James P. Grover

Resource Competition in a Variable Environment: Phytoplankton Growing According to the Variable-Internal-Stores Model

Many studies in population ecology and competition theory are based on models in which the consumption rate of a resource is some function of resource availability and in which the yield of new consumers is constantly proportional to the amount of resource consumed. For such models, the best competitors at equilibrium have low resource requirements, and the best competitors in nonequilibrium habitats have high maximal growth rates. In this study, I abandon the assumption of constant yield and allow consumption rates to be a function of the consumers internal state with respect to a resource, as well as a function of external resource availability. Specifically, I study functions and parameters describing the competition of planktonic algae for a dissolved nutrient, phosphorus, that is supplied in periodic pulses. In this model, trade-offs between competitive abilities in equilibrium and nonequilibrium habitats can arise in several ways. The most important trade-offs are likely to involve the capacity to store nutrient and the relation between consumption rate and internal nutrient state. It is possible that a species may have unimpressive rates of growth and consumption as functions of resource availability but nevertheless be a superior competitor. It is also possible that no trade-offs between competitive abilities in equilibrium and nonequilibrium habitats arise: that the same species is the best competitor in all habitats. Which of these possibilities is prevalent in natural phytoplankton can only be determined empirically, by adequate knowledge of the constraints on and correlations among the physiological properties of algae.

GROWTH AND TOXICITY OF PRYMNESIUM PARVUM (HAPTOPHYTA) AS A FUNCTION OF SALINITY, LIGHT, AND TEMPERATURE

The growth rate, stationary cell concentration, and toxicity of Prymnesium parvum N. Carter were measured using a strain isolated from Texas inland waters. We used a multifactor experimental approach with multiple regression analysis to determine the importance of environmental factors, including temperature, light, and salinity to these algal measurements. Exponential growth rate was unimodal in relation to temperature, salinity, and irradiance, with an estimated maximal growth of 0.94 d−1 occurring at 27°C, 22 practical salinity units (psu), and 275 μmol photons·m−2·s−1. Stationary cell concentrations also had unimodal responses to temperature and salinity but increased with irradiance. Maximal cell concentrations were estimated to occur at 26°C and 22 psu. Both maximum growth rate and highest stationary cell concentrations were measured at levels of each factor resembling warm, estuarine conditions that differ from the conditions under which blooms occur in inland waters in the southwestern United States. Acute toxicity to fish was highest at the lowest salinity and temperature levels, conditions not optimal for exponential growth but similar to those under which blooms occur in inland waters. Our results imply that summer blooms could occur in inland waters of the southwestern United States. Generally, they have not, suggesting that factors other than those investigated in this research influence bloom dynamics.

Dynamics of competition among microalgae in variable environments: experimental tests of alternative models

Two species of green microalgae, Scenedesmus quadricauda var. longispina and chlorella sp., were grown together in cntinuous cultures under conditions of phosphorus-limitation. This resource was supplied as a series of discrete periodic pulse, preventing achievment of a strict equilibrium. Two mathematical models, based on the equations of Monod and Droop were parameterized from earlier, single-species experiments. These models were then tested for their ability to predict competitivedynamics in these and similar competition cultures. Chlorella outcompeted Scenedesmus in all cultures, regardless of the period of phosphorus pulses

Resource Competition in a Variable Environment: Phytoplankton Growing According to Monod's Model

Theoretical models of interspecific competition often assume equilibrium in population and resource dynamics, an assumption that is often criticized. Departures from equilibrium are hypothesized to reduce interspecific competition and facilitate coexistence. When competition between two algae for one resource is modeled using Monod equations, the species that wins at equilibrium does not usually win when resources are supplied as periodic pulse, if the second competitor has the higher maximal growth rate. For model parameters typical of nitrogen and phosphorus limitation, this selection for species with high maximal growth rates is strong. A review of laboratory studies of algal competition is largely inconclusive concerning this prediction. Theory suggests, however, two reasons to doubt that selection for species with high maximal growth rates occurs by this mechanism in nature. First, when variability in resource supply occurs as sinusoidal fluctuations, rather than as pulses, selection for species with high maximal growth rates is weak. Second, when intracellular storage of resources occurs, selection for species with high maximal growth rates is also weak. Under either or- these conditions, it is common for algae that are superior competitors at equilibrium to be superior competitors under nonequilibrium conditions also. Under all conditions tested, coexistence of two competitors on one resource is less common than competitive exclusion. Lack of equilibrium may, however, be a likely explanation for species diversity in the more complicated ecosystems with more than one resource, or more than one trophic level, that are nearly universal in nature.

