Scott T. Larned

Nitrogen Export from Forested Watersheds in the Oregon Coast Range: The Role of N2-fixing Red Alder

AbstractVariations in plant community composition across the landscape can influence nutrient retention and loss at the watershed scale. A striking example of plant species importance is the influence of N2-fixing red alder (Alnus rubra) on nutrient cycling in the forests of the Pacific Northwest. To understand the influence of red alder on watershed nutrient export, we studied the chemistry of 26 small watershed streams within the Salmon River basin of the Oregon Coast Range. Nitrate and dissolved organic nitrogen (DON) concentrations were positively related to broadleaf cover (dominated by red alder: 94% of basal area), particularly when near-coastal sites were excluded (r 2 = 0.65 and 0.68 for nitrate-N and DON, respectively). Nitrate and DON concentrations were more strongly related to broadleaf cover within entire watersheds than broadleaf cover within the riparian area alone, which indicates that leaching from upland alder stands plays an important role in watershed nitrogen (N) export. Nitrate dominated over DON in hydrologic export (92% of total dissolved N), and nitrate and DON concentrations were strongly correlated. Annual N export was highly variable among watersheds (2.4–30.8 kg N ha−1 y−1), described by a multiple linear regression combining broadleaf and mixed broadleaf–conifer cover (r2 = 0.74). Base cation concentrations were positively related to nitrate concentrations, which suggests that nitrate leaching increases cation losses. Our findings provide evidence for strong control of ecosystem function by a single plant species, where leaching from N saturated red alder stands is a major control on N export from these coastal watersheds.

A prospectus for periphyton: recent and future ecological research

Abstract The presence, abundance, composition, and growth of periphyton are controlled or influenced by 5 broad classes of environmental variation: disturbances, stressors, resources, hydraulic conditions, and biotic interactions. In turn, periphyton communities affect water chemistry, hydraulic conditions, habitat availability, and foodweb dynamics. This review focuses on responses of periphyton communities to environmental variation. A specific objective of the review is to identify robust periphyton–environment relationships and insightful concepts. Contributors to J-NABS have led the field in testing and expanding concepts in periphyton ecology. J-NABS papers about periphyton patch dynamics, light- and nutrient-limited periphyton growth, and the effects of disturbances on periphyton structure and function have been particularly influential. However, many topics in periphyton ecology remain unexplored and underexplored. These topics include resource colimitation, physiological responses to stressors, allelopathy, competitive inhibition and exclusion, and the effects of drag forces and turbulence. Periphyton ecology studies in J-NABS tend to be multivariable, phenomenological, and nonmechanistic. Such studies provide information about temporal and spatial patterns, but rarely provide evidence for the causes of those patterns. These studies are often impaired by low statistical power and insufficient experimental control. Periphyton ecology needs more rigorous manipulative experiments, particularly experiments that generate clear relationships between environmental drivers and ecological responses.

Water quality in low‐elevation streams and rivers of New Zealand: Recent state and trends in contrasting land‐cover classes

Abstract River water quality in New Zealand is at great risk of impairment in low elevation catchments because of pervasive land‐use changes, yet there has been no nationwide assessment of the state of these rivers. Data from the surface‐water monitoring programmes of 15 regional councils and unitary authorities, and the National River Water Quality Network were used to assess the recent state (1998–2002) and trends (1996–2002) in water quality in low‐elevation rivers across New Zealand. Assessments were made at the national level, and within four land‐cover classes (native forest, plantation forest, pastoral, and urban). Finer‐scaled assessments were made by subdividing the large number of pastoral sites into six climate classes, and seven stream orders. At the national level, median concentrations of the faecal indicator bacterium Escherichia coli, and dissolved inorganic nitrogen and dissolved reactive phosphorus exceeded guidelines recommended for the protection of aquatic ecosystems and human health. Water quality state varied widely within land‐cover classes: E. coli and dissolved nitrogen and phosphorus concentrations in the pastoral and urban classes were 2–7 times higher than in the native and plantation forest classes, and median water clarity in the pastoral and urban classes was 40–70% lower than in the native and plantation forest classes. Water quality state in the pastoral class was not statistically different from that of the urban class, and water quality state in the plantation forest class was not statistically different from that of the native forest class. Significant trends in low‐elevation rivers were limited to four parameters: flow (trending down in all instances), and temperature, clarity, and conductivity (trending up in all instances). The trends in flow, temperature, and clarity were apparent at the national scale, and within the pastoral class. The magnitudes of these trends were very low, corresponding to changes of ≤0.5%/ year in parameter medians.

