Spatiotemporal variation in aquatic ecosystem heterotrophy in freshwater subtropical wetlands is driven by water and organic matter availability

Background/Question/Methods

Variation in carbon (C) quantity and quality drive ecosystem trophic state (net autotrophy or heterotrophy). Labile flocculent organic matter (floc) derived from algae and detritus is an abundant C source that forms the basis of food web production in carbonate subtropical wetlands, but the spatiotemporal patterns of floc and its effects on ecosystem trophic state are poorly quantified. Quantifying how changes in the proportion of floc with declining dry-season water depth interactively drive water column metabolism in wetlands is critical for understanding C source-sink dynamics. We estimated aquatic ecosystem metabolism (gross primary productivity, GPP; ecosystem respiration, ER) during wet and dry seasons from continuous measurements of diel water column dissolved oxygen, photosynthetically active radiation (PAR), temperature, and water depth in ridge and slough wetlands of the Florida Everglades. We also measured floc-specific metabolism (GPP and ER) by incubating benthic floc collected from ridge and slough habitats using settlement traps. Estimates of GPP, ER, and reaeration rate were modeled from light, temperature, and water depth using non-linear minimization and maximum likelihood.

Results/Conclusions

Changes in water depth drove changes in PAR and the proportion of benthic floc in both habitat types, which explained differential spatiotemporal patterns in aquatic ecosystem metabolism. ER was highest in open slough habitats with less submerged aquatic vegetation (SAV) and trapped floc (range: -11.7 to -2.5 g O₂ m^-2 d^-1), and ER and GPP were positively correlated (R² = 0.81, P < 0.05). Slough habitats with higher SAV and proportion of benthic floc had comparatively lower ER (range: -5.2 to -0.02 g O₂ m^-2 d^-1), and ER was not related to GPP. ER in ridge habitats (range: -10.4 to -0.9 g O₂ m^-2 d^-1) was highest at intermediate water depth and positively related to GPP from 0.3-0.5 m depth (R² = 0.60, P < 0.05). Below 0.3-m water depth, metabolism in ridge habitats was dominated by ER (GPP ~ 0 g O₂ m^-2 d^-1), due to increased proportion of benthic floc. Temperature-corrected, floc-specific GPP (11.50 mg g AFDM^-1 h^-1  ± 0.28, 11.02 ± 0.38) and ER (25.22 mg g AFDM^-1 h^-1 ± 0.27, 25.70 ± 0.26) were similar in ridge and slough habitats. Enhanced ecosystem heterotrophy likely occurs through biogeochemical priming with increased dry-season floc densities.