in the watersheds(37–76%) is considerably higher than theproportion of N input to an individual reachthat is removed in that reach (generally<20%) because of the cumulative effect ofcontinued nitrogen removal along the entireflow path in downstream reaches. Thisgenerally has not been recognized in previousstudies, but is critical to an evaluation ofthe total amount of N removed within a rivernetwork. At the river network scale,reservoirs were predicted to have a minimaleffect on N removal. A fairly modest decrease(<10 percentage points) in the N removed atthe river network scale was predicted when athird of the direct watershed loading was tothe two highest orders compared to a uniformloading.     

Abundance patterns of filter-feeding Caddisflies (Trichoptera : Hydropsychidae) and seston in a Montana (U.S.A.) lake outlet

The abundance patterns of four species of net-spinning caddisflies (Trichoptera : Hydropsychidae) were studied in a Montana (U.S.A.) lake outlet stream. Total Hydropsychidae reached very high densities (X > 1400 / 0.2 m²) near the lake outfall but declined precipitously downstream. Abundance patterns generally agree with a published model of filter-feeder / seston interactions in lake outlets. Larval densities were greater on moss-covered substrates and one species occupied habitats of lower water velocity compared to other species. Seston concentrations increased downstream contrary to expectation. Nutrient quality of seston may be important in predicting abundance of stream filter-feeders.     

Temporal and spatial patterns in littoral-zone fish assemblages of a reservoir (Lake Texoma,Oklahoma-Texas,U.S.A.)

Synopsis We sampled the littoral-zone fish fauna of Lake Texoma reservoir by electrofishing from January through December 1986 to examine species abundance, species associations and assemblage structure. Although total fish abundance differed significantly across seasons, only one common species (Dorosoma cepedianum) exhibited significant seasonal movement into or out of the littoral zone. Overall littoral-zone assemblage structure (based on rank order of species abundance) was concordant across seasons and habitat types, (vegetation, wood, open). However, within individual seasons and habitat types, assemblage structure was likely influenced by temporal and spatial differences in habitat availability and physicochemical conditions. Associations characteristic of species in natural aquatic environments were not well developed among species in this partly artificial, evolutionarily short-lived reservoir assemblage. Conditions related to water-level fluctuation appeared to deter the formation of persistent species associations and assemblage structure, especially in vegetation and open littoral zone habitats of this multi-purpose reservoir.Senior author     

Nitrogen uptake and transformation in a midwestern U.S. stream: A stable isotope enrichment study

This study presents a comprehensive analysis ofnitrogen (N) cycling in a second-order forestedstream in southern Michigan that has moderatelyhigh concentrations of ammonium (mean,16 g N/L) and nitrate (17 g N/L). Awhole-stream ¹⁵NH₄ ⁺ addition wasperformed for 6 weeks in June and July, and thetracer ¹⁵N was measured downstream inammonium, nitrate, and detrital and livingbiomass. Ancillary measurements includedbiomass of organic matter, algae, bacteria andfungi, nutrient concentrations, hydrauliccharacteristics, whole-stream metabolism, andnutrient limitation assays. The resultsprovide insights into the heterotrophic natureof woodland streams and reveal the rates atwhich biological processes alter nitrogentransport through stream systems.Ammonium uptake lengths were 766–1349 m anduptake rates were 41–60 g N m^–2min^–1. Nitrate uptake could not bedetected. Nitrification rates were estimatedfrom the downstream increase in¹⁵N-enriched nitrate using a simulationmodel. The ammonium was removed bynitrification (57% of total uptake),heterotrophic bacteria and fungi associatedwith detritus (29%), and epilithic algae(14%). Growth of algae was likely limited bylight rather than nutrients, and dissolvedO₂ revealed that the stream metabolism washeterotrophic overall (P:R = 0.2). Incubationsof detritus in darkened chambers showed thatuptake of ¹⁵N was mostly heterotrophic.Microbial N in detritus and algal N inepilithon appeared to reach isotopic steadystate with the dissolved ammonium, but theisotopic enrichment of the bulk detritus andepilithon did not approach that of ammonium,probably due to a large fraction of organic Nin the bulk samples that was not turning over. The actively cycling fraction of total N inorganic compartments was estimated from theisotopic enrichment, assuming uptake ofammonium but not nitrate, to be 23% forepilithon, 1% for fine benthic organic matter,5% for small woody debris, and 7% for leaves. These percentages agree with independentestimates of epilithic algal biomass, whichwere based on carbon:chlorophyll ratios in bulksamples and in algal fractions separated bydensity-gradient centrifugation in colloidalsilica, and of microbial N in the detritus,which were based on N released by chloroformfumigations.     

Seasonal changes in the chemistry and biology of a meromictic lake (Big Soda Lake,Nevada, U.S.A.)

