Walter K. Dodds

Community interactions between the filamentous alga Cladophora glomerata (L.) Kuetzing, its epiphytes, and epiphyte grazers

SummaryInteractions between epiphytes, epiphyte grazers and the filamentous green alga Cladophora glomerata (L.) Kuetzing were explored with smaples from rivers in Montana. Extracts of C. glomerata lowered photosynthetic rates of Nitzschia fonticola Grunow (an epiphytic diatom). Nutrient enrichment showed that C. glomerata from the Madison River was N deficient and its epiphytes were P deficient on 2 dates and N deficient on one date, while no nutrient deficiencies were detected in samples from 3 other rivers; this implies there was little nutrient competition between the epiphytes and C. glomerata. Epiphytes lowered drag on C. glomerata tufts and current velocity inside the tufts, apparently by decreasing the effective surface area. Lower drag may decrease detachment, but lowering current velocity from 8 to 0 cm s-1 resulted in a 100 % decrease in photosynthesis. Light absorption by epiphyte pigments may lower photosynthetic rate of C. glomerata when irradiance is below 200–500 μE m-2 s-1, and protect against photoinhibition above this irradiance range. Invertebrate grazers (predominantly Baetis tricaudatus Dodds, Trycorythodes minutus Traver and Brachycentrus occidentalis Banks) at high densities removed 75% of epiphytes and B. occidentalis grazed on C. glomerata. Invertebrates regenerated a mean of 0.16 μmol NHinf4sup+individual-1 d-1 which could have enhanced growth of downstream C. glomerata. Competition and grazing were not the only interactions in the C. glomerata community, positive (mutualistic) interactions were also important.

Factors associated with dominance of the filamentous green alga Cladophora glomerata

Abstract Cladophora glomerata is a filamentous green alga dominant in many large rivers of Montana, U.S.A. I studied several factors which may be associated with nuisance (> 100 mg m−2 chl a, Welch et al., Hydrobiologia157, 161–168, 1988) C. glomerata biomass: temperature, pH, conductivity, NH4+, soluble reactive phosphorus (SRP), NO3−, flow and epiphyte load. Samples were collected at 6 sites, ranging from an irrigation ditch to large rivers. Low flow, high NH4+ and low epiphyte loads were all correlated (P

Photosynthesis of two morphologies of Nostoc parmelioides (Cyanobacteria) as related to current velocities and diffusion patterns

Colonies of the stream‐inhabiting cyanobacterium Nostoc parmelioides Kützing often contain a single endosymbiotic dipteran larva Cricotopus nostocicola (Wirth), which induces a morphological change from small, spherical colonies to larger, ear‐shaped colonies. At a current velocity of 0 cm · s−1, whole colonies containing the midge showed overall rates of 14CO2 uptake and nitrogenase activity that were higher than those when the midge was absent (sphere‐shaped colonies). Spherical colonies incubated at current velocities of 5‐10 cm · s−1did not show higher rates of 14CO2 or 15N2 incorporation than those with the larvae (ear‐shaped colonies). Ear‐shaped colonies extended well into regions of higher current velocity, whereas spherical colonies did not. Photosynthesis of ear‐shaped colonies was stimulated by increased current velocity, increased inorganic C and decreased O2 concentrations. Moreover, levels of O2 at the surface of midge‐inhabited colonies decreased with increased current velocity. The morphological change induced by the larva is detrimental (lowers photosynthesis and N2 fixation) in quiescent water but not at current velocities above 10 cm · s−1. This is probably a result of higher diffusion of O2 and CO2 associated with the midge‐induced morphology.

AMMONIUM, NITRATE, PHOSPHATE, AND INORGANIC CARBON UPTAKE IN AN OLIGOTROPHIC LAKE: SEASONAL VARIATIONS AMONG LIGHT RESPONSE VARIABLES1

The dependence of substrate saturated uptake of 15NH4+, 15NO3−, 32PO43−, and 14CO2 on photosynthetic photon flux density (PPFD or photsynthetically active radiation, 400–700 nm) was characterized seasonally in oligotrophic Flathead Lake, Montana. PO43− uptake was not dependent upon PPFD at any time of the year, whereas NH4+, NO3−, and CO2 uptake were consistently dependent on PPFD over all seasons. Maximal rates of NH4+, NO3− and CO2 uptake usually occurred near 40% of surface PPFD, which corresponded to about 5 m in the lake; inhibition was evident at PPFD levels greater than 40%. NH4+, NO3− and PO43− were incorporated in the dark at measurable rates most of the year, whereas dark CO2 uptake was always near 0 relative to light uptake. CO2 and NO3− uptake were more strongly influenced by PPFD than was NH43− uptake. The PPFD dependence of PO43−, NH4+, NO3− and CO2 uptake may affect algal growth and nutrient status by influencing the balance in diel and seasonal C:N:P uptake ratios.