Aquatic invertebrate community structure along an intermittence gradient: Selwyn River, New Zealand

Abstract Changes in community structure and life-history traits of benthic invertebrates were examined along a longitudinal intermittence gradient in an alluvial river. The gradient was characterized with modeled and measured hydrologic, chemical, and physical environmental variables. The invertebrates were collected in the Selwyn River, southeastern New Zealand, at multiple sites in each of 4 river sections with distinct hydrological conditions (perennial-losing, ephemeral, intermittent, perennial-gaining). Values of hydrological metrics for each site were generated with an empirical model developed for the Selwyn River. The metrics included 4 that characterized intermittent flow (flow permanence, flow duration, drying frequency, distance to nearest perennial site). Most invertebrate richness and density metrics were significantly higher in the perennial-losing and perennial-gaining sections than in the ephemeral and intermittent sections. A principle components analysis (PCA) separated invertebrate samples from the 4 sections along 2 primary factors. Nine of 13 hydrological metrics, including the 4 intermittence metrics, were correlated with the PCA site scores. Linear regressions indicated that most taxon-richness metrics and some density metrics were related to flow permanence, flow duration, or both. Based on the regression analysis, we predicted that 1.9 taxa/m2 are added with each 10% increase in flow permanence, and 0.5 taxa/m2 are added with each 10-d increase in flow duration. Results from a nestedness analysis indicated that communities at ephemeral and intermittent sites were nested subsets of the communities at perennial sites, and the nesting order of sites was related to both flow permanence and flow duration. Assemblages of taxa with particular life-history traits (life span, fecundity, maximum size, and voltinism) varied linearly with flow permanence and flow duration. The variation in invertebrate communities along the Selwyn River was primarily the result of progressive removal of desiccation-sensitive taxa with increasing intermittence, not to selection for desiccation-resistant specialists. Quantitative intermittence–ecology relationships are needed to predict the consequences of future changes in flow intermittence, but such relationships are rare. The univariate relationships reported in our study contribute to a small but growing array of intermittence–ecology relationships.

Invertebrate and microbial responses to inundation in an ephemeral river reach in New Zealand : effects of preceding dry periods

Abstract.Inundation marks the shift from a terrestrial ecosystem to an aquatic ecosystem in ephemeral rivers. The forms and rates of responses by aquatic invertebrates and sediment microbes to inundation depend on desiccation resistance during preceding dry periods. We assessed invertebrate and microbial responses to inundation over a range of preceding dry periods in an ephemeral reach of the Selwyn River, New Zealand. Microbial response variables were dissolved oxygen consumption and non-specific esterase activity. Sampling sites along the reach had been continuously dry for 1–592 d prior to sample collection. The onset of flow simulated by an experimental inundation led to the appearance of aquatic invertebrates in all samples, but the assemblages varied with the length of the preceding dry period. Taxon richness decreased linearly with dry period length while density decreased exponentially. These patterns indicate that a large number of individuals from desiccation-sensitive taxa were eliminated soon after flow ceased, and a low-density assemblage composed of a small number of desiccation-resistant taxa persisted during prolonged dry periods. As with invertebrate density, sediment respiration and nonspecific esterase activity decreased with length of dry period, and were characterized by exponential decay functions. The results of the inundation experiments suggest that a temporal ecotone exists for about one week after the disappearance of flowing water, and before the terrestrial system stabilizes.

