Robert D. Doyle

Modeling submersed macrophyte growth in relation to underwater light climate: modeling approaches and application potential

The underwater light climate is one of the most important determinants of submersed aquatic vegetation. Because of the recent, large-scale, declines in aquatic vegetation, largely attributed to deterioration of the underwater light climate, interest in tools to predict the wax and wane of aquatic macrophyte populations has greatly increased. This paper summarizes two modeling approaches that can be applied to assess impacts of changes in underwater light climate on submersed vegetation. The first, stand-alone, model type focuses on metabolism and biomass formation of submersed freshwater macrophytes with difference in phenologies. This type is illustrated by examples from various sites using models developed for the freshwater macrophytes Hydrilla verticillata (L.f.) Royle (HYDRIL) and Myriophyllum spicatum L. (MILFO), and also by an example ecological risk assessment. The models (HYDRIL and MILFO) track carbon flow through the vegetation in meter-squared (m2) water columns. The models include descriptions of various factors that affect biomass dynamics, such as site-characteristic changes in climate, latitude, light attenuation within the water column, carbon assimilation rate at light saturation, temperature, wintering strategies, grazing and mechanical control (removal of shoot biomass). Simulated biomass, net assimilation and maintenance respiration over a relatively short (1–5 year) period agree well with measured values. The models are, therefore, believed to be suitable for predicting plant community production, growth and survival characteristics over relatively short periods over a large range of sites. The feasibility of using a macrophyte growth model of the HYDRIL type for ecological risk assessment is demonstrated. It is used to evaluate the consequences of management changes in large rivers for the survival of submersed vegetation. The current assessment evaluates the potential impact of increased commercial navigation traffic on the growth of Potamogeton pectinatus L. in Pool 4 of the Upper Mississippi River, U.S.A. In this case, navigational traffic scenarios were translated into suspended solids concentrations and underwater light climate, with the latter being used as inputs into the aquatic plant growth model. Model results demonstrate that the scenario increases in commercial traffic cause minimal decreases in growth and vegetative reproduction. Results indicate that this growth model can be a useful tool in ecological risk assessment, since the required stress-response relationships could be established. The second, integrated, model type focuses on the role of seagrass and other primary producers in estuarine littoral zone material cycling (carbon and nitrogen) at the Goodwin Islands, Virginia, U.S.A. The latter model was used to explore the effects of changes

PERIPHYTON NUTRIENT LIMITATION AND NITROGEN FIXATION POTENTIAL ALONG A WETLAND NUTRIENT-DEPLETION GRADIENT

While intensified nutrient limitation of periphyton has been reported along wetland nutrientdepletion gradients, changes to the specific nutrient that limits periphyton growth are not documented. In this study, we used artificial nutrient-diffusing substrata to determine nutrient limitation status of periphyton along a nitrogen- and phosphorus-depletion gradient in a freshwater marsh during the growing season of 2003. We also characterized water-column nutrient content, N:P ratio of dissolved nutrients, and periphytic N2 fixation potential along the gradient. Dissolved inorganic nitrogen concentrations consistently decreased (60%–95%) from inflow to outflow during all bioassays, while soluble reactive phosphorus concentrations decreased during the April (49%) and September (39%) bioassays but increased in the July bioassay (51%). Unequal N and P retention resulted in a general decrease in DIN:SRP mass ratio from 20.2 ± 5.0 to 3.8 ± 1.6 between inflow and outflow, respectively. Periphyton at the wetland inflow never responded to N additions alone, while periphyton at the outflow always responded to N enrichment. Periphyton at the inflow were either not limited by nutrients (September) or co-limited by N+P (April and July). At the outflow, periphyton were either N-limited (April and September) or strongly co-limited by N+P (July). A significant increase in N2 fixation potential (p<0.05) from inflow to outflow locations was noted for all measured events. Our results suggest that, in addition to the severity of nutrient limitation, some wetlands may display spatial heterogeneity in the specific nutrient that limits periphyton growth. Further, these shifts influence the structure and function of wetland periphyton assemblages.

Impact of herbivory by Hydrellia pakistanae (Diptera: Ephydridae) on growth and photosynthetic potential of Hydrilla verticillata

The impacts of varying levels of herbivory by Hydrellia pakistanae on the dioecious ecotype of Hydrilla verticillata were evaluated by conducting a 10-week growth experiment within mesocosm tanks. The observed leaf damage to H. verticillata stems was highly correlated with the total number of immature H. pakistanae in H. verticillata tissue at the time of harvest (P 0.80). Increasing levels of insect herbivory significantly impacted biomass and growth morphology of H. verticillata. Relative to control tanks, plants under intermediate or high levels of herbivory produced progressively less biomass. Insect herbivory also significantly impacted investment of energy in sexual and asexual reproduction. Plants under an intermediate or high level of herbivory produced fewer than 15% of the number of pistillate flowers produced by plants in control tanks. Furthermore, plants subject to high insect herbivory produced fewer and smaller tubers than control tanks. Finally, herbivory had a strong impact on the photosynthetic potential of stems. With 10–30% leaf damage, the maximum rate of light-saturated photosynthesis was reduced 30–40% relative to undamaged controls. Total daily photosynthetic production in these stems was estimated to balance, just barely, the daily respiratory needs of stems. Photosynthetic rate was reduced by about 60% in stems showing 70–90% leaf damage. This level of photosynthetic reduction would make continued survival of the plants unlikely since they would be unable to meet daily respiratory demands.

