Bottom type

Abstract: Remote sensing is a valuable tool for rapid identification of benthic features in coastal environments Past applications have been limited, however, by multispectral models that are typically difficult to apply when bottom types are heterogeneous and complex We attempt to overcome these limitations by using a spectral library of remote sensing reflectance ( Rrs), generated through radiative transfer computations, to classify image pixels according to bottom type and water depth Rrs spectra were calculated for water depths ranging from 05 to 20 m at 05- to 10-m depth intervals using measured reflectance spectra from sediment, seagrass, and pavement bottom types and inherent optical properties of the water To verify the library, computed upwelling radiance and downwelling irradiance spectra were compared to field measurements obtained with a hyperspectral tethered spectral radiometer buoy (TSRB) Comparisons between simulated spectra and TSRB data showed close matches in signal shape and magnitude The library classification method was tested on hyperspectral data collected using a portable hyperspectral imager for low light spectroscopy (PHILLS) airborne sensor near Lee Stocking Island, Bahamas Two hyperspectral images were classified using a minimum-distance method Comparisons with ground truth data indicate that library classification can be successful at identifying bottom type and water depth information from hyperspectral imagery With the addition of diverse sediments types and different species of corals, seagrass, and algae, spectral libraries will have the potential to serve as valuable tools for identifying characteristic wavelengths that can be incorporated into bottom classification and bathymetry algorithms

Abstract: Measurements of inherent optical properties (IOP) were conducted over bottoms with different substrates by use of a sampling package mounted on and operated by a SCUBA diver. It was found that in areas of low ambient currents the distribution of IOP varies with bottom type in (1) its value relative to a nearby bottom of different type, (2) its vertical gradient, and (3) its variability. This implies that radiative transfer modeling in shallow environments may need to include, besides the bottom characteristics, the bottom effect on in-water IOP. In tidally flushed shallow banks, vertical and horizontal gradients over scales of O(1, 10 m), respectively, are as large as temporal gradients over scales of minutes and cannot be separated in our measurements. However, bottom-substrate‐ related processes over the banks result in gradients over large horizontal spatial scales and tidal timescales. The distribution of IOP is consistent with several biogeochemical processes that may be active at a given bottom substrate and suggest that optical measurements may provide a useful tool to infer and quantify bulk rates of biogeochemical processes.

Abstract: The Hawaiian monk seal (Monachus schauinslandi) is thought to be a foraging generalist, preying on numerous species in diverse habitats of the subtropical Northwestern Hawaiian Islands. At the atoll of French Frigate Shoals, recent evidence of emaciation and low survival in monk seals prompted a search for their specific prey communities and foraging habitat. A video camera (National Geographic Television's CRITTERCAM) fitted to 24 adult male seals documented benthic and demersal foraging on the deep slopes (50-80 m) of the atoll and neighboring banks. The number of bottom searches for prey was compared by year, time of day, type of bottom, individual seal, and length of bottom time. Analysis of variance identified a significant interaction of seal and bottom type, explaining 65% of the total variance. Seals fed on communities of cryptic fauna (fish and large invertebrates) in transitional “ecotone” regions of low relief where consolidated substrate, rubble, and talus bordered areas of sand. Independent areal surveys of bottom types throughout the atoll and neighboring banks suggest that the type of bottom selected as foraging habitat represents a relatively small percentage of the total benthic area available.

Abstract: Surface waves play an important role for the sediment distribution in the shallow Baltic Sea. This paper presents the large-scale spatio-temporal distribution of wave-induced bottom friction velocity, u * , based on modelled wave data for the years 1999 and 2000. The highest values of u * are found along the eastern coasts of the Baltic Proper and Bothnian Sea—areas characterised by long fetches for the dominant winds. Temporally, the dynamics follow that of the wind climate with higher velocities during winter and lower during summer. A smooth bottom is assumed for the calculations. To test this assumption, u * is compared to other estimates of u * assuming rough bottoms. The spatio-temporal patterns are similar, although the present approach gives a slight underestimation of u * at areas with coarse grain sizes. To compare the results, the co-variation between the u * distribution and bottom type distribution from a digitised sediment map is analysed. It shows upon a good agreement. This is also found when comparing critical levels for resuspension found in the literature with the same from modelled u * . In addition, other processes important for bottom stress, such as mesoscale eddies and coastal jets, are discussed.