Nitrogen and phosphorus in New Zealand streams and rivers: Control and impact of eutrophication and the influence of land management

Abstract Given sufficient light and heat, the growth of aquatic macrophytes and algae associated with eutrophication is generally controlled by the concentration, form and ratio between nitrogen (N) and phosphorus (P). Data from 1100 freshwater sites monitored for the last 10 years by New Zealands regional councils and unitary authorities were assessed for streams and rivers with mean nitrate/ nitrite‐N (NNN), dissolved reactive P (DRP), total N (TN) and total P (TP) concentrations in excess of New Zealand guidelines, and to generate a data set of N:P ratios to predict potential periphyton response according to the concentration of the limiting nutrient. The frequency of sites exceeding the guidelines varied from 0 to 100% depending on the parameter and region, but South Island regions were generally more compliant. The dissolved inorganic N (DIN) to dissolved reactive P (DRP) ratio was used to group data into three nutrient limitation classes: <7:1 (N‐limited), between 7:1 and 15:1 (co‐limited), and>15:l (P‐limited), by mass. P‐limitation was the most frequent scenario in New Zealand streams (overall, 76% of sites were P‐limited, 12% N‐limited, and 12% co‐limited). The mean concentration of the limiting nutrient for each site was combined with empirical relationships to predict periphyton densities (the average of N‐and P‐limited growth was used for sites with co‐limitation). This assessment predicted that 22 sites were likely to exceed the periphyton guideline for protecting benthic biodiversity (50 mg chlorophyll a m−2), but this assessment is likely to be highly changeable in response to climatic conditions and present and future land use. As an example, we modelled N and P losses from an average sheep and a dairy farm in Southland (South Island, New Zealand) in 1958, 1988, 2008 and 2028. We predicted that with time, as farm systems have and continue to intensify, N losses increase at a greater rate than P losses. Since the pathway for N to reach fresh waters may be more tortuous and take longer than P to reach a stream or river, focusing mitigation on P losses may have a quicker effect on potential algal growth. In addition, with time, it is expected that P‐limitation in New Zealands rivers and streams will be more widespread as N‐losses are unabated. Hence, although strategies to decrease N losses should be practised, mitigating P losses is also central to preventing eutrophication.

Natural variation in immersion and emersion affects breakdown and invertebrate colonization of leaf litter in a temporary river

Flow pulses that alternately immerse and expose benthic habitats are widely recognized as key determinants of biodiversity and ecosystem functioning in rivers. Terrestrial leaf litter input, colonization, and breakdown are also key processes in river ecosystems, but little is known about the effects of alternating immersion and emersion on these processes. We used litterbags to examine breakdown, microbial activity, and colonization of Populus sp. leaves by invertebrates along a natural gradient in immersion and emersion (i.e., submergence and exposure to air) in a temporary river. Rates of leaf litter mass loss, microbial activity and colonization by invertebrates differed among litterbags that were permanently immersed, intermittently immersed and permanently emersed, and breakdown rate coefficients (k) decreased with increasing cumulative emersed duration (the total number of day of emersion during the experiment). In contrast, the frequency of emersed periods had no detectable effects on these variables. k was positively correlated with the density of invertebrate shredders in immersed litterbags, with microbial activity and shredder density in intermittent litterbags, and with microbial activity in emersed litterbags. These correlations suggest that the relative importance of microbial activity on k increases with emersed duration, due to the periodic elimination of aquatic shredders and the scarcity of terrestrial detritivores. The fact that leaf litter breakdown was detectable under permanently emersed conditions indicates that mechanisms other than shredding by invertebrates, such as leaching and photodegradation, are dominant in dry river habitats.

Dynamics of coarse riparian detritus in a Hawaiian stream ecosystem: a comparison of drought and post-drought conditions

