Small-scale heterogeneity in the geochemistry of seagrass vegetated and non-vegetated estuarine sediments: causes and consequences

Abstract

In addition to nutrient and light availability, sedimentary biogeochemical processes can play an essential role in seagrass productivity. Previous investigations of the interactions between seagrasses and their underlying sediments have failed to clearly identify the spatio-temporal variability of the major geochemical parameters involved. Dissolved and solid phase chemical parameters in eelgrass vegetated and nearby non-vegetated sediments were investigated in this study to determine their vertical, lateral, and temporal distributions. Solid-state microelectrodes were used to investigate dissolved O 2 , SH 2 S, Fe 2+ , and Mn 2+ on mm space scales. In this study, spatial heterogeneity was assessed and diurnal ''ventilation'' by seagrass productivity (i.e., the translocation of photosynthetically produced oxygen to the anoxic sedimentary environment) was not observed probably because benthic infaunal activity (bioturabation and bioirrigation) and microzones established by microbial processes led to highly heterogeneous sediment geochemistry where temporal variability was obscured by small-scale spatial variability. Non-vegetated sediments were less 2006 geochemically variable laterally than vegetated sediments, however, in some cases, they had similar vertical variability, possibly because they had been vegetated at an earlier time. This study demonstrates that in vegetated sediments where there is also substantial benthic macrofaunal activity it is difficult to separate the impacts of the two types of biota on sediment geochemistry and their spatial patterns, and it also raises the question of the applicability of traditional one-dimensional diagenetic models for such spatially-temporally complex sediments.

Benthic microbial metabolism in seagrass meadows along a carbonate gradient in Sulawesi, Indonesia

“…This is in addition to acid generation via aerobic respiration. Regardless of the driving mechanism, our observations are consistent with evidence of the occurrence of carbonate dissolution in other seagrass sediments (Burdige & Zimmerman 2002, Holmer et al 2003, Hebert et al 2007). Rates of carbonate dissolution estimated from the Ca fluxes ranged from 14 to 109 mmol m -2 d -1 (Table 7), coincident with increasing carbonate content from the inshore to the outer bay seagrass beds.…”

“…) in other studies (Burdige & Zimmerman 2002, Holmer et al 2003, Hebert et al 2007), but rates of sulfate reduction were lower in these other locations. Comparing these estimates to the rates of total carbon decomposition (assuming sulfate reduction is the only major process), on average, carbon dissolution equates to ∼40% of total carbon oxidation, with an increase in this proportion from ∼35% at Stn SG1 to ∼40% at Stn SG2 to ∼70% at Stn SG3.…”

Analyzing sediment dissolved oxygen based on microprofile modeling

“…However, in real sediments, larger variation may be expected in the DO microprofile because the sediment structure is heterogeneous. In some cases, the oxygen levels may even show peaks below the SWI as a result of the burrowing effect of macroinvertebrates (Hebert et al 2007). In our study, the prepared sediment was homogeneous and no macroinvertebrates existed, so there were no large fluctuations in the DO microprofiles.…”

Mechanisms for copepod resting egg accumulation in seagrass sediments

“…Sulfide concentrations and oxygen availability in seagrass and bare sediments can be expected to vary seasonally in the study area. During summer months, microbial activity in seagrass sediments would be stimulated by high temperatures and organic material trapped by the canopy, leading to lower oxygen conditions and higher sulfide concentrations than in the bare sand (Agawin and Duarte 2002;Hebert et al 2007). In the winter, cool temperatures, less production in the water column, and the absence of seagrass canopy trapping effects would depress microbial activity in both environments and allow more similar sediment conditions to develop between them (Pomeroy and Wiebe 2001;Pringault et al 2008).…”