J. Thad Scott

It Takes Two to Tango: When and Where Dual Nutrient (N & P) Reductions Are Needed to Protect Lakes and Downstream Ecosystems

Preventing harmful algal blooms (HABs) is needed to protect lakes and downstream ecosystems. Traditionally, reducing phosphorus (P) inputs was the prescribed solution for lakes, based on the assumption that P universally limits HAB formation. Reduction of P inputs has decreased HABs in many lakes, but was not successful in others. Thus, the P-only paradigm is overgeneralized. Whole-lake experiments indicate that HABs are often stimulated more by combined P and nitrogen (N) enrichment rather than N or P alone, indicating that the dynamics of both nutrients are important for HAB control. The changing paradigm from P-only to consideration of dual nutrient control is supported by studies indicating that (1) biological N fixation cannot always meet lake ecosystem N needs, and (2) that anthropogenic N and P loading has increased dramatically in recent decades. Sediment P accumulation supports long-term internal loading, while N may escape via denitrification, leading to perpetual N deficits. Hence, controlling both N and P inputs will help control HABs in some lakes and also reduce N export to downstream N-sensitive ecosystems. Managers should consider whether balanced control of N and P will most effectively reduce HABs along the freshwater-marine continuum.

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.

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.

Physical Factors Control Phytoplankton Production and Nitrogen Fixation in Eight Texas Reservoirs

We compared regression tree analyses and multiple linear regression models to explore the relative importance of physical factors, land use, and water quality in predicting phytoplankton production and N2 fixation potentials at 85 locations along riverine to lacustrine gradients within eight southern reservoirs. The regression tree model (r2xa0=xa00.73) revealed that differences in phytoplankton production were primarily a function of water depth. The highest rates of production (mg Cxa0m−3xa0h−1) occurred at shallow sites (<0.9xa0m), where rates were also related to total phosphorus (TP) levels. At deeper sites, production rates were higher at sites with relative drainage area (RDA, ratio of drainage area to water surface area) below 45, potentially due to longer hydraulic residence times. In contrast, multiple linear regression selected TP, RDA, dissolved phosphorus, and percent developed land as significant model variables (r2xa0=xa00.63). The regression tree model (r2xa0=xa00.67) revealed that N2 fixation potentials (mg Nxa0m−3xa0h−1) were substantially higher at sites with relatively smaller drainage areas (RDAxa0<xa045). Within this subgroup, fixation rates were additionally related to TP values (thresholdxa0=xa041xa0μgxa0l−1). The multiple linear regression model (r2xa0=xa00.67) also selected RDA as the primary predictor of N2 fixation. Regression tree models suggest that nutrient controls (phosphorus) were subordinate to physical factors such as depth and RDA. We concluded that regression tree analysis was well suited to revealing nonlinear trends in data (for example, depth), but yielded large uncertainty estimates when applied to linear data (for example, phosphorus).

Carbon sink to source: longitudinal gradients of planktonic P:R ratios in subtropical reservoirs

Spatial patterns of planktonic production and respiration in the surface mixed layer were examined in eight Texas, USA reservoirs to test the hypothesis that P:R ratios are lowest in upreservoir inflow zones and highest in downreservoir open-water zones, as predicted by the heuristic reservoir zonation model. We measured summer planktonic metabolism with light–dark bottles and physical–chemical conditions in epilimnetic water at 85 sites distributed among sixteen longitudinal transects within the eight reservoirs (2 transects per reservoir). Volumetric production and plankton biomass were lowest in the open-water zones and increased upreservoir; however, that pattern was reversed for areal production due to greater photic depths at open-water sites. Volumetric respiration was similar in the three zones; however, corresponding planktonic P:R ratios in the surface mixed layer were significantly lower at open-water sites, which is opposite than hypothesized. Based on linear regressions of production and respiration rates on chlorophyll a, open-water sites were net heterotrophic during the summer regardless of trophic state; whereas inflow and mid-reservoir zone sites were heterotrophic when chlorophyll concentrations were respectively less than 9.5 and 35xa0mgxa0m−3. Although variation among reservoirs was high, five of the eight reservoirs had inflow zones that were net carbon sinks while seven had open-water zones that were carbon sources. Mean (±standard error) carbon flux rates of inflow, mid-reservoir, and open-water zones were −0.22xa0±xa00.12 (C sink), 0.39xa0±xa00.44 (moderate C source), and 1.33xa0±xa00.50 (strong C source) g C m−2 day−1 respectively. Inflow and mid-reservoir zones comprised approximately 45% of the total reservoir area studied. Therefore, omitting their contribution as often done when a single open-water site is sampled may substantially overestimate reservoir carbon flux.

