Plant litter dynamics in the forest-stream interface: precipitation is a major control across tropical biomes.

Riparian plant litter is a major energy source for forested streams across the world and its decomposition has repercussions on nutrient cycling, food webs and ecosystem functioning. However, we know little about plant litter dynamics in tropical streams, even though the tropics occupy 40% of the Earth's land surface. Here we investigated spatial and temporal (along a year cycle) patterns of litter inputs and storage in multiple streams of three tropical biomes in Brazil (Atlantic forest, Amazon forest and Cerrado savanna), predicting major differences among biomes in relation to temperature and precipitation regimes. Precipitation explained most of litter inputs and storage, which were generally higher in more humid biomes (litterfall: 384, 422 and 308 g m-2 y-1, storage: 55, 113 and 38 g m-2, on average in Atlantic forest, Amazon and Cerrado, respectively). Temporal dynamics varied across biomes in relation to precipitation and temperature, with uniform litter inputs but seasonal storage in Atlantic forest streams, seasonal inputs in Amazon and Cerrado streams, and aseasonal storage in Amazon streams. Our findings suggest that litter dynamics vary greatly within the tropics, but point to the major role of precipitation, which contrasts with the main influence of temperature in temperate areas.

The effects of spatial scale on breakdown of leaves in a tropical watershed.

The objective was to assess the effects of natural variation in the physical structure of the environment on biological communities and on the processing of Eucalyptus cloeziana and Inga laurina and to identify the controlling factors at different scales along stream order gradients. The study area consisted of 14 sampling sites distributed within a tropical watershed (1st, 2nd, 3rd and 4th order streams replicated in 4 sub-basins). Our samples consisted of 3 g of leaves of E. cloeziana (high-quality) and I. laurina (low-quality) placed in 252 bags with 10mm mesh (measured by the chemical composition of the detritus). Four samples of each leaf type were collected periodically (three times) over a period of 75-125 days and washed on a sieve to separate the invertebrates. A series of leaf disks were cut to determine ash-free dry mass, polyphenol, lignin, cellulose, total microbial biomass and fungal biomass, and the remaining material was oven-dried to determine the dry weight. We performed analyses within and between spatial scales (regional and local) to assess which watershed scale was the more import determinant of the leaf breakdown rate (k). The microbial and shredder were most influenced at the local scale (stream order). Shredders were influenced by microorganisms, with stronger interactions between them than were found to drive the k at the local scale. Moreover, differences in the overall k and abiotic variables were more strongly influenced at the regional scale (sub-basin), showing that the study scale alters the response of the studied variables. We found higher k values at higher values of water velocity, dissolved oxygen and temperature, all of which accelerate biological metabolism in response to variations on the regional scale. Watersheds with warmer microclimates and streams with higher nutrient levels and oxygen could be accelerating the ecosystem metabolism, independent of the detritus quality.

Dynamics of planktonic prokaryotes and dissolved carbon in a subtropical coastal lake.

To understand the dynamics of planktonic prokaryotes in a subtropical lake and its relationship with carbon, we conducted water sampling through four 48-h periods in Peri Lake for 1 year. Planktonic prokaryotes were characterized by the abundance and biomass of heterotrophic bacteria (HB) and of cyanobacteria (coccoid and filamentous cells). During all samplings, we measured wind speed, water temperature (WT), pH, dissolved oxygen (DO), precipitation, dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), and carbon dioxide (CO2). DOC was higher in the summer (average = 465 µM - WT = 27°C) and lower in the winter (average = 235 µM - WT = 17°C), with no significant variability throughout the daily cycles. CO2 concentrations presented a different pattern, with a minimum in the warm waters of the summer period (8.31 µM) and a maximum in the spring (37.13 µM). Daily trends were observed for pH, DO, WT, and CO2. At an annual scale, both biological and physical-chemical controls were important regulators of CO2. HB abundance and biomass were higher in the winter sampling (5.60 × 10(9) cells L(-1) and 20.83 µmol C L(-1)) and lower in the summer (1.87 × 10(9) cells L(-1) and 3.95 µmol C L(-1)). Filamentous cyanobacteria (0.23 × 10(8)-0.68 × 10(8) filaments L(-1)) produced up to 167.16 µmol C L(-1) as biomass (during the warmer period), whereas coccoid cyanobacteria contributed only 0.38 µmol C L(-1). Precipitation, temperature, and the biomass of HB were the main regulators of CO2 concentrations. Temperature had a negative effect on the concentration of CO2, which may be indirectly attributed to high heterotroph activity in the autumn and winter periods. DOC was positively correlated with the abundance of total cyanobacteria and negatively with HB. Thus, planktonic prokaryotes have played an important role in the dynamics of both dissolved inorganic and organic carbon in the lake.