Leaves in Streams I
Presiding: J Kominoski, University of Georgia; J Jackson, Stroud Water Research Center
NB41B-01 08:30h
Microbial Dynamics on Decaying Leaf Litter in an Ohio Stream
Deciduous leaf litter is an important source of energy for many streams. Fungi (aquatic hyphomycetes) and bacteria play important roles in litter processing, but the colonization dynamics of these groups across different leaf types is not well described. We examined fungal and bacterial colonization on sugar maple and white oak leaves in a hardwater stream in Northeastern Ohio from November 2003 through May 2004. Triplicate samples of each leaf type were collected from litter bags approximately monthly. The dry weight and organic content of the leaves were measured. Fungal biomass was determined from ergosterol concentrations in the samples. Image analysis of DAPI-stained cells and standard conversion factors were used to calculate bacterial biomass. On most dates, fungal biomass was similar on leaf types, and 10-fold higher than bacterial biomass. Throughout the study, bacterial numbers and biomass were greater on sugar maple than white oak. Sugar maple leaves decayed faster than oak leaves, but this was not reflected in the biomass of aquatic hyphomycetes, rather in the abundance of bacteria. Although fungi are considered the main organisms in litter breakdown, bacteria showed a greater response to leaf quality in this study, and may be more important than the biomass suggests.
NB41B-02 08:45h
Habitat Complexity of Stream Leaf Packs: Effects on Benthic Macroinvertebrates and Leaf Litter Breakdown
We investigated two attributes of leaf-pack complexity (i.e., leaf-pack mass and leaf surface area) on fish predation, colonization of benthic macroinvertebrates, and leaf breakdown rates in a coldwater Michigan stream. We manipulated three factors using a factorial design: fish (exclusion or control cage), leaf-pack mass (1, 3, or 5 g dry mass), and leaf surface area (<7, 7-10, or >10 cm leaf width). Acer leaves were fastened into leaf packs. Exclusion cages had mesh on all sides; control cages lacked mesh on two sides to provide access to fishes. Two replicate leaf packs were randomly collected after 25-31 d from two sections of the stream (n = 4). Common shredders were Gammarus, Pycnopsyche, and Lepidostoma. We did not detect a significant effect of fish predation on benthic macroinvertebrates or leaf breakdown (i.e., mass loss). Colonization of benthic macroinvertebrates appeared proportional to leaf-pack mass but was unaffected by the surface area of leaves. Leaf breakdown was more rapid among leaf packs with fewer leaves (i.e., leaves with large surface area and leaf packs with low mass) and greater numbers of shredders. We suspect that physical fragmentation is the primary mechanism for higher breakdown rates among leaf packs with fewer leaves.
NB41B-03 09:00h
Effects of Leaf Litter Species Diversity on Decomposition in a Forested Watershed in the Southern Appalachians, USA
Decomposition of leaf litter influences nutrient cycling and energy flow in terrestrial and aquatic ecosystems. Diversity of decomposing leaves in streams is a function of riparian plant composition and temporal and spatial heterogeneity. Leaf species have different decay rates, and leaf litter input to streams varies seasonally. Leaves with decreased lignin, lower phenolics, and lower C:N:P typically exhibit faster rates of decay and are more bioaccessible. Our collaborative project at Coweeta Hydrologic Laboratory, North Carolina, compares leaf litter decomposition in a stream and riparian zone. We selected four leaf litter species representing a gradient of resource qualities and decay rates to develop a 2 x 2 factorial design with 15 species combinations. We examined effects of species diversity and time on leaf litter decay rate (k), phytochemistry, and microbial and macroinvertebrate colonization. Results for in-stream leaf litter indicate a positive, non-additive effect of species diversity on k. AFDM remaining of mixed and single species packs was unpredictable. Defense (e.g., phenolics) and structural (e.g., fiber) compounds varied in single and mixed species packs throughout decay; however, leaf packs with Rhododendron maximum had significantly less associated fungal biomass than all other single and mixed species packs (p < .0001).
