Jerry W. Elwood

THE LIMITING ROLE OF PHOSPHORUS IN A WOODLAND STREAM ECOSYSTEM: EFFECTS OF P ENRICHMENT ON LEAF DECOMPOSITION AND PRIMARY PRODUCERS'

The limiting role of phosphorus on leaf decomposition and primary producers was investigated in a second-order woodland stream in Tennessee by experimentally enriching, for 95 d, adjacent reaches with an average of 60 and 450 ,ug PO4-P/L, respectively, over upstream control levels of -4 ,ug/L. Red oak (Quercus rubra) leaf packs in the enriched sections lost mass 24% faster than control packs (P .05). Respiration rates of subsampled leaf discs were significantly higher than control rates only at the high level of enrichment. The increased respiration rates in the low and high enrichments accounted for 10 and 34% of the increased mass loss in the respective enriched sections, suggesting that the en- richment also produced increases in mechanical breakdown through faster microbial conditioning, increases in macroinvertebrate feeding, or both. Effects of the enrichment on aufwuchs initially consisted of increased chlorophyll a levels, followed by increased aufwuchs biomass levels. Dense growth of filamentous algae, including some Oscillatoria, which may be a nitrogen fixer, developed immediately downstream of P inputs. In addition, Nostoc, a known nitrogen-fixing blue-green alga, sampled after the enrichment, was significantly more abundant in the enriched sections than the control (P < .05). Densities of the snail, Goniobasis clavaeformis, a grazer-shredder sampled after the enrichment, also were significantly greater in the enriched reaches, suggesting that the lack of a sustained response of chlorophyll a to the enrichment may have been a result of increased grazing on algal biomass. These findings indicate that nutrient limitation of detrital processing is a significant factor in natural streams. The apparent increases in densities of benthic macroinvertebrates in the enriched sections, along with reported relationships between detrital food richness and macroinver- tebrate growth and survivorship, suggest that nutrient limitation in streams also has ramifications on higher trophic levels.

Top-down and bottom-up control of stream periphyton: effects of nutrients and herbivores

We conducted two experiments to determine the relative effects of herbivory and nutrients on an algal community in Walker Branch, a stream having effectively two trophic levels: primary producers and herbivorous snails. The first study (1989), performed in streamside channels, tested the effects of three factors: (1) stream water nitrogen (N), (2) phosphorus (P), and (3) snail grazing, on periphyton biomass, productivity, and community composition. The second study (1990), conducted in situ, tested the effects of snail grazing and nutrients (N + P). In the 1989 study, nutrients had positive effects, and herbivores had negative effects, on algal biomass (chlorophyll a, ash-free dry mass, total algal biovol? ume) and primary productivity (area- and chlorophyll-specific). Likewise, both nutrients and snail grazing exerted effects (+ and -, respectively) on biomass measured in the 1990 study (chlorophyll a, algal biovolume). Grazed communities were dominated by chloro- phytes and cyanophytes, which were overgrown by diatoms when herbivores were removed. Algal species that were reduced most by herbivores were increased most by nutrient ad? dition, and vice versa, suggesting a trade-off between resistance to herbivory and nutrient- saturated growth rates. Increases in algal biomass and productivity were slight with the addition of either N or P compared to responses observed when both nutrients were added together, suggesting that both nutrients were at growth-limiting levels. The greatest changes in periphyton structure or function were observed when both nutrients were added and simultaneously, grazers were removed, in contrast to lesser effects when nutrients were added under grazed conditions or grazers were removed at low nutrient levels, indicating dual control by both factors. Nutrient addition also positively affected snail growth in both experiments, indicating tight coupling between herbivore and algal growth (top-down ef? fects) and that bottom-up factors that directly affected plant growth could also indirectly affect consumers belonging to higher trophic levels. Indices quantifying the direct effects of top-down factors relative to bottom-up factors (top-down index, TDI) and the importance of interactions between these factors (interaction coefficient, IC) were computed. These indices showed that the relative strength of top-down and bottom-up factors varied among biomass and productivity parameters and that top-down and bottom-up effects, alone, were less important than their combined effects.

NUTRIENT SPIRALLING IN STREAMS: IMPLICATIONS FOR NUTRIENT LIMITATION AND INVERTEBRATE ACTIVITY

Nutrient cycling in streams occurs in conjunction with downstream transport as spatially distributed process that has been termed spiralling. The intensity of reutilization of nutrients as they pass downstream can be quantified in terms of the length of stream required for a nutrient atom to complete one (abstract) cycle; this distance is termed the spiralling length. Our model for steady-state spiralling of a limiting nutrient predicts that most of the downstream transport of nutrient occurs in particulate or unavailable form when nutrient limitation is severe; in this case, transportability of particulates is a major determinant of spiralling length. On the other hand, when nutrient limitation is moderated by density-dependent mechanisms, transport in the dissolved phase dominates, and transportability of particles has little influence on spiralling length. The potential role of invertebrate consumers in controlling spiralling was investigated by considering their influence on regeneration, transportability, and uptake of nutrients. Functional processes of grazing and filter feeding appear most likely to shorten spiralling length when nutrient limitation is severe, while the process of shredding is more likely to shorten spiralling length when nutrient limitation is weak. In some cases there may be levels of consumer activity at which spiralling length is minimized.

