James B. Cotner

Fish Assemblage Structure in Relation to Environmental Variation among Brazos River Oxbow Lakes

Abstract Fish assemblages and habitat conditions of oxbow lakes and the main channel of the middle-lower Brazos River, a meandering lowland river in east central Texas, were investigated during summer 1994. All oxbows were eutrophic, with chlorophyll-a levels of up to 640 μg/L. Assemblage structure showed large between-lake variation that was explained by both physical and biotic variables, with combinations of water depth, dissolved oxygen, dissolved nutrients, turbidity, and plankton densities accounting for 45–59% of the variation in abundance of the dominant species. Water depth and dissolved nutrient concentrations were the best predictors of species diversity and fish abundance. Periodic desiccation of shallow, vegetated oxbows created harsh conditions that favored small fishes that are efficient colonizers. The two youngest oxbows were relatively deep and contained a high diversity and biomass of fishes. Of the 42 fish species collected, several were largely restricted to oxbow lakes, and others we...

Nutrient Limitation of Heterotrophic Bacteria in Florida Bay

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.

Effects of Zebra Mussels (Dreissena polymorpha) on Bacterioplankton: Evidence for Both Size-Selective Consumption and Growth Stimulation

Abstract Zebra mussels had significant direct and possible indirect effects on heterotrophic bacteria in two contrasting sites in Saginaw Bay. At a eutrophic site in the inner portion of Saginaw Bay, mussels fed directly on bacterial-sized particles and had a negative impact on bacterial abundances. Mussels removed large bacteria (> 0.9xa0μm) more effectively than small bacteria at this site. Individual mussels cleared from 37–89xa0ml per day. Results using different sizes of fluorescent microspheres suggest that zebra mussels have a lower limit for particle size removal that is less than 0.4xa0μm. Contradictory to inner bay results, mussels at an outer bay oligotrophic site had a positive impact on heterotrophic bacterial abundance, perhaps as a result of indirect effects, such as nutrient or organic carbon excretion by the mussels. Differences in the impact of mussels on the bacterial communities of the inner bay and outer bay probably result from differences in trophic state and bacterial community structure. A hypothesized smaller size of bacteria at outer bay sites may enable them to escape heavy predation pressure from mussels and the high rates of mussel nutrient excretion may facilitate their growth in these nutrient depleted conditions.

Seasonally variable riverine production in the Venezuelan llanos

Abstract We examined primary production, respiration, and nutrient dynamics in littoral areas of the mainstem and lagoons of the Cinaruco, a nutrient-poor river in the Venezuelan llanos. Gross primary productivity (GPP) was relatively high, given the poor nutrient conditions in this river. Seasonal variability in net ecosystem production (NEP) was also high, with highest values occurring in the dry season (March–April) when fish biomass and chlorophyll a (both water-column and benthic) levels were greatest (dry-season NEP = 542 mg C m−2 d−1, wet-season NEP = 303 mg C m−2 d−1). NEP and algal biomass (measured as chlorophyll a) were higher in lagoons than at river sites, with more pronounced differences between these 2 habitats during the dry season. Strong N limitation was evident. Dissolved inorganic N (DIN) concentrations always were <2 μM and typically were <0.5 μM. Molar ratios of DIN:SRP (soluble reactive P) varied little seasonally and were always <8. As in other Neotropical aquatic systems, water-column productivity was an important source of organic matter and was >2× benthic productivity, even in shallow regions of the riverine ecosystem. Low nutrient levels combined with high rates of autochthonous productivity in the littoral zone of this river suggest extremely rapid nutrient cycling rates and support the view that the littoral regions may be important in providing the organic matter that maintains secondary production, consistent with the Riverine Productivity Model and other observations in the Orinoco Basin.

Mineralization of Organic Material and Bacterial Dynamics in Mississippi River Plume Water

Net remineralization rates of organic matter and bacterial growth rates were observed in dark-bottle incubation experiments conducted in July–August and February with water samples collected from sites in the Mississippi River plume of the Gulf of Mexico. Our objectives were to measure site-specific degradation rates of labile dissolved and particulate organic matter, quantify the potential importance of bacteria in these processes, and examine the kinetics of degradation over time. Unfiltered samples, and samples treated to remove (or dilute out) particles larger than bacteria, were enclosed in 9-1 bottles and incubated in the dark for 3–5 d. Respiration rates and inorganic compound accumulation rates were higher in summer than in winter and were highest in unfiltered surface samples at sites of intermediate salinities where phytoplankton were most abundant. The ratio of ammonium accumulation to oxygen removal in summer experiments suggested that the mineralized organic material resembled “Redfield” stoichiometry. Chemical fluxes were greater in bottles containing large (>1–3 μm) particles than in the bottles with these particles removed, but bacterial activities were generally similar in both treatments. These results suggest that particle consumers were an important component of total organic matter degradation. However, these experiments may have underestimated natural bacterial degradation rates because the absence of light could affect the production of labile organic substrates by phytoplankton. In agreement, with this hypothesis, bacterial growth rates tended to decrease over time in summer in surface plume waters where phytoplankton were abundant. In conjunction with other data, our results indicate that heterotrophic processes in the water column are spatially and temporally dependent on phytoplankton production.

