J. Denis Newbold

Contributions of microbial biofilms to ecosystem processes in stream mesocosms

In many aquatic ecosystems, most microbes live in matrix-enclosed biofilms and contribute substantially to energy flow and nutrient cycling. Little is known, however, about the coupling of structure and dynamics of these biofilms to ecosystem function. Here we show that microbial biofilms changed the physical and chemical microhabitat and contributed to ecosystem processes in 30-m-long stream mesocosms. Biofilm growth increased hydrodynamic transient storage—streamwater detained in quiescent zones, which is a major physical template for ecological processes in streams—by 300% and the retention of suspended particles by 120%. In addition, by enhancing the relative uptake of organic molecules of lower bioavailability, the interplay of biofilm microarchitecture and mass transfer changed their downstream linkage. As living zones of transient storage, biofilms bring hydrodynamic retention and biochemical processing into close spatial proximity and influence biogeochemical processes and patterns in streams. Thus, biofilms are highly efficient and successful ecological communities that may also contribute to the influence that headwater streams have on rivers, estuaries and even oceans through longitudinal linkages of local biogeochemical and hydrodynamic processes.

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.

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.

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.