Nitrogen and phosphorus dynamics in hot desert streams of Southwestern U.S.A.

Grimm, Nancy B.; Fisher, Stuart G.; Minckley, W. L.

doi:10.1007/bf00008281

Cited by 81 publications

(50 citation statements)

References 35 publications

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“…In some cases, this geometry of the SRP signal is strikingly similar to the 8 hour lag pattern observed here. For example, Grimm et al (1981) observed weak diel P variation in a Sonoran Desert stream that was out-of-phase with expectations of synchronized uptake of mineral P and C. Notably, diel removal peaks occurred at night and were bi-modal, potentially indicating geochemical interactions early in the day. Kimura et al (1999) report that an estuarine alga assimilated P at night during diel migrations out of the photic zone, and used that P during the day.…”

Section: Timing Of Assimilatory P Retentionmentioning

confidence: 80%

Diel phosphorus variation and the stoichiometry of ecosystem metabolism in a large spring‐fed river

Cohen

Kurz

Heffernan

et al. 2013

Ecological Monographs

107

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Abstract. Elemental cycles are coupled directly and indirectly to ecosystem metabolism at multiple time scales. Understanding coupling in lotic ecosystems has recently advanced through simultaneous high-frequency measurements of multiple solutes. Using hourly in situ measurements of soluble reactive phosphorus (SRP), specific conductance (SpC), and dissolved oxygen (DO), we estimated phosphorus (P) retention pathways and dynamics in a large (discharge, Q ' 7.5 m 3 /s) spring-fed river (Ichetucknee River, Florida, USA). Across eight multi-day deployments, highly regular diel SRP variation of 3-9 lg P/L (mean ;50 lg P/ L) was strongly correlated with DO variation, suggesting photosynthetic control directly via assimilation, and/or indirectly via geochemical reactions. Consistent afternoon SRP maxima and midnight minima suggest peak removal lags gross primary production (GPP) by ;8 hours. Two overlapping processes were evident, one dominant with maximum removal near midnight, the other smaller with maximum removal near midday. Hourly [Ca] measurements during three 24-hour deployments showed consistent afternoon minima, suggesting that calcite precipitates as GPP increases pH and mineral saturation state. Resulting P coprecipitation was modeled using SpC as a [Ca] proxy, yielding a diel P signal adjusted for the primary geochemical retention pathway. P assimilation, the dominant diel signal, was computed by interpolating between daily concentration maxima, both with and without geochemical adjustment, and converting concentration deficits to fluxes using discharge and benthic area. Adjusting for calcite co-precipitation yielded assimilation rates (13.7 6 5.8 mg PÁm À2 Ád À1) strongly correlated with GPP, accounting for 72% 6 9% of gross removal, and ecosystem C:P stoichiometry (466 [6 12]:1) consistent with dominant vascular autotrophs (478 [6 24]:1). Without adjusting for co-precipitation, covariance with GPP was weaker, and C:P implausibly high. Consistent downstream P accumulation, likely from P-rich porewater seepage, varied across deployments (3-22% of total flux) and co-varied with discharge and respiration. Asynchronous N and P assimilation may arise from differential timing of protein and ribosome production, with the latter requiring maximal carbohydrate stores and minimal photolytic interference. This study demonstrates direct and indirect coupling of biological, hydrological, and geochemical processes and the utility of high-resolution time series of multiple solutes for understanding these linkages.

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Section: Timing Of Assimilatory P Retentionmentioning

confidence: 80%

Diel phosphorus variation and the stoichiometry of ecosystem metabolism in a large spring‐fed river

Cohen

Kurz

Heffernan

et al. 2013

Ecological Monographs

107

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“…Given N assimilation rates reported by Heffernan and Cohen (in press) (Grimm et al 1981, Grimm andFisher 1986). Benthic systems in other large rivers exhibit nutrient saturation at low concentrations; in the Athabasca River (Alberta, Canada), for example, P demand is saturated at concentrations as low as 0.003 mg PO 4 -P/L (Scrimgeour and Chambers 1997).…”

Section: ) Thus Downstream Declines In Nomentioning

confidence: 96%

Algal blooms and the nitrogen‐enrichment hypothesis in Florida springs: evidence, alternatives, and adaptive management