Dynamics of Competition in a Variable Environment: Experiments with Two Diatom Species

The hypothesis that environmental variability promotes phytoplankton species diversity was investigated experimentally using phosphorus-limited continuous cultures. Variability was introduced as a series of phosphorus pulses delivered at 8-d intervals. The growth of Synedra sp. and Fragilaria crotonensis was examined in cultures of natural phytoplankton, in cultures containing both species but no other phytoplankton, and in monocultures. In cultures inoculated with comparable densities of both species, Synedra was com- petitively dominant to Fragilaria in both constant and varying cultures, but the rate of competitive exclusion was slower in varying cultures. Synedra was able to invade both constant and varying monocultures of Fragilaria when injected into those monocultures, but Fragilaria could not invade monocultures of Synedra. In monocultures, the average yield of Fragilaria was increased by variability, but that of Synedra was not. During pulses, Fragilaria had a higher uptake rate per cell than Synedra, but the two species had similar specific uptake rates. Previous studies (Tilman 198 1) suggest that Synedra is a better steady-state competitor for phosphorus than Fragilaria. Although Fragilaria may be better able to exploit phos- phorus pulses than Synedra, this advantage is apparently too weak to prevent the dominance of Synedra under the regime of pulsed phosphorus supply used here.

The Impact of Variable Stoichiometry on Predator‐Prey Interactions: A Multinutrient Approach

A model for prey and predators is formulated in which three essential nutrients can limit growth of both populations. Prey take up dissolved nutrients, while predators ingest prey, assimilate a fraction of ingested nutrients that depends on their current nutrient status, and recycle the balance. Although individuals are modeled as identical within populations, amounts of nutrients within individuals vary over time in both populations, with reproductive rates increasing with these amounts. Equilibria and their stability depend on nutrient supply conditions. When nutrient supply increases, unusual results can occur, such as a decrease of prey density. This phenomenon occurs if, with increasing nutrient, predators sequester rather than recycle nutrients. Furthermore, despite use of a linear functional response for predators, high nutrient supply can destabilize equilibria. Responses to nutrient supply depend on the balance between assimilation and recycling of nutrients by predators, which differs depending on the identity of the limiting nutrient. Applied to microbial ecosystems, the model predicts that the efficiency of organic carbon mineralization is reduced when supply of mineral nutrients is low and when equilibria are unstable. The extent to which predators recycle or sequester limiting nutrients for their prey is of critical importance for the stability of predator‐prey systems and their response to enrichment.

Competition along a Spatial Gradient of Resource Supply: A Microbial Experimental Model

In a set of laboratory experiments, we examined competition for phosphorus between algae and bacteria under various carbon:phosphorus (C:P) supply ratios in spatially homogeneous and heterogeneous microcosms. Experimental results were compared to those predicted by theoretical models of resource competition. In the spatially heterogeneous microcosm, algae that were inferior competitors for P persisted in vessels with high local C:P supply ratios that would cause exclusion in the spatially homogeneous microcosms. Resource competition theory, adapted to this system, provided a starting point for explaining these results. Spatial structure can enhance local diversity because locally inferior competitors are transported from source habitats into sink habitats where they would otherwise be excluded. Such local sources were determined by their resource supply ratios. These results verify the hypothesis that spatial processes enhance local diversity when a system of local habitats is divided into sources and sinks in such a way that each persisting species has at least one source within the system. However, existing theoretical models did not accurately predict distributions of competitor abundance within this experimental system.