Nitrogen Dynamics in Sandy Freshwater Sediments (Saginaw Bay, Lake Huron)

Sediment-water nitrogen fluxes and transformations were examined at two sites in Saginaw Bay, Lake Huron, as a model for sandy freshwater sediments. Substantial ammonium release rates (74 to 350 μmole NH4+/m2/h1) were observed in flow-through cores and in situ benthic chamber experiments. Sediment-water ammonium fluxes were similar at the inner and outer bay stations even though inner bay waters are enriched with nutrients from the Saginaw River. The high net flux of remineralized ammonium into the overlying water from these sandy sediments resembles typical data for marine systems (11 to 470 μmole NH4+/m2/h1) but were higher than those reported for depositional freshwater sediments (0 to 15 μmole NH4+/m2/h1; Seitzinger 1988). Addition of montmorillonite clay (ca. 1 kg dry weight/m2) to the top of the sandy cores reduced ammonium flux. Mean “steady-state” ammonium flux following clay addition was 46 ± 2 (SE) % of the initial rates as compared to 81 ± 8% of the initial rates without clay addition. Zebra mussel excretion dominanted ammonium regeneration in the inner bay where the bivalve was abundant, but addition of zebra mussel feces/psuedofeces (3.0 g dw/m2) to sediments did not increase ammonium or nitrate flux. Partial nitrification of ammonium at the sediment-water interface was suggested by removal of added 15NH4+ from lake water passing over dark sediment cores. Sediment-water fluxes of nitrogen obtained from flow-through sediment cores resembled those from in situ benthic chambers. However, extended static incubations in gas-tight denitrification chambers caused more of the regenerated nitrogen to be nitrified and denitrified than occurred with the other two measurement systems.

Temporal changes in oxygen demand and bacterial sulfate reduction in inland shrimp ponds

Abstract Changes in sediment oxygen demand, sulfate reduction rates, and sulfate reducing bacteria (SRB) abundances were monitored in nine 0.04–0.08 ha earthen shrimp ponds during a 17-week growing season. Organic matter (feed) was added at two rates (standard and high) in a block design. The aquaculture site was located in an inland region and utilized saline groundwater of lower total dissolved salts and a proportionally higher sulfate concentration than undiluted seawater. Sediment oxygen demand was measured in situ in an enclosed chamber, sulfate reduction rates were measured in cores using injected 35 SO 4 – , and SRB counts were estimated using most probable number analyses. Over the growing season, sediment oxygen demand increased four-fold, SRB populations increased by more than one order of magnitude, and sulfate reduction rates increased over two orders of magnitude, with the highest values occurring at the end of the season. Sulfide flux measurements within chambers demonstrated that sulfide re-oxidation represented a greater proportion of sediment oxygen demand at the end of the season (mean 84%) than at the beginning ( r 2 = 0.66). Furthermore, significant differences ( P

The Effects of Benthivorous Smallmouth Buffalo (Ictiobus bubalus) on Water Quality and Nutrient Cycling in a Shallow Floodplain Lake

ABSTRACT Experimental mesocosm studies revealed that the presence of benthivorous smallmouth buffalo (Ictiobus bubalus) significantly enhanced turbidity, phytoplankton biomass, ammonium (NH4 +), and total nitrogen (N) and phosphorus (P) concentrations in a shallow, aerobic, hypereutrophic oxbow lake. The effects of Ictiobuson water quality are similar to the results of experiments performed on other benthivorous fish species. Prior studies have suggested that enhanced phytoplankton biomass in the presence of benthivorous fishes is a result of P excretion by fish and physical relocation of sediment P to the water column. However, our results revealed that enhanced microbial NH4 + regeneration and subsequent uptake by phytoplankton may also be an important factor causing increased phytoplankton biomass levels in the presence of benthivorous fishes. Net sediment plus water column particle NH4 + fluxes were over six times higher in the presence of Ictiobus, relative to the control. Increased biological demand...

Zooplankton Community Structure of Lake Livingston, Texas, as Related to Paddlefish Food Resources

ABSTRACT The zooplankton community of Lake Livingston was examined in terms of composition, abundance, distribution, and appropriateness as a food resource for paddlefish, Polyodon spathula. By comparing the range of interraker distances of paddlefish (0.06–0.08 mm) to size measurements of different zooplankton taxa, we determined that paddlefish do not consume small rotifers and copepod nauplii as efficiently as they do larger, copepods and cladocerans. All zooplankton taxa (rotifers, cladocerans, and copepods) were comprised of relatively small organisms, and community abundance and biomass were dominated by small rotifers and copepod nauplii most of the year. In fall and winter, however, the biomass was dominated by copepods. A bioenergetics model predicted the paddlefish biomass that could be supported in Lake Livingston at a density of 1.17 kg ha−1, with a range of 0.06 to 10.45 kg ha−1.