Landscapes export nutrients to aquatic ecosystems at rates and ratios that are strongly influenced by land use practices. After entry into aquatic ecosystems, nutrients can be processed, retained, lost to the atmosphere, or exported downstream. The stoichiometry of nutrients as they travel from terrestrial landscapes through aquatic systems can influence ecosystem services such as water quality, nutrient limitation, biodiversity, eutrophication and the sequestration of nutrients and carbon in sediments. However, we know little about how nutrient stoichiometry varies along the pathway from terrestrial landscapes through aquatic systems. We studied the stoichiometry of nutrients (nitrogen and phosphorus) exported by three watersheds of contrasting land use (forest, mixed forest/agriculture, and agriculture) and in downstream lakes (reservoirs). Specifically, we related N:P stoichiometry of watershed exports via streams to N:P stoichiometry in the water columns and buried in the sediments of the downstream lakes. We also related stoichiometry to phytoplankton nutrient limitation and the abundance of heterocystous cyanobacteria.
Results/Conclusions
The total N:P of stream exports varied greatly among catchments, being 18, 54 and 153 (molar) in the forested, mixed-use, and agricultural catchment, respectively. Total N:P in the mixed layers of the lakes was less variable but ordered similarly: 35, 52, 132 in the forested, mixed-use and agricultural lake, respectively. Phytoplankton in the forested lake were consistently N-limited, those in the agricultural lake were consistently P-limited, and those in the mixed-use lake seasonally shifted from P- to N-limitation. Total phytoplankton and cyanobacteria biomass were highest in the agricultural lake, but potentially N-fixing (heterocystous) cyanobacteria were most abundant in the forested lake, corresponding to low N:P ratios. The N:P ratio of sediments buried in reservoirs varied with catchment input N:P, but differences were very slight: N:P was 4.3, 5.6, and 7.3 in sediments of the forested, mixed and agricultural lakes. Additional studies in the agricultural lake showed that the N:P ratio of settling particles (from sediment traps) was less than that of seston and that nutrients were released (in dissolved form) from sediments at a high N:P ratio. Both mechanisms can help explain the relatively low N:P of buried sediment. Overall, our results show congruence between the watershed export N:P, reservoir N:P, and phytoplankton N vs. P limitation. However, the N:P stoichiometry of sediments retained in the lakes was relatively insensitive to catchment stoichiometry, suggesting more efficient retention of P and/or greater loss of N in these lake systems.