Big Soda Lake is an alkaline, saline lake with a permanent chemocline at 34.5 m and a mixolimnion that undergoes seasonal changes in temperature structure. During the period of thermal stratification, from summer through fall, the epilimnion has low concentrations of dissolved inorganic nutrients (N, Si) and CH₄, and low biomass of phytoplankton (chlorophyll a ca. 1 mgm ^-3). Dissolved oxygen disappears near the compensation depth for algal photosynthesis (ca. 20 m). Surface water is transparent so that light is present in the anoxic hypolimnion, and a dense plate of purple sulfur photosynthetic bacteria (Ectothiorhodospira vacuolata) is present just below 20 m (Bchl a ca. 200 mgm^-3). Concentrations of N H₄ ⁺, Si, and CH₄ are higher in the hypolimnion than in the epilimnion. As the mixolimnion becomes isothermal in winter, oxygen is mixed down to 28 m. Nutrients (NH₄ ⁺, Si) and CH₄ are released from the hypolimnion and mix to the surface, and a diatom bloom develops in the upper 20 m (chlorophyll a > 40 mgm^-3). The deeper mixing of oxygen and enhanced light attenuation by phytoplankton uncouple the anoxic zone and photic zone, and the plate of photosynthetic bacteria disappears (Bchl a ca.10mgm^-3). Hence, seasonal changes in temperature distribution and mixing create conditions such that the primary producer community is alternately dominated by phytoplankton and photosynthetic bacteria: the phytoplankton may be nutrient-limited during periods of stratification and the photosynthetic bacteria are light-limited during periods of mixing.     

Stream macroinvertebrate response to catchment urbanisation (Georgia, U.S.A.)

SUMMARY 1. The effects of catchment urbanisation on water quality were examined for 30 streams (stratified into 15, 50 and 100 km² ± 25% catchments) in the Etowah River basin, Georgia, U.S.A. We examined relationships between land cover (implying cover and use) in these catchments (e.g. urban, forest and agriculture) and macroinvertebrate assemblage attributes using several previously published indices to summarise macroinvertebrate response. Based on a priori predictions as to mechanisms of biotic impairment under changing land cover, additional measurements were made to assess geomorphology, hydrology and chemistry in each stream. 2. We found strong relationships between catchment land cover and stream biota. Taxon richness and other biotic indices that reflected good water quality were negatively related to urban land cover and positively related to forest land cover. Urban land cover alone explained 29–38% of the variation in some macroinvertebrate indices. Reduced water quality was detectable at c. >15% urban land cover. 3. Urban land cover correlated with a number of geomorphic variables such as stream bed sediment size (–) and total suspended solids (+) as well as a number of water chemistry variables including nitrogen and phosphorus concentrations (+), specific conductance (+) and turbidity (+). Biotic indices were better predicted by these reach scale variables than single, catchment scale land cover variables. Multiple regression models explained 69% of variation in total taxon richness and 78% of the variation in the Invertebrate Community Index (ICI) using phi variability, specific conductance and depth, and riffle phi, specific conductance and phi variability, respectively. 4. Indirect ordination analysis was used to describe assemblage and functional group changes among sites and corroborate which environmental variables were most important in driving differences in macroinvertebrate assemblages. The first axis in a non‐metric multidimensional scaling ordination was highly related to environmental variables (slope, specific conductance, phi variability; adj. R²=0.83) that were also important in our multiple regression models. 5. Catchment urbanisation resulted in less diverse and more tolerant stream macroinvertebrate assemblages via increased sediment transport, reduced stream bed sediment size and increased solutes. The biotic indices that were most sensitive to environmental variation were taxon richness, EPT richness and the ICI. Our results were largely consistent over the range in basin size we tested.     

Organic matter processing by a stream-segment ecosystem: Fort River,Massachusetts, U.S.A.

An annual organic matter budget for a 1700 m segment of Fort River (Massachusetts, USA) is presented. Primary production in this fourth order stream exceeds litter input annually, however ecosystem P/R is 0.5. Respiration in excess of gross primary production is supported by allochthonous organic matter imported from upstream reaches. The relative contribution of organic matter size fractions to stream consumers depends upon biologic lability, rate of input, and residence time in the ecosystem. Particles of seston size (1 μm to 1 mm) are most heavily used by consumers, however dissolved organic matter represents the largest input component. Microorganisms are the predominant consumers in this soft-water, nutrient-poor stream ecosystem. A conceptual model for assessing the processing efficiency of stream ecosystems is presented and discussed in terms of several headwater to estuary gradients.     

Experimental studies of zooplankton–phytoplankton–nutrient interactions in a large subtropical lake (Lake Okeechobee, Florida, U.S.A.)