Evaluation of Phytoplankton–Limiting Factors in Lake Chapala, México: Turbidity and the Spatial and Temporal Variation in Algal Assay Response

ABSTRACT Laboratory algal bioassays using both cultures of Ankistrodesmus bibraianus and natural phytoplankton, and large, in-lake, container assays with natural populations, were used to determine the factor most limiting phytoplankton production in Lake Chapala, Mexico. Both types of laboratory culture assays showed that nitrogen was the principal limiting nutrient at each station across this very large lake in all seasons. The growth response of natural phytoplankton was similar to that of A. bibraianus. However, management practices to regulate the lakes productivity based solely upon this laboratory information would be inappropriate because the natural population assays showed that the ultimate limiting factor in situ is illumination controlled by the high clay turbidity. Rarely, if ever, was phytoplankton production controlled by the laboratory-determined limiting nutrient, nitrogen, expressed in the lake. The importance of performing algal assays extensively through time and space also was demons...

ARE WATERSHED AND LACUSTRINE CONTROLS ON PLANKTONIC N2 FIXATION HIERARCHICALLY STRUCTURED

N2 fixation can be an important source of N to limnetic ecosystems and can influence the structure of phytoplankton communities. However, watershed-scale conditions that favor N2 fixation in lakes and reservoirs have not been well studied. We measured N2 fixation and lacustrine variables monthly over a 19-month period in Waco Reservoir, Texas, USA, and linked these data with nutrient-loading estimates from a physically based watershed model. Readily available topographic, soil, land cover, effluent discharge, and climate data were used in the Soil and Water Assessment Tool (SWAT) to derive watershed nutrient-loading estimates. Categorical and regression tree (CART) analysis revealed that lacustrine and watershed correlates of N2 fixation were hierarchically structured. Lacustrine conditions showed greater predictive capability temporally. For instance, low NO3(-) concentration (<25 microg N/L) and high water temperatures (>27 degrees C) in the reservoir were correlated with the initiation of N2 fixation seasonally. When lacustrine conditions were favorable for N2 fixation, watershed conditions appeared to influence spatial patterns of N2 fixation within the reservoir. For example, spatially explicit patterns of N2 fixation were correlated with the ratio of N:P in nutrient loadings and the N loading rate, which were driven by anthropogenic activity in the watershed and periods of low stream flow, respectively. Although N2 fixation contributed <5% of the annual N load to the reservoir, 37% of the N load was derived from atmospheric N2 fixation during summertime when stream flow in the watershed was low. This study provides evidence that watershed anthropogenic activity can exert control on planktonic N2 fixation, but that temporality is controlled by lacustrine conditions. Furthermore, this study also supports suggestions that reduced inflows may increase the propensity of N2-fixing cyanobacterial blooms in receiving waters of anthropogenically modified landscapes.

Nitrogen fixation and phosphatase activity in periphyton growing on nutrient diffusing substrata: evidence for differential nutrient limitation in stream periphyton

Abstract We explored N2 fixation and alkaline phosphatase activity (APA) in periphyton from a N-limited stream ecosystem by coupling measurements of these processes with nutrient diffusion substrata (NDS) experiments. We measured periphyton biomass accumulation (as ash-free dry mass [AFDM] and chlorophyll a [CHLA]), N2 fixation, and APA to evaluate the relative importance of N2 fixation as an N source to the periphyton community and APA as an indicator of P deficiency in a seemingly N-limited system. We used fritted-glass-disc NDS and estimated AFDM, CHLA, N2 fixation, and APA on days 6, 18, and 29 after deployment. Periphyton AFDM steadily increased on NDS over time, but was not influenced by nutrients. CHLA was elevated in the N treatment on days 18 and 29, indicating autotrophic N limitation. Consistent with N limitation, N2 fixation was high but not different in the control and P treatments and was virtually undetectable on the N treatment. N2 fixation in control and P treatments was detectable in both light and dark incubations, and dark rates were 4 to 73% of the light rates on days 18 and 29. The average contribution of total N2 fixation to periphyton in control and P treatments was 0.93 mg N/m2 on day 18 and 1.0 mg N/m2 on day 29. APA was significantly elevated on the control and was highest in the N treatment despite no apparent P limitation of periphyton biomass accumulation. P enrichment always decreased APA. Measurable N2 fixation and the change in CHLA suggest that autotrophs were primarily N limited. However, APA observed in controls demonstrated that some portion of the periphyton community was experiencing P deficiency. This result suggests that periphyton metabolism was related to both N and P availability, but that biomass accumulation might have been limited primarily by N. One explanation for these findings is that different organisms, perhaps occupying different trophic positions within the community, might have been limited by different elements.