Substituting values for censored data from Texas, USA, reservoirs inflated and obscured trends in analyses commonly used for water quality target development

We calculated four median datasets (chlorophyll a, Chl a; total phosphorus, TP; and transparency) using multiple approaches to handling censored observations, including substituting fractions of the quantification limit (QL; dataset 1u2009=u20091QL, dataset 2u2009=u20090.5QL) and statistical methods for censored datasets (datasets 3–4) for approximately 100 Texas, USA reservoirs. Trend analyses of differences between dataset 1 and 3 medians indicated percent difference increased linearly above thresholds in percent censored data (%Cen). This relationship was extrapolated to estimate medians for site-parameter combinations with %Cenu2009>u200980%, which were combined with dataset 3 as dataset 4. Changepoint analysis of Chl a- and transparency-TP relationships indicated threshold differences up to 50% between datasets. Recursive analysis identified secondary thresholds in dataset 4. Threshold differences show that information introduced via substitution or missing due to limitations of statistical methods biased values, underestimated error, and inflated the strength of TP thresholds identified in datasets 1–3. Analysis of covariance identified differences in linear regression models relating transparency-TP between datasets 1, 2, and the more statistically robust datasets 3–4. Study findings identify high-risk scenarios for biased analytical outcomes when using substitution. These include high probability of median overestimation when %Cenu2009>u200950–60% for a single QL, or when %Cen is as low 16% for multiple QL’s. Changepoint analysis was uniquely vulnerable to substitution effects when using medians from sites with %Cenu2009>u200950%. Linear regression analysis was less sensitive to substitution and missing data effects, but differences in model parameters for transparency cannot be discounted and could be magnified by log-transformation of the variables.

Comparing two periphyton collection methods commonly used for stream bioassessment and the development of numeric nutrient standards

Periphyton is an important component of stream bioassessment, yet methods for quantifying periphyton biomass can differ substantially. A case study within the Arkansas Ozarks is presented to demonstrate the potential for linking chlorophyll-a (chl-a) and ash-free dry mass (AFDM) data sets amassed using two frequently used periphyton sampling protocols. Method A involved collecting periphyton from a known area on the top surface of variably sized rocks gathered from relatively swift-velocity riffles without discerning canopy cover. Method B involved collecting periphyton from the entire top surface of cobbles systematically gathered from riffle-run habitat where canopy cover was intentionally avoided. Chl-a and AFDM measurements were not different between methods (pxa0=xa00.123 and pxa0=xa00.550, respectively), and there was no interaction between method and time in the repeated measures structure of the study. However, significantly different seasonal distinctions were observed for chl-a and AFDM from all streams when data from the methods were combined (pxa0<xa00.001 and pxa0=xa00.012, respectively), with greater mean biomass in the cooler sampling months. Seasonal trends were likely the indirect results of varying temperatures. Although the size and range of this study were small, results suggest data sets collected using different methods may effectively be used together with some minor considerations due to potential confounding factors. This study provides motivation for the continued investigation of combining data sets derived from multiple methods of data collection, which could be useful in stream bioassessment and particularly important for the development of regional stream nutrient criteria for the southern Ozarks.

The synergistic effect of elevated CO2 and phosphorus on reservoir eutrophication

ABSTRACT Winston B, Scott JT, Pollock E. 2016. The synergistic effect of elevated CO2 and phosphorus on reservoir eutrophication. Lake Reserve Manage. 32:373–385. Increased delivery of carbon (C) and phosphorus (P) as a result of climate change is predicted to significantly increase algal biomass and alter algal nutrient content. We conducted a 2-phased experiment to determine the following: (1) whether carbon dioxide partial pressure (PCO2) in the water column of reservoirs dropped low enough to limit algal biomass, (2) whether increasing the PCO2 in the water column could alleviate C limitation and increase algal biomass, and (3) whether the simultaneous increase in PCO2 and P alter algal nutrient content. In the first phase we reconstructed PCO2 in the water column of several northwest Arkansas reservoirs on timescales ranging from days to decades. During fall, winter, and spring, PCO2 was greater than atmospheric levels, suggesting the reservoirs were sources of C released to the atmosphere. During summer, PCO2 was less than atmospheric levels, suggesting that reservoir PCO2 fell low enough to potentially limit algal biomass. In the second phase, we conducted an enrichment experiment along a PCO2 gradient representative of preindustrial (250 µatm), current (400 µatm), and 2050 projected levels from the Intergovernmental Panel on Climate Change (550 µatm) along a P gradient. Our results showed that relative to preindustrial and current levels, elevated PCO2 had no significant effect on algal biomass and nutrient content. Instead, biomass increased significantly with increasing levels of P, supporting predictions that even under elevated PCO2 and global climate change scenarios, P might still be the major nutrient limiting algal growth in these reservoirs.