NB41B-04 09:15h
Does Initial Leaf Chemistry Affect the Contribution of Insects, Fungi, and Bacteria to Leaf Breakdown in a Lowland Tropical Stream?
We examined effects of initial leaf chemistry of six common riparian species on the relative contribution of fungi, bacteria, and invertebrates to leaf breakdown in a lowland stream in Costa Rica. We hypothesized that fungi and bacteria would contribute more to the breakdown of species with low concentrations of secondary (tannins and phenolics) and structural (cellulose and lignin) compounds, while invertebrates would be more important in the processing of species with high concentrations of secondary and structural compounds. We incubated single species leaf bags of six common riparian species, representing a range in secondary and structural compounds, in a third-order stream at La Selva Biological Station, Costa Rica. We measured leaf chemistry during the breakdown process. We determined fungal biomass using ergosterol methods, bacteria using DAPI counts, and invertebrate biomass using length-weight regressions. We then used biomass estimates for each group to determine their contribution to the overall breakdown process. Breakdown rates ranged from very fast (Trema integerima, k = 0.23 day-1) to slow (Zygia longifolia , k = 0.011 day-1). While analyses are still under way, preliminary results support our initial hypothesis that fungi contribute more to the break down of leaves from tree species with low concentrations of secondary and structural compounds.
NB41B-05 09:30h
Spatial Variability of Leaf-litter Decomposition in Streams
Most studies of leaf decomposition in streams quantify decomposition rates in single riffles or short stream reaches and so we still know little about how decomposition varies at larger spatial scales. We used a leaf-bag approach and a hierarchical experimental design to investigate decomposition across three spatial scales spanning five orders of magnitude. In each of four replicate fourth-order watersheds in the Black Forest of Germany we selected three third-order streams. Each of the 12 streams contained four replicate riffles in which we installed four coarse- and fine-mesh bags (384 bags in total). Mean decomposition rates were highly consistent among watersheds (a range of less than 3% across all four watersheds) for both mesh types. However, significant differences in mean decomposition rates were observed among streams and riffles in coarse-mesh bags. Decomposition rates in fine-mesh bags did not vary at any spatial scale. This discrepancy between mesh types suggests that patchy distribution of invertebrate feeding accounts for observed differences in decomposition whereas the more homogeneous distribution of microbes made microbial decomposition highly predictable. These results show that 1) variability of leaf decomposition depends on scale and 2) microbial decomposition responds differently to a change in scale than decomposition involving invertebrates.
NB41B-06 09:45h
Annual Variation in Climatic Conditions and Leaf Litter Inputs Into Two Tropical Dry Forest Streams -- a Response to the El Nino Southern Oscillation (ENSO)
Tropical dry forests are defined by a severe seasonal drought that results from warm air temperature and extremely seasonal rainfall. We measured daily air temperature, rainfall, and leaf litterfall from 1991-2004 at a dry forest site in northwestern Costa Rica. Rainfall ranged from 1-121 mm in the driest month (April) to 185-1051 mm in the wettest month (October). Annual rainfall ranged from 2229-3676 mm. Air temperature varied little (6.7°C diel, 1.5°C interannual). Annual climatic differences appear related to ENSO conditions - El Nino years are 1-2°C warmer and receive 1400 mm less rainfall than La Nina years. Monthly litterfall collected with wire baskets suspended over two nearby streams ranged from 30-136 g AFDM/m2 while annual litterfall ranged from 714-968 g/m2. Leaves comprised 78% of litterfall. Annual litterfall during El Nino years was 30% greater (250 g/m2) than during La Nina years, presumably because greater moisture stress increased leaf shedding from streamside forests. This increase was only apparent during the driest months (February-May). These results suggest that annual variation in allochthonous organic matter input into these streams reflects, in part, a response to the ENSO - a large-scale climatic phenomenon with a multi-year return interval.