Phosphorus Spiralling in a Woodland Stream: Seasonal Variations

Four radiotracer releases were performed over an annual period in 1981-1982 to de? termine seasonal variation in indices and pathways of phosphorus spiralling in Walker Branch, a small woodland stream in eastern Tennessee, USA. Each release consisted of an addition of ^370 MBq each of carrier-free 32P04 and 3H20 over a 1 -h period during baseflow. Concentrations of 32P and 3H dissolved in stream water were measured intensively at several stations downstream from the radio? tracer input during and immediately following each release. Activity of 32P in coarse particulate organic matter (CPOM), fine particulate organic matter (FPOM), and aufwuchs was measured 2-3 h after each release and at various intervals for 7 wk. Total biomass of CPOM, FPOM, and aufwuchs at the time of each release was also measured. Uptake of 32P04 from the water was greatest in November and lowest in August. Uptake length {Sw) of phosphorus, defined as the average distance travelled by a P04 ion dissolved in water, varied from 22 m in November to 97 m in August. Uptake of 32P04 by CPOM was generally greatest, with ~50% of total uptake, while that by aufwuchs was lowest, with < 15% ofthe total. CPOM abundance was the major determinant of whole-stream P04 uptake rate and Sw. Turnover length {Sp) of phos? phorus, defined as the average distance traveled by an atom of P taken up by particulate material, was short compared to Sw, varying from 1 m in November to 3 m in January. Consequently total spiralling length {S) of P varied from 23 m in November, just after peak autumn leaf fall, to 99 m in August, and reflected primarily the travel of P in the dissolved form. Our results indicate that the greatest increase in Sw (and consequently in S) in Walker Branch occurs in late autumn or winter after storms reduce the abundance of CPOM in the lower portions of the stream bed. Although we calculate that Sp may increase by one to two orders of magnitude for short periods during storms, the greatest effect of storms on P spiralling over the long term is their impact on the quality and quantity of CPOM and FPOM in the stream bed after the return to baseflow. For most of the year, detrital organic carbon probably influences phosphorus spiralling more than phosphorus spiralling influences the processing of organic carbon in Walker Branch. Only during the fall and early winter periods, when CPOM abundance is high and Sw is short, does phosphorus spiralling exert strong control over biotic processes downstream.

Elemental dynamics in streams

We discuss elemental dynamics in streams and seek to identify areas where there are critical gaps in our understanding. Both landscape-level processes (e.g., geology, land-use practices, vegetation) and heterogeneous in-stream processes influence the supply and availability of elements to the stream biota. Stream ecologists need to consider the relative availability of different compounds or groups of compounds to the biota rather than lumping all forms of an element into operationally-defined units such as dissolved organic nitrogen or carbon. The impact of short-term events like storms on the elemental dynamics in streams needs to be assessed and compared with other controls. The relative importance of longitudinal (upstream), lateral (riparian zone, flood-plains), and in-stream controls of supply and availability of elements needs to be compared in a variety of streams. Availability of essential elements is a key factor controlling rates of primary productivity and decomposition in streams. Whole system manipulations offer a valuable tool for understanding the interactions between elements and all components of the stream food web. We include an action plan of developments that would assist researchers in addressing some of the critical gaps we have identified in our understanding of elemental dynamics in streams.

Role of Nutrient Cycling and Herbivory in Regulating Periphyton Communities in Laboratory Streams

In this study we examined the role of nutrient cycling and herbivory in regulating stream periphyton communities. Population, community, and ecosystem—level properties were studied in laboratory stream channels that had nutrient inputs reduced compared to channels where ambient nutrient levels were maintained. We reduced nutrient inputs in four of eight channels by recirculating 90% of the flow, whereas the other four channels received once—through flow of spring water. We examined the interaction between herbivory and nutrients by varying the number of snails (Elimia clavaeformis) among streams with different nutrient input (circulation) regimes. Reduction in nutrient input viar recirculation resulted in lower concentrations of nutrients in the water but did not result in significant differences in biomass, carbon fixation, or algal taxonomic composition. However, herbivory had large effects on these characteristics by reducing biomass and areal rates of carbon fixation and simplifying periphyton taxonom...