Mesocosm studies on the influence of phosphate enrichment on ammonium and nitrate flux in an oligotrophic lake

The fate of 15NHinf4sup+and 15NOinf3− was followed in control and POinf4sup3−enriched 1570 l mesocosms filled with epilimnetic water from an oligotrophic Rocky Mountain lake. Volumetric incorporation of 15NHinf4sup+and 15NOinf3sup−into phytoplankton and bacterioplankton (particulates between 280 and 0.7 µm), and crustacean zooplankton > 80 µm was enhanced by POinf4sup3−, but no increase in biomass specific rates of uptake by phytoplankton and bacteria occurred for either form of 15N. Dilution of both 15NHinf4sup+and 15NOinf3sup−by 14NHinf4sup+and 14NOinf3sup−, respectively, was evident indicating regeneration of these nutrients, but regeneration rates were not effected by POinf4sup3−enrichment. The results illustrate the strong trophic coupling between N dynamics and POinf4sup3−enrichment in this system.

Hydrodynamic constraints on evolution of chemically mediated interactions between aquatic organisms in unidirectional flows

Two hydrodynamic habitats can be defined in unidirectional flow: (1) A boundary where molecular diffusion dominates is found within roughly 1 mm of solid substrates. The actual thickness of this diffusion boundary is a function of current velocity and topography of the substrates. (2) A zone of turbulent flow is present outside the diffusion boundary where chemicals are rapidly diluted and transported downstream. The mode of chemically mediated interaction between organisms in streams is constrained differently depending on which of these two habitats they occupy. Within a diffusion boundary, reciprocal interaction between small organisms is possible because mixing is low, diffusion flux high, and organisms “share” the same water. Outside a diffusion boundary, in unidirectional flow, organisms unable to move effectively against flow are only chemically influenced by upstream organisms and can only influence downstream organisms. Organisms that are able to move upstream can interact reciprocally with other organisms, even if one or both organisms are found primarily in areas of turbulent mixing.

Nitrogenase activity, photosynthesis, and the degree of heterocyst aggregation in the cyanobacterium Anabaena flos-aquae

The nitrogen‐fixing cyanobacterium Anabaena flosaquae Lyngb.) De Breb. exhibited aggregation of heterocysts from different filaments in a eutrophic lake and when grown in unialgal culture. The resulting aggregated filaments formed unialgal flocculent masses having a thickness of several centimeters that apparently resulted from cohesive mucilage surrounding heterocysts. We tested the effects of heterocyst aggregation on nitrogenase activity (NA) and photosynthesis in relation to microscale environmental O2 gradients. The redox indicator 2,3,5‐triphenyl tetrazolium chloride showed that aggregated heterocysts had lower intracellular redox potential than those that were dispersed. Microelectrode measurements showed that heterocyst aggregates in actively photosynthesizing flocculent masses were surrounded by a microzone of O2 30% higher than in the surrounding water: dispersed cells exhibited no such elevated O2 microzone. Despite high levels of O2, NA was greater in aggregated than dispersed samples, Microscale irradiance measurements made with a fiber optic light probe showed that 40% of the incident light was absorbed within the first 3 mm of a 1‐cm‐thick flocculent mass. The microscale irradiance data, together with nitrogenase and photosynthesis versus irradiance data, imply that the ratio of N:C fixation is lowest in filaments on the outside of 1.5–2.0‐cm masses and increases toward the center.

Zooplankton induced decrease in inorganic phosphorus uptake by plankton in an oligotrophic lake

Experiments conducted on samples collected from a large oligotrophic lake revealed the following: (1) excretion rates of POinf4sup3−by single Daphnia thorata were below detection (5 pmol animal−1 min−1) in 20 ml of oligotrophic lake water over a period of 10 min, (2) experimental addition of D. thorata to 20 ml aliquots of lake water decreased community-wide microbial uptake of POinf4sup3−on two occasions (as measured by 32POinf4sup3−incorporation), and (3) the presence of D. thorata increased uptake by organisms smaller than 1µm, and decreased uptake by large phytoplankton. The specific mechanism for these responses remains unclear, but the results imply that when phytoplankton larger than 1µm encounter cm scale patches of water recently occupied by Daphnia they may experience decreased POinf4sup3−availability rather than elevated concentrations of POinf4sup3−caused by excretion. We show that 32P uptake experiments using natural plankton assemblages can be influenced by the presence or absence of large zooplankton, and that neither grazing, turbulence, nor POinf4sup3−excretion can account for this influence.

Development and application of a technique for estimating nutrient deficiency in soft sediments

A diffusion enrichment technique is presented which allows for chemical enrichment of soft surficial and shallow subsurface sediments and subsequent measurement of O2 production. The sediment is enriched by inserting a perforated tube containing dialysis tubing filled with a nutrient/agar mixture. O2 production by surficial sediment is measured using an inverted, translucent, polyethylene chamber over the sediment. The inside of the chamber contains a collapsible bag connected to the water outside the chamber. When water overlying the sediment is withdrawn from a sampling port, it is displaced with water from outside the chamber, thus preventing contamination of water samples with pore water from below. The technique was tested by enriching near-shore sediments in a large oligotrophic lake with inorganic N and P. NHinf4/p+ additions significantly stimulated benthic primary production as measured by 02 production, whereas enrichment with POinf4/3- had no effect.