Water Quality Reporting Limits, Method Detection Limits, and Censored Values: What Does It All Mean?

The Arkansas Water Resources Center (AWRC) maintains a fee-based water-quality lab that is certified by the Arkansas Department of Environmental Quality (ADEQ). The AWRC Water Quality Lab analyzes water samples for a variety of constituents, using standard methods for the analysis of water samples (APHA 2012). The lab generates a report on the analysis, which is provided to clientele, and reports the concentrations or values as measured. Often times the concentrations or values might be very small, even zero as reported by the lab – what does this mean? How should we use this information? This document is intended to help our clientele understand the analytical report, the values, and how one might interpret information near the lower analytical limits. Every client wants the analysis of their water sample(s) to be accurate and precise, but what do we really mean when we say those two words? These words are often used synonymously or thought of as being the same, but the two words mean two different things. Both are equally important when analyzing water samples for constituent concentrations

Beaver Lake Numeric Chlorophyll-a and Secchi Transparency Standards, Phases II and III: Uncertainty and Trend Analysis

The objective of Phases II and III of this study were to 1) assess the variation in chl‐a and ST across multiple spatial and temporal scales in Beaver Lake in order to validate the assessment method, and 2) quantify trends in chl‐a, ST, and nutrient (total phosphorus and total nitrogen) concentrations in Beaver Lake and the major inflowing rivers to verify any potential water quality impairment

Nutrient Criteria Development Mini-Workshops for USEPA Region VI States

The U.S. Environmental Protection Agency (USEPA) has been encouraging states to accerlate their efforts in developing numeric nutrient criteria to promulgate within respective water quality standards and regulations. Based on USEPA recommendations, many states are pursuing nutrient criteria development by assessing frequency distributions and stressor‐response relationships in historical databases, various special studies and the literature. The mini‐workshops described here were targeted toward state agency personnel and stakeholder groups with an interest in understanding statistical techniques that could be used to aide, guide and support numeric nutrient criteria. An initial workshop was conducted at the USEPA Regional Technical Assistance Group meeting in February 2010, but it was poorly attended because of the development of harsh weather and travel conditions. The limited state and USEPA personnel in attendance gave a positive response, and subsequent workshops were planned for each state agency with regulatory authority to develop water quality standards. The workshops were designed to be one working day, starting at 0900 and ending at 1600, and the topics covered included: workshop goals, objectives and tasks; measurement and ecological indicators of water quality impairment by nutrients; basic statistical methods; advanced statistical methods; and application of the statistical methods and interpretation of results. The number of participants varied from 5 at one workshop (participants specific to state agency developing numeric nutrient criteria) to over 20 (where various state agencies and stakeholder groups were represented). The series of topic presentations were followed by two group exercises that put the statistical tools discussed into action, where smaller working groups had to suggest numeric nutrient criteria for a watershed with ample data and then one with a very limited amount of data. The case studies were hypothetical in nature, but the stressor‐ response relationships shown mimicked those observed in aquatic systems and reported in the literature. The numeric values were defined only as nutrient, not specifically nitrogen or phosphorus; the intent here was to keep this a more open and unbiased exercise, especially for states that might have promulgated specific nutrient criteria. The results of the workshops were not specifically recorded, allowing the participants freedom to present, discuss and criticize without formal record. The proposed numeric nutrient criteria did vary between smaller working groups within a given workshop, but the range proposed was similar across workshops. The workshops were not intended to provide training so that the participants would walk away with the knowledge to actively pursue the techniques discussed. Rather, they were intended to provide a foundation for participants to understand what tools are available for the process of numeric nutrient criteria development and to be exposed to the potential pitfalls and benefits of various approaches. Overall, the feedback from all of these workshops was overwhelmingly positive by the state

Managing Lake Fertility within the Guidelines of a Nutrient Management Plan and based on Algal Nutrient Limitation

The specific objectives were to first, monitor nutrients, algal biomass, and water clarity in lakes Cove, Spring, and Wedington. Second, evaluate whether algal growth in each of the lakes was limited by N, P, or both N and P. This research was conducted to help USFS better manage lake fertilization to maximize algal growth and improve the fisheries within these lakes

Nitrogen fixation: A poorly understood process along the freshwater-marine continuum