A mechanistic explanation for pH-dependent ambient aquatic toxicity of Prymnesium parvum carter

The harmful algal bloom species Prymnesium parvum has caused millions of dollars in damage to fisheries around the world. These fish kills have been attributed to P. parvum releasing a mixture of toxins in the water. The characterized toxins, reported as prymnesin-1 and -2, have structural similarities consistent with other known ionizable compounds (e.g., ammonia). We investigated whether pH affects the toxicity of P. parvum under conditions representative of inland Texas reservoirs experiencing ambient toxicity from bloom formation. We evaluated pH influences on toxicity in laboratory and field samples, and modeled the physicochemical properties of prymnesins. Aquatic toxicity to a model fish and cladoceran was reduced by lowering pH in samples obtained from reservoirs experiencing P. parvum blooms; similar observations were confirmed for experiments with laboratory cultures. A pKa value of 8.9 was predicted for the prymnesins, which suggests that ionization states of these toxins may change appreciably over surface water pH of inland waters. These findings indicate that ionization states of toxins released by P. parvum may strongly influence site-specific toxicity and subsequent impacts to fisheries. Consequently, these results emphasize the importance of understanding processes that affect pH during P. parvum blooms, which may improve predictions of ambient toxicity.

LIMITING RESOURCES, DISTURBANCE, AND DIVERSITY IN PHYTOPLANKTON COMMUNITIES

Phytoplankton diversity, limiting resources, and disturbance were studied in two reservoirs, Eagle Mountain Lake (EML) and Joe Pool Lake (JPL), in north Texas, USA, for three summer growing seasons and two winters. Availabilities of phosphorus, nitrogen, and silicon were measured as chemical concentrations, and availability of light was measured as irradiance during the photoperiod, averaged over the depth of the surface mixed layer. A resource was defined as limiting whenever its availability fell below a threshold, treated as a parameter to be fitted. Depth of the surface mixed layer and wind speed were taken as indices of disturbance associated with episodic mixing. Hydrological disturbance was gauged by rainfall, inflow, and the variation of lake level. We found that diversity was strongly and significantly correlated with the number of limiting resources in one lake (JPL), but not the other (EML). The onset of nitrogen and phosphorus limitation during the summer growing season is associated with increased phytoplankton diversity in JPL. Regression modeling detected decreasing relationships between diversity and disturbance indices in one lake (JPL), but concave-down relationships in the other (EML), though the peaks of these relationships did not lie within the usual range of disturbance. Predictive regressions for diversity incorporating both limiting resources and disturbance were partitioned into unique and shared effects of these factors. There was a large unique effect of the number of limiting resources in JPL, but otherwise the shared effects were large relative to unique effects.

An exploratory study of air emissions associated with shale gas development and production in the Barnett Shale

Information regarding air emissions from shale gas extraction and production is critically important given production is occurring in highly urbanized areas across the United States. Objectives of this exploratory study were to collect ambient air samples in residential areas within 61 m (200 feet) of shale gas extraction/production and determine whether a “fingerprint” of chemicals can be associated with shale gas activity. Statistical analyses correlating fingerprint chemicals with methane, equipment, and processes of extraction/production were performed. Ambient air sampling in residential areas of shale gas extraction and production was conducted at six counties in the Dallas/Fort Worth (DFW) Metroplex from 2008 to 2010. The 39 locations tested were identified by clients that requested monitoring. Seven sites were sampled on 2 days (typically months later in another season), and two sites were sampled on 3 days, resulting in 50 sets of monitoring data. Twenty-four-hour passive samples were collected using summa canisters. Gas chromatography/mass spectrometer analysis was used to identify organic compounds present. Methane was present in concentrations above laboratory detection limits in 49 out of 50 sampling data sets. Most of the areas investigated had atmospheric methane concentrations considerably higher than reported urban background concentrations (1.8–2.0 ppmv). Other chemical constituents were found to be correlated with presence of methane. A principal components analysis (PCA) identified multivariate patterns of concentrations that potentially constitute signatures of emissions from different phases of operation at natural gas sites. The first factor identified through the PCA proved most informative. Extreme negative values were strongly and statistically associated with the presence of compressors at sample sites. The seven chemicals strongly associated with this factor (o-xylene, ethylbenzene, 1,2,4-trimethylbenzene, m- and p-xylene, 1,3,5-trimethylbenzene, toluene, and benzene) thus constitute a potential fingerprint of emissions associated with compression. Implications: Information regarding air emissions from shale gas development and production is critically important given production is now occurring in highly urbanized areas across the United States. Methane, the primary shale gas constituent, contributes substantially to climate change; other natural gas constituents are known to have adverse health effects. This study goes beyond previous Barnett Shale field studies by encompassing a wider variety of production equipment (wells, tanks, compressors, and separators) and a wider geographical region. The principal components analysis, unique to this study, provides valuable information regarding the ability to anticipate associated shale gas chemical constituents.