Light, nutrients and grazing interact to determine stream diatom community composition and functional group structure

Abstract

1. Benthic algal communities are shaped by the availability of nutrients and light and by herbivore consumption. Many studies have examined how one of these factors affects algal communities, but studies simultaneously addressing all three are rare. 2. We investigated the effects of nutrients, light and a herbivore (the snail Potamopyrgus antipodarum) on benthic stream algae in a fully factorial experiment in 128 circular streamside channels. Four nutrient levels (none added to highly enriched), four snail grazing pressures (no snails to 777 individuals m )2 ) and two light levels (ambient and 65% reduced) were applied. Colonising algae were dominated by diatoms (Bacillariophyta), which were determined to species using acid-cleaned samples and assigned to functional groups according to their physiognomic growth form. 3. Diatom community structure changed considerably in response to our manipulations. Light had the strongest influence (as indicated by M AN O V A M AN O V A effect size), whereas nutrients had an intermediate effect and grazing was fairly weak. Relative abundances of six common diatom taxa decreased under reduced light, whereas five others became more prevalent. Eight taxa benefitted from nutrient enrichment, while three became rarer. Grazing affected the relative density of only one common taxon, which increased at higher grazing pressure. 4. Diatom functional groups also responded strongly. 'Low profile' taxa dominated at low resource levels (nutrients and especially light), whereas 'high profile' and 'motile' taxa became markedly more prevalent at higher resource levels. 5. Two-way interactions between experimental factors were quite common. For example, Planothidium lanceolatum and Rossithidium petersenii responded more strongly to nutrient enrichment at reduced than at ambient light, whereas Cocconeis placentula responded more strongly at ambient light. For diatom functional groups, the benefit of nutrient enrichment for 'motile' diatoms was greater at ambient than at reduced light. 6. Our results imply that multifactor experiments are required to determine the main forces driving the composition of benthic algal communities. Further, our findings highlight the considerable potential of using functional algal groups as indicators of changing environmental conditions to complement the traditional taxonomic approach.

Nutrient and light limitation of periphyton in the River Thames: Implications for catchment management

“…These reductions in direct sunlight monitored within the flumes were closely comparable with those observed under partial and full tree canopy (Table 4). Shading was also found to reduce the proportion of motile diatoms in a recirculating flume study by Lange et al, (2011). As in the current study (Figure11), the motile diatom Nitzschia palea was less abundant in the shaded treatments, whereas the adnate diatoms…”

“…Cocconeis placentula and Achnanthidium minutissimum occurred at higher abundances under reduced light (Lange et al, 2011). Motile diatoms are generally at an advantage within thick biofilms (e.g.…”

The roles of environment and space in shaping stream diatom communities

“…The occurrence of benthic diatoms in streams is the outcome of complex interactions between abiotic factors, such as hydrological and chemical, and biotic factors (Stevenson et al, 1996;Lange et al, 2011). Local environmental variables that may influence diatom communities include water chemistry, current velocity, light, grazing, temperature and substrate.…”

“…Of the local proximate factors, water chemistry and amount of light are often the most important variables influencing the density of diatom populations (Stevenson et al, 1996;Lange et al, 2011). Temperature interacts with these variables to affect the rates of metabolism and growth.…”

Changes in the epipelic diatom assemblage in nutrient rich streams due to the variations of simultaneous stressors

“…They also concluded that as the number of stressors in a system increase, stressor pair interactions become increasingly complex and more commonly synergistic. In laboratory settings, few studies have been conducted that analyze the responses of the diatom assemblage to the effects of multiple A c c e p t e d M a n u s c r i p t variables combined (such as Navarro Rodriguez, 1998;Rier & Stevenson, 2002;Lange et al, 2011), a situation that is more likely to occur in a natural stream since environmental changes rarely affect a single variable.…”