Relative strengths of benthic algal nutrient and grazer limitation along a lake productivity gradient

Abstract

The relative effects of nutrients and herbivores on primary producers are rarely compared across ecosystems that vary in potential primary productivity. Furthermore, proposed mechanisms to explain such patterns remain understudied. Here, I examine the strength of nutrient and grazer (herbivore) limitation (i.e., the extent to which producers' growth is limited by insufficient nutrient supply or herbivory) of benthic algae across 13 southwest Michigan lakes that vary widely in productivity (i.e., resource supply). I compare the observed patterns of algal limitation and species composition to those predicted by two simple models: one that includes multiple species and species' traits (the food-web model) and one that includes no variation in species or traits (the food-chain model). Species in the food-web model are assumed to display a tradeoff between resource competitive ability and resistance to herbivory (the "keystone predator" tradeoff). Among these lakes, benthic algal nutrient limitation was positive (x=0.083 day-1) and declined significantly along a lake N:P gradient. In contrast, grazer limitation was negative (x=-0.019 day-1) and was not significantly related to any of the measured lake productivity variables. Negative grazer limitation indicated that the removal of grazers caused unexpected declines in algal biomass, which were potentially due to indirect, positive effects (e.g., nutrient recycling) of grazers. Nutrient limitation was significantly stronger than grazer limitation across lakes, which was more consistent with the food-web versus food-chain model. Changes in algal composition were also broadly consistent with predictions of the food-web model in that vulnerable, superior nutrient competitors dominated in low productivity lakes and more grazer-resistant species were observed in high productivity lakes. In general, these results point to the importance of examining limiting factors across systems and the consideration of key species' traits when predicting and interpreting patterns.

Response of lotic producer and consumer trophic levels to gradients of resource supply and predation pressure

“…The interactive effects of light and nutrients appeared to switch on at a riparian reserve width of 10‐m, which corresponds to a light flux of about 75 μmol m −2 s −1 . This result is qualitatively similar to studies in stream, lake, marine, and terrestrial systems that found nutrients interact with ambient environmental conditions to influence plant growth (Rosemond 1993, Wootton et al 1996, D'Antonio et al 2001, Brewer 2003, Darcy‐Hall 2006). For example, Taulbee et al (2005) found that primary producer biomass increased as light levels increased but only on nitrogen‐enriched substrates.…”

“…Resource supply and predation pressure vary within and among natural systems, likely contributing to the relative importance of these factors in shaping trophic level biomass and abundance. It is logistically difficult to manipulate more than one ecological gradient simultaneously, but such experiments are critical if we are to have any confidence in predicting the effects of environmental change on community structure or ecosystem function (Hunter and Price 1992, Chase 2003, Darcy‐Hall 2006). I describe results from a study that examined the main and interactive effects of light, nutrients and predator abundance in affecting primary producers and consumers.…”

Food Web Architecture and Basal Resources Interact to Determine Biomass and Stoichiometric Cascades along a Benthic Food Web

“…It has been suggested that a substantial part of the photosynthesis in clear-water lakes occurs in benthic algal communities, and the fraction of benthic productivity that is transferred to consumers decreases as the amount of dissolved carbon increases in the water column [14]. Consequently, changes in light abundances due to dissolved colored carbon can weaken the strength of trophic interactions by limiting the productivity of basal trophic levels [37], as we observed even in high nutrient conditions. This result highlights that mild nutrient loads that can increase the relative importance of predators' indirect effects on basal trophic levels [10] may be diminished in dark-water lakes.…”

Substrate influence and temporal changes on periphytic biomass accrual and metabolism in a tropical humic lagoon

“…Herbivory may also play an important role regulating periphytic algal biomass, although not measured in this experiment. However, experimental studies conducted with benthic algal communities showed that grazer limitation is weak compared with nutrient limitation at low productivity sites (Darcy-Hall 2006). Thus, grazer regulation of benthic algal biomass is expected to be weak in this experiment.…”