Input, standing crop, and export of coarse (>4 mm2) particulate organic matter (CPOM) was measured in Kaiwiki Stream, a forested stream on the Island of Hawaii. The study was conducted during and after a severe drought associated with the 1997–1998 El Niño/Southern Oscillation event. During the drought, stream discharge was reduced by ∼70% and no spates occurred. Hawaiian streams draining windward mountain slopes normally experience >20 spates per year. During the drought, the mean rate of CPOM export was 8% of the mean input rate; following the drought, the export rate increased to 40% of input. Rates of marked leaf transport within the study area increased 30- to 50-fold following the drought, and rates of stem transport increased 10- to 30-fold. Lower rates of CPOM export and transport during the drought suggested that retention and potential availability to detritivores were increased compared to the post-drought period. Differences in CPOM standing crops during and after the drought were not significant however, and the difference in turnover times was small, suggesting that the quantity of detrital food did not change substantially. Results of litter bag experiments indicated that detritivores had little effect on leaf processing. Although detritus was mainly composed of leaves and stems in Kaiwiki Stream, these items are not important food resources for macroscopic detritivores. In contrast, detritivores in the stream frequently fed on detrital fruit. Year-round availability and seasonally high input rates raised the possibility that detrital fruit is an important food resource in Hawaiian streams. CPOM input during the drought was dominated by direct litterfall from the riparian forest, with lower rates of input from upstream reaches and lateral movement from the forest floor. CPOM input increased significantly when normal discharge resumed. Rates of input to the study area from upstream reaches equaled direct litterfall, but rates of lateral input remained low. Although leaf input decreased following the drought, CPOM input increased because of increased fruit, stem, and flower inputs. Input of detrital fruit to the stream was continuous and, at peak fruiting periods, input rates and standing crops equaled or exceeded all other forms of detritus.

Interactions between the mat-forming alga Didymosphenia geminata and its hydrodynamic environment

Benthic autotrophs in oligotrophic rivers must adapt to and modify their hydrodynamic environment to balance the conflicting requirements of minimal drag (to minimize detachment risks) and maximal exposure to turbulent flow (to maximize nutrient acquisition). We explored flow–organism interactions using the benthic, freshwater alga Didymosphenia geminata. D. geminata forms large mats in swift, oligotrophic alluvial rivers. The physical properties that allow D. geminata to resist detachment and proliferate under these harsh conditions are unknown. We transplanted cobbles with attached D. geminata mats from a riverbed to a flume and used velocimetry and microelectrode profiling to measure hydrodynamic and transport conditions above and within the mats over a wide range of flows. We then removed the mats from the cobbles and repeated the velocimetry measurements. Experiment results indicated that D. geminata mats reduce form-induced stresses and near-bed turbulent velocity fluctuations, which may reduce the risk of detachment. D. geminata mats also increase turbulent shear stress just above mat surfaces, which may enhance water column–mat solute exchange. High friction associated with flow at mat surfaces leads to very low velocities and predominantly diffusive transport within mats, which may in turn favor the retention of solutes derived from organic matter within and below mats. Enhanced mass transfer at mat surfaces and effective solute retention in mat matrices suggest a mechanism by which D. geminata cells acquire nutrients from different sources: advection-dominated transport of water-column nutrients to cells at mat surfaces, and diffusion-dominated transport from decomposing organic matter within mats, with minimal advective losses.

Light- and nutrient-limited periphyton in low order streams of Oahu, Hawaii

To date, most studies of light- and nutrient-limited primary productivity in forested streams have been carried out in deciduous forests of temperate, continental regions. Conceptual models of light and nutrient limitation have been developed from these studies, but their restricted geographic range reduces the generality of such models. Unlike temperate continental streams, streams on tropical high islands are characterized by flashy, unpredictable discharge and riparian canopies that do not vary seasonally. These contrasting conditions suggest that patterns of light and nutrient limitation in tropical streams may differ from those in temperate streams. The effects of light, and nitrogen and phosphorus availability on periphyton accrual (measured as chlorophyll a per unit area) were investigated using field experiments in 4 low-order streams on the island of Oahu, Hawaii. Levels of chlorophyll a in partially-shaded stream pools were significantly greater than in heavily-shaded pools, and nutrient-enrichment increased the level of chlorophyll a in partially-shaded pools but not in heavily-shaded pools. In each stream, phosphate enrichment resulted in an increase in the level of chlorophyll a, but nitrate enrichment had no effect. Spates following rainstorms occur frequently in these streams, and may increase periphyton productivity by increasing the flux of nutrients to algal cells. However, differences in inorganic nitrogen and phosphorus concentrations measured during spates and baseflow were small, and during some spates, concentrations of these two nutrients declined relative to baseflow concentrations. These observations suggest that phosphorus limitation was not alleviated by spates.