Print PageLimited aquatic primary productivity is often cited as a factor behind low oxygen levels observed in forested blackwater rivers. However, submerged trunks and knees of the same trees that limit light with their canopy also provide stable substrate for growth of aquatic bryophytes. We use laboratory and field studies to demonstrate that oxygen release by the aquatic liverwort Porella pinnata counteracts a significant portion of oxygen demand generated by decomposing organic matter in the Little River in southern Georgia, USA. To model total oxygen release by bryophytes under varying river conditions, we calculated surface area supporting bryophyte growth using a modified point-centered quarter method incorporating differences in tree trunk and cypress (Taxodium distichum) knee circumference with depth at points along transects. Because river levels vary seasonally, liverwort biomass was measured on tree trunks relative to water depths simulated with self-leveling rotary lasers. Water depths were then measured at the same transect points at a range of river discharge levels during multiple wet seasons.
Results/Conclusions
In laboratory chambers, photosynthesis-irradiance curves generated for P. pinnata indicate that photosynthetic release of oxygen exceeds uptake by respiration at levels of irradiance above 25 µmol m-2 s-1, which is lower than the range of average light levels (35-272 µmol m-2 s-1) measured within the vertical growth zone of liverworts under a full forest canopy. Oxygen, temperature, and discharge continuously measured downstream from the swamp-forest from 2006-2009 reveal highest amplitudes of diel oxygen curves at river discharge levels submerging bryophytes but maximizing their exposure to light. At low discharge, water drops below the vertical range of bryophyte growth, significantly lowering river oxygen concentrations. High temperatures allow microbial respiration to exceed photosynthetic oxygen release, but at moderate flows aquatic bryophytes have strong influences on ecosystem metabolism. Oxygen release by bryophytes was modeled under three scenarios: 1) forest composition as measured in the field, with surface area of cypress knees included and 2) with surface area of knees removed, and 3) a forest consisting entirely of gum (Nyssa sp.). Results suggest that forest composition plays an additional role in determining aquatic primary production: with buttressed trunks and knee production, a swamp dominated by cypress produces significantly greater surface area capable of supporting aquatic primary production. This study demonstrates the importance of aquatic primary production in forested blackwater rivers and also shows how cypress, a foundation tree species that is intensively harvested, can significantly modify ecosystem function.