N2 fixation is a major component of the global N cycle and has been extensively studied in open-ocean and terrestrial ecosystems. Yet rates and ecological dynamics remain virtually unknown for the inland and coastal aquatic ecosystems (lakes, wetlands, rivers, streams, and estuaries) that connect terrestrial and marine biomes. This is due to the diversity of these habitats as well as the traditional paradigm that N2 fixation rates were low to nonexistent, and therefore not important, in these ecosystems. We identify three major research themes to advance understanding of aquatic N2 fixation: (1) the biological diversity of diazotrophs and variability of N2 fixation rates, (2) the ecological stoichiometry of N2 fixation, and (3) the upscaling of N2 fixation rates from genes to ecosystems. Coordinating research across these areas will advance limnology and oceanography by fully integrating N2 fixation into ecological dynamics of aquatic ecosystems from local to global scales

Variable Stoichiometry and Homeostatic Regulation of Bacterial Biomass Elemental Composition

Prokaryotic heterotrophs (hereafter, bacteria) represent a large proportion of global biomass, and therefore bacterial biomass stoichiometry likely exerts control on global phosphorus (P), carbon (C), and nitrogen cycling and primary productivity. In this study we grew recently isolated freshwater heterotrophic bacteria across an ecologically relevant range of resource C:P ratios (organic C to P ratio in available resources) to quantify the P requirements of these organisms and examine the degree to which they regulated their P content under P-sufficient and P-deficient conditions. Bacterial biomass was only limited by P when resource C:P was greater than 250 (by atoms). Bacterial C:P ranged from 71 to 174 under P sufficiency and from 252 to 548 under P deficiency. Bacteria exhibited very little C:P homeostasis under P-sufficient growth conditions, greater C:P homeostasis under P-deficient conditions, and the ability of bacteria to outcompete one another in short-term experiments depended on a tradeoff between storing excess P for later use under P-deficient conditions or immediately using P to produce more biomass. These results indicate that freshwater heterotrophic bacteria are not as P-rich as previously thought and that homeostatic regulation of C:P stoichiometry depends on the individual taxa and what resource (organic C or available P) is limiting bacterial growth. Individual bacterial populations can vary between strong C:P homeostasis under P deficiency to virtually no C:P homeostasis under P sufficiency, but variation between taxa and the effect this has on competitive ability may dampen the signal in C:PB at the bacterial community level. Nevertheless, the prevalence of homeostatic and non-homeostatic strategies in a bacterial community should have important implications for nutrient regeneration and carbon cycling

Sestonic Chlorophyll-a and Nutrient Relationships Across the Red River Basin, USA

The Red River is a trans‐boundary, multi‐jurisdictional basin, where water‐quality standards often change at the state lines. The state agencies with USEPA Region VI focused resources to organize water‐quality data from within this basin and have it statistically analyzed to evaluate the relationships between nutrients and sestonic chlorophyll‐a (chl‐a). There were 152 sites within the Red River Basin that had nutrient and sestonic chl‐a data, and these sites were narrowed down to 132 when a minimum number of observations was required. Sestonic chl‐a levels increased with increasing nutrient concentrations; these significant regressions were used to predict nutrient concentrations at 10 µg chl‐a L⁻¹. Total nitrogen (TN) and phosphorus (TP) concentrations (at 10 µg chl‐a L⁻¹ ) varied across the Red River Basin and its eco‐regions from 0.10‐0.22 mg TP L⁻¹ and 0.75‐2.11 mg TN L⁻¹ . Nutrient thresholds were also observed with sestonic chl‐a at 0.14 mg TP L⁻¹ and 0.74 mg TN L⁻¹ using categorical and regression tree analysis (CART). CART analysis also revealed that hierarchical structure was important when attempting to predict sestonic chl‐a from TN, TP and conductivity. The ranges of TN and TP concentrations that resulted in chl‐a concentrations which exceeded 10 µg chl‐a L⁻¹ were similar in magnitude to the threshold in TN and TP that resulted in increased sestonic chl‐a. This corroborating evidence provides useful guidance to the states with jurisdiction within the Red River Basin for establishing nutrient criteria, which might be similar when the Red River and its tributaries cross political boundaries