Hyperspectral imaging of the microscale distribution and dynamics of microphytobenthos in intertidal sediments

Arjun Chennu, Paul Färber, Nils Volkenborn, Mohammad Al-Najjar, Felix Janssen, Dirk De Beer, Lubos Polerecky
2013 Limnology and Oceanography: Methods   unpublished
511 Microphytobenthos (MPB), consisting of benthic pho-totrophs such as microalgae, cyanobacteria, and dinoflagel-lates, is the fundament of the trophic food web in coastal ecosystems. In shallow-water environments, such as intertidal flats and estuaries, MPB represents a considerable portion of the autotrophic biomass, accounting for up to 50% of the primary production (MacIntyre et al. 1996; Nozais et al. 2001; Spilmont et al. 2006). MPB is the predominant food source for many deposit-feeding
more » ... organisms, and the activity and distribution of MPB profoundly affect nutrient fluxes across the sediment-water interface, sediment geochemistry, as well as sediment morphology and stability (Sundbäck et al. 1991; Montagna et al. 1995; Miller et al. 1996; Stal 2010). The spatio-temporal organization of a community is a prominent issue in ecology. Levin (1992) argued that analysis of large-scale (regional) patterns must integrate effects occurring at smaller scales. Generally, the distribution of MPB is affected by both abiotic processes (e.g., nutrient availability, hydrodynamic exposure, sediment type) and biotic processes (e.g., grazing, competition) through a complex network of interactions. In their reductionist approach to benthic ecology , Miller et al. (1996) argued that studies of interactions with a direct effect on the MPB distribution (so called "isolated first-order interactions") are required to disentangle the complex relationships between the sediment bed, infaunal organisms , and the water column, and for a comprehensive picture Abstract We describe a novel, field-deployable hyperspectral imaging system, called Hypersub, that allows noninva-sive in situ mapping of the microphytobenthos (MPB) biomass distribution with a high spatial (sub-millimeter) and temporal (minutes) resolution over areas of 1 × 1 m. The biomass is derived from a log-transformed and near-infrared corrected reflectance hyperspectral index, which exhibits a linear relationship (R 2 > 0.97) with the chlorophyll a (Chl a) concentration in the euphotic zone of the sediment and depends on the sediment grain size. Deployments of the system revealed that due to factors such as sediment topography, bioturbation, and grazing, the distribution of MPB in intertidal sediments is remarkably heterogeneous, with Chl a concentrations varying laterally by up to 400% of the average value over a distance of 1 cm. Furthermore, due to tidal cycling and diel light variability, MPB concentrations in the top 1 mm of sediments are very dynamic, changing by 40-80% over a few hours due to vertical migration. We argue that the high-resolution hyperspectral imaging method overcomes the inadequate resolution of traditional methods based on sedimentary Chl a extraction, and thus helps improve our understanding of the processes that control benthic primary production in coastal sediments.
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