Effects of Acidification on Leaf Decomposition in Streams

Effects of acidification on leaf decomposition in streams were studied in four second-order streams in the Great Smoky Mountains National Park. The streams ranged in pH from 4.5 to 6.4 at baseflow. Mass loss of leaves incubated in mesh bags placed in pools in each stream was measured periodically over 15 wk beginning in late August. Measurements were also made of C, N, P, and Al in leaves, microbial biomass (adenosine triphosphate [ATP]) and respiration rate and bacterial production (thymidine uptake) associated with leaves, and the number and biomass of macroinvertebrates in leaf bags. Rates of leaf mass loss were significantly lower in streams with pH ≤5.7 compared with a stream with pH 6.4. Although rate of leaf mass loss among the streams varied directly with pH, differences between streams with pH values between 4.5 and 5.7 were not significant. Microbial ATP and respiration rates and bacterial production rates followed the same pattern as leaf mass loss rate, i.e., low for more acidic streams and highest in the stream with the highest pH. Accumulation of aluminum by the leaf-microbe complex was also greatest in the most acidic streams. The number and biomass of macroinvertebrate shredders found in leaf bags was lowest at the highest pH site and therefore cannot account for the higher rate of leaf mass loss found at this site. Our results suggest that the lower rate of leaf decomposition in the more acidic streams is due largely to low rates of microbial activity.

Periphyton production and grazing rates in a stream measured with a 32p material balance method

Net production rates and standing crops of periphyton and grazing rates on periphyton were measured in a small, woodland stream in southeastern United States using a material balance method. The material balance of radioactive phosphorus was followed at three times during the year in periphyton, consumer organisms, and stream water for up to six weeks following a one-hr release of 32PO4 to the stream. Rates of decrease of 32p in periphyton per unit weight, per unit area of substrate, and in the entire study reach of stream were used to compute biomass turnover rates of periphyton and fractions of periphyton standing crop grazed per unit area per unit time. Periphyton standing crops in July, September, and November were estimated at 200, 198, and 658 mg ash-free dry weight m-2, respectively, while estimates of net production rates were 22, 24, and 16 mg ash-free dry weight m-2 day-1, respectively. Estimated grazing rates on periphyton during these periods were 23, 15, and 14 mg ash-free dry wt m-2 day-1, respectively. Biomass turnover rates of periphyton were in the range of those computed from other stream ecosystems which had higher standing crops and production rates of periphyton. This suggests that grazing limited periphyton production rates in this stream by controlling the standing crop of periphyton. The method is of wide applicability since less than maximum permissible concentrations of radioactive phosphorus were used in the spike releases.

The effect of grazing intensity on phosphorus spiralling in autotropic streams

The effect of grazing on primary productivity and phosphorus cycling in autotrophic streams was studied using the snail Goniobasis clavaeformes. Snails were added to each of three replicate laboratory stream channels, receiving once-through flow of groundwater, in densities of 2.1, 3.0, and 4.2 g ash free dry mass (AFDM)/m2. A fourth channel received no snails and served as an ungrazed control.Presence of snail grazers resulted in a large reduction in aufwuchs biomass, primary productivity, and biotic phosphorus uptake; a modest reduction in fine particulate organic matter (FPOM); and an increase in the fraction of stream particulate organic matter (POM) exported as seston. Although primary production and aufwuchs biomass continued to decline with increasing snail density, phosphorus uptake increased. This increased phosphorus uptake is attributed to abiotic sorption to inorganic surfaces exposed as a result of efficient removal of aufwuchs at high snail densities. Although snail densities were chosen to bracket the density measured in a natural stream, the experimental densities may result in considerably higher grazing pressure on aufwuchs due to the absence of alternate food sources (e.g., coarse particulate organic matter) usually found in natural streams.Presence of snail grazers increased the spiralling length of phosphorus, primarily by reducing aufwuchs biomass and consequently reducing uptake of phosphorus from the water. Presence of snails also increased downstream transport velocity of phosphorus bound to organic particles. These results follow the patterns predicted in a previous theoretical analysis for mildly phosphorus-limited streams.

Effect of a leaf-shredding invertebrate on organic matter dynamics and phosphorus spiralling in heterotrophic laboratory streams

SummaryThe effect of invertebrate shredders on organic matter dynamics and phosphorus spiralling was studied over a 30-week period in laboratory streams. The streams were fed by groundwater, layered with cobble and gravel from a natural stream, covered with opaque material to eliminate algal growth, and initially contained 195 g/m2 of autumn-shed leaves. Four weeks after leaf addition, leaf-shredding snails (Goniobasis clavaeformis) were added to each of three streams in densities of 75, 220, and 800/m2. A fourth stream contained no snails and served as a control.Presence of snails increased the loss rates of coarse particulate organic matter (CPOM) and total organic matter (TOM), primarily by increasing leaf fragmentation and seston export. Although snail feeding increased specific metabolism of microbes associated with CPOM and cobble surfaces, it was not enough to compensate for reduction in bacterial cell numbers per unit surface area and in stream TOM. Consequently mineralization of detritus and whole stream phosphorus utilization rate were maximum in the stream with no snails and decreased with increasing snail density. From previous simulations of a stream model based on the nutrient spiralling concept, we predicted that there should be an intermediate shredder density which would minimize phosphorus spiralling length (maximize phosphorus utilization) in a natural stream nearby. Our current results conflict with the model-based predictions primarily because the increase in microbial metabolism was less important than reduction in bacterial cell numbers and total benthic organic matter resulting from snail feeding. Although our results indicate macroinvertebrate shredders reduce phosphorus utilization in headwater streams, shreders may increase nutrient utilization downstream where riparian inputs are lower, thus linking low- and high-order streams.