1. Over a 1-year period, twenty controlled experiments were performed using small mesocosms (20-l clear plastic carboys) and plankton communities collected from four sites in shallow, subtropical Lake Okeechobee, Florida. In replicated treatments, macrozooplankton grazers were excluded by size fractionation (115 μm), and/or nutrients (N and P) were added, and impacts on phytoplankton biomass and productivity were measured after 3-day incubations.
2. In most experiments (fifteen out of twenty), there was no significant effect of zooplankton exclusion on phytoplankton biomass or productivity, but there were significant increases in those attributes due to nutrient additions. The magnitude of the responses was a function of light availability at the collection sites.
3. In three experiments, zooplankton exclusion led to declines in phytoplankton biomass and productivity, suggesting that animals may sometimes have net positive effects on the phytoplankton, perhaps via nutrient recycling.
4. In only two experiments was there evidence of net negative impacts of grazers on the phytoplankton. In both instances, cladocerans ( Daphnia ambigua and Eubosmina tubicen ) were dominant in the zooplankton. However, the increases in chlorophyll a due to zooplankton exclusion were small (5–20%), probably because of the small size and relatively low grazing rates of the cladocerans.
5. The results support the hypothesis that phytoplankton biomass in Lake Okeechobee is little affected by herbivorous macrozooplankton. This may be a common feature of lowland tropical and subtropical lakes.     

Experimental studies of zooplankton–phytoplankton–nutrient interactions in a large subtropical lake (Lake Okeechobee, Florida, U.S.A.)

1. Over a 1-year period, twenty controlled experiments were performed using small mesocosms (20-l clear plastic carboys) and plankton communities collected from four sites in shallow, subtropical Lake Okeechobee, Florida. In replicated treatments, macrozooplankton grazers were excluded by size fractionation (115 μm), and/or nutrients (N and P) were added, and impacts on phytoplankton biomass and productivity were measured after 3-day incubations.
2. In most experiments (fifteen out of twenty), there was no significant effect of zooplankton exclusion on phytoplankton biomass or productivity, but there were significant increases in those attributes due to nutrient additions. The magnitude of the responses was a function of light availability at the collection sites.
3. In three experiments, zooplankton exclusion led to declines in phytoplankton biomass and productivity, suggesting that animals may sometimes have net positive effects on the phytoplankton, perhaps via nutrient recycling.
4. In only two experiments was there evidence of net negative impacts of grazers on the phytoplankton. In both instances, cladocerans ( Daphnia ambigua and Eubosmina tubicen ) were dominant in the zooplankton. However, the increases in chlorophyll a due to zooplankton exclusion were small (5–20%), probably because of the small size and relatively low grazing rates of the cladocerans.
5. The results support the hypothesis that phytoplankton biomass in Lake Okeechobee is little affected by herbivorous macrozooplankton. This may be a common feature of lowland tropical and subtropical lakes.     

Epizoan associations in a lower mississippian paleocommunity (borden group,Indiana, U.S.A.)

Epizoans were part of an epifaunal suspension‐feeding paleocommunity of the interchannel mudstone fades of the Borden delta (Edwardsville Formation, Mississippian, Indiana, USA). The paleocommunity was composed predominantly of bryozoans, crinoids, and brachiopods. A total of 26 epizoans and 42 potential hosts were present with Composita globosa, crinoids, and Cladochonus beecheri the principal hosts. Associations are classified as obligate, preferred, random, variable, and limited data. Stenoporid sp. B, Phosphannulus sp., and Platyceras spp. had obligate relationships with crinoids; Cornulitella sp. and Dendrina sp. had obligate relationships with Composita globosa; Ropalonaria sp. had an obligate relationship with Cladochonus beecheri; and Amplexizaphremis? sp. had an obligate relationship with fenestrate bryozoans. The following taxa had preferred associations to more than one host: Ascodictyon sp., Cheilotrypa sp., Corynotrypa sp. A, Corynotrypa sp. B, Fistulipora sp., Heteralosia keokuk, stenoporid sp. A, and fenestrates. Totypammina sp. displayed a nearly random pattern of settlement; and Cladochonus beecheri, Spirorbis sp., Thuramminoides sp., and crinoids have variable associations with hosts between the two localities studied.     

The macrophytic community of an acidic lake in adirondack (New York,U.S.A.): A new depth record for aquatic angiosperms

The submerged vegetation of Silver Lake (Herkimer County, NY, U.S.A.) was extensively surveyed to help to determine the effects of low pH on aquatic biota. This extra-ordinarily clear lake (Secchi disk > 18 m, the maximum depth of the lake), was low in conductivity (27 μS cm^?1), pH (4.8) chlorophyllα (0.11 μg l[^?1), and total phosphorus (<1 μg 1^?1).The vegetation was dominated, in terms of percentage of the total plant community, by Utricularia spp. (47%), Sphagnum spp. (23%), and Eriocaulon septangulare With. (17%). Distinct depth zonation was evident, with U. geminiscapa Benj. extending to the deepest part of the lake (18 m). This represents a new maximum depth penetration for aquatic angiosperms. The mechanisms by which angiosperms are limited in depth-distribution are discussed. It is concluded that single factor theories, those which ascribe the maximum depth of penetration to a single variable (e.g., light, temperature, hydrostatic pressure), are all deficient, and that the actual limitation is brought about by a combination of factors.