Heffernan

Liebowitz

Frazer

et al. 2010

Ecological Applications

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Abstract. Contradictions between system-specific evidence and broader paradigms to explain ecosystem behavior present a challenge for natural resource management. In Florida (USA) springs, increasing nitrate (NO 3 À ) concentrations have been implicated as the cause of algal overgrowth via alleviation of N-limitation. As such, policy and management efforts have centered heavily on reduction of nitrogen (N) loads. While the N-limitation hypothesis appears well founded on broadly supported aquatic eutrophication models, several observations from Florida springs are inconsistent with this hypothesis in its present simplified form. First, NO 3 À concentration is not correlated with algal abundance across the broad population of springs and is weakly negatively correlated with primary productivity. Second, within individual spring runs, algal mats are largely confined to the headwater reaches within 250 m of spring vents, while elevated NO 3 À concentrations persist for several kilometers or more. Third, historic observations suggest that establishment of macroalgal mats often lags behind observed increases in NO 3À by more than a decade. Fourth, although microcosm experiments indicate high thresholds for N-limitation of algae, experiments in situ have demonstrated only minimal response to N enrichment. These muted responses may reflect large nutrient fluxes in springs, which were sufficient to satisfy present demand even at historic concentrations. New analyses of existing data indicate that dissolved oxygen (DO) has declined dramatically in many Florida springs over the past 30 years, and that DO and grazer abundance are better predictors of algal abundance in springs than are nutrient concentrations. Although a precautionary N-reduction strategy for Florida springs is warranted given demonstrable effects of nutrient enrichment in a broad suite of aquatic systems worldwide, the DO-grazer hypothesis and other potential mechanisms merit increased scientific scrutiny. This case study illustrates the importance of an adaptive approach that explicitly evaluates paradigms as hypotheses and actively seeks alternative explanations.

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“…We used data for small streams in the desert southwest, Arizona, USA (sites 5, 6, 7, and 25 in Grimm et al [1981] We also assessed potential groundwater nutrients by collecting original samples from point-source hill-slope surface waters (seeps and springs, n ¼ 10) and mixing wells placed into stream sediments (n ¼ 80) following Brookshire et al (2005) and through the use of piezometers (n ¼ 4, at Noland Divide) and published values for spring, soil lysimeter, and riparian well water (Grimm et al 1981, Mulholland 1992, Qualls et al 2000, Van Miegroet et al 2001, Yeakley et al 2003. We further explored the implications of in-stream nutrient removal for watershed nutrient balance (atmospheric input minus stream output) at sites for which long-term inorganic N budgets have been constructed and across which deposition varies from ;5 to 32 kg NÁha …”

Section: Field Measuresmentioning

confidence: 99%

Maintenance of terrestrial nutrient loss signatures during in‐stream transport

Brookshire

Valett

Gerber

2009

Ecology

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Abstract. Small streams account for the majority of channel length in river basins worldwide and are the primary conveyors of terrestrial nutrients to rivers and ultimately the oceans. The controls of stream nutrient fluxes, however, are debated. Classical models emphasize that nutrient transport in streams integrates nutrient cycling in the terrestrial watershed while others argue that in-stream processes control nutrient flux. Recent studies have shown that in-stream cycling can be important in determining downstream nutrient fluxes, but results have not been reconciled with mass-balance calculations at the smallwatershed scale. Here we use a simple analytical framework to assess nutrient cycling in streams and show that, under most conditions, longitudinally static nutrient concentrations reflect in-stream biotic balance between uptake and regeneration and groundwater inputs. Using measures of nutrient concentrations in small streams across four biomes, we provide evidence for generality of biogeochemical steady state (inputs ¼ outputs) in stream ecosystems: overall, longitudinal profiles were flat for nitrogen and phosphorus and were similar in concentration to soil and ground waters. Deviation from flat longitudinal profiles was associated with seasonal or successional biomass growth and small groundwater inputs relative to in-stream sink strength. We conclude that streams tend strongly toward nutrient balance, allowing use of their chemistry as an integrated measure of terrestrial nutrient losses.

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Nitrogen and phosphorus dynamics in hot desert streams of Southwestern U.S.A.

Cited by 81 publications

References 35 publications

Diel phosphorus variation and the stoichiometry of ecosystem metabolism in a large spring‐fed river

Diel phosphorus variation and the stoichiometry of ecosystem metabolism in a large spring‐fed river

Algal blooms and the nitrogen‐enrichment hypothesis in Florida springs: evidence, alternatives, and adaptive management

Maintenance of terrestrial nutrient loss signatures during in‐stream transport

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