Nitrogen fixation by periphyton and plankton on the Amazon floodplain at Lake Calado

Nitrogen fixation by periphyton and plankton was measured on the Amazon flood-plain using the acetylene reduction method calibrated with15N-N2. The average ratio (± SD) of moles C2H4 reduced per mole N2-N fixed was 3.4 ± 0.7, similar to other studies. Periphyton and plankton had high rates of light-dependent nitrogen fixation, with dark nitrogen fixation averaging 26% of the average rates in the light. The average daily (24 h) rates for periphyton nitrogen fixation in 1989 and 1990 were 1.79 and 0.51 mmol N2-N·m−2·d−1 respectively, which are comparable to summer rates in many temperate cyanobacterial assemblages. Nitrogen fixation was depressed at N03− concentrations as low as 0.5 μM, and was below detection limits at concentrations of 4 μM, which occurred during periods of river flooding. Planktonic nitrogen fixation rates were high (0.5–0.8 mmol N2-N·m−2·d−1) during the high-water and drainage phases of the annual hydrograph when the floodplain waters were draining towards the river (low NO3−), but rates were undetectable (< 0.05 mmol N2-N·m−2·d−1) when there was river flooding (high NO3−). Nitrogen fixation by periphyton and plankton in 1989–1990 accounted for approximately 8% of previously reported total annual nitrogen inputs to the floodplain at Lake Calado.

Establishment of Native Aquatic Plants for Fish Habitat: Test Plantings in Two North Texas Reservoirs

ABSTRACT Test plantings of native aquatic plant species were made in two Texas reservoirs. Founder populations of three native submersed or floating-leaved species were established within small (2- × 2-m) exclosures utilizing actively growing transplants. Herbivory and excessive sedimentation proved to be deterrents to plant survival and expansion. Nine small founder populations of Vallisneria americana were established within North Lake, a small reservoir with limited water level fluctuations and a developing community of native pioneer aquatic plants. Plants within all nine exclosures successfully established and began vegetative growth. In intact exclosures, the plants rapidly covered the sediment surface within the exclosures. Expansion beyond the exclosures was variable and occurred primarily during the cooler portions of the year when herbivory was assumed to be low. During the spring of the second growing season, herbivores cropped most of the previous expansion leaving only a narrow fringe of plan...

River–reservoir transition zones are nitrogen fixation hot spots regardless of ecosystem trophic state

Reservoir hydrodynamics may create heterogeneity in nitrogen (N) fixation along the riverine–transition–lacustrine gradient. In particular, N fixation may be highest in reservoir transition zones where phytoplankton biomass is also expected to be relatively high. We investigated spatial patterns of N fixation in three Texas (USA) reservoirs of varying trophic state. We sampled 6–9 stations along the longitudinal axes of the major inflows and measured N fixation using the acetylene reduction method. Total N, total phosphorus (P), and algal biomass (as chlorophyll-a) were also measured at each sample location. Measurable N fixation was observed in all reservoirs and was light-dependent. Nitrogen fixation was consistently low in the riverine zone, highest in the transition zone, and low in lacustrine zone of all reservoirs. The absolute magnitude of N fixation was similar in two relatively unproductive reservoirs and an order of magnitude higher in the eutrophic reservoir. A similar pattern was observed in mean nutrient and chlorophyll-a concentrations among reservoirs. However, chlorophyll-a concentrations were highest in the riverine zone of each reservoir and exhibited a monotonic decrease in the downstream direction. Maximum chlorophyll-a concentrations did not coincide with maximum N fixation rates. Results of our study indicate that reservoir transition zones can be biogeochemical hot spots for planktonic N fixation, regardless of trophic state. Therefore, transition zones may be the most at risk locations for water quality degradation associated with increased reservoir productivity. Water quality managers and aquatic scientists should consider the spatial heterogeneity imposed by unique hydrodynamic controls in reservoir ecosystems.

Impacts of Water Column Turbidity on the Survival and Growth of Vallisneria americana Winterbuds and Seedlings

ABSRACT Survival and growth of Vallisneria americana winterbuds was significantly related to both initial winterbud size and to the water column turbidity under which the plants were grown. Larger winter buds showed better survival and better growth than did smaller ones. Turbidity likewise significantly impacted the survival and growth of the plants. Over the turbidity range of 0.2–45 NTU (53–7% total incident light), the plants were shown to have progressively poorer survival and to produce fewer rosettes and total number of leaves. Vallisneria americana seedlings were likewise influenced by turbidity. Under high turbidity conditions the seedlings had significantly higher mortality, while surviving plants produced fewer rosettes and accumulated less biomass than seedlings grown under low turbidity conditions. In addition, under turbid conditions the seedlings had to invest proportionally more energy into above-ground tissues.