Is co-limitation of primary production in headwater streams by light and nutrients a product of aggregate single-factor limitation?
Limitation of primary production by two or more factors together (i.e., co-limitation) is increasingly recognized as the predominant condition at the ecosystem scale. In stream ecosystems, nutrients and light availability are the two dominant factors limiting primary production. Numerous studies have documented co-limitation by nitrogen and phosphorus, and others have found light and nutrient co-limitation in forested streams. While nutrient concentrations can vary somewhat along a stream reach, variability in stream light can be much greater and can occur over short distances, especially in stream reaches bordered by complex riparian forests. Nutrient limitation rarely manifests under low light conditions so the amount of light can exert a key control whether a given area experiences nutrient versus light limitation. We hypothesized that local light conditions would influence the presence of light vs. nutrient limitation for periphyton at small spatial scales and that high variability in light could create simultaneous light and nutrient limitation in nearby sections of the same stream.
In this study, we used sets of 6 to 12 nutrient diffusing substrate arrays paired with stream light measurements to determine light versus nutrient limitation of stream periphyton at multiple locations along four streams in the Cascade Mountains in Oregon. The streams encompassed a range of background nutrient levels with mean mid-summer nitrate nitrogen concentrations as low as 3ug*L-1 and as high as 65ug*L-1. In three of the four reaches, we also measured light every 5m for 300m to quantify local light variability.
In all four streams, we identified areas of the stream benthos where periphyton experienced light limitation, even when background nutrients were low. And, in three of the four streams, we also identified areas of the stream benthos where periphyton was primarily nutrient limited. Heterogeneity in stream light led to variability in the limiting factor for periphyton growth along these three stream reaches. Based on responses across the set of nutrient diffusing substrate arrays in each stream, we identified a potential light availability threshold range in each system where periphyton shifted between being predominantly light limited and predominantly nutrient limited. Given the variability in light availability along these streams (based on the extensive light measurements in three sites), we concluded that these ecosystems are collectively co-limited by light and nutrients but that co-limitation at the reach scale may be a product of spatial variability in single-factor limitation.