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Five predictor categories

PREHAB has evaluated the usefulness of five broad predictor categories for modelling benthic species and habitats. Below, you find some general information about each category.

As a spatial measure, geographical location can be a useful predictor of benthic organisms, even though this type of variable has no direct mechanistic relevance and is not of general importance as explanatory variable. Latitude and longitude values are given using nautical charts and point sampling with a GPS receiver.

Bottom topography
One of the most powerful predictors of benthic species distribution is water depth. High-quality depth data can be extremely useful and strongly increase the probability of species occurrence in models, although patterns are often driven by species-specific requirements. Depth is a strong indicator of species’ upper and lower limitations in the water column, zonation and light requirements. Slope, aspect of a terrain (direction) and terrain curvature are other predictors related to the seafloor morphology, which may be useful as explanatory predictors of benthic organisms. Direct measurements or estimations of depth and other seafloor morphometrics can be derived using e.g. shipboard sonar, acoustic ecosounders (e.g. high-resolution multibeam) or laser technique (e.g. airborne hydrographic LiDAR).

Wave exposure
Benthic communities are frequently affected by hydrodynamic forces, including both waves and currents. Wave exposure is a physical measure that governs the prevalence and distribution of organisms in shallow coastal environments, and can often improve predictive models on benthic species distribution. For instance, wave exposure is considered a primary factor limiting the distribution of submerged macrovegetation either directly by e.g. physical disturbance or indirectly through e.g. reduced light availability due to sediment resuspension. Wave exposure is often qualitatively described and categorized into subjectively decided groups like “sheltered”, “moderately exposed” and “exposed”, while more sophisticated ways to quantify wave exposure goes through wave exposure models using e.g. fetch indices.

Bottom substrate
The type of substrate – whether the sediment bottom is rocky or soft – is a significant determinant of benthic species assemblage composition. If a bottom is soft or hard is actually the strongest substrate predictor, while also within these two main types of bottom type extremes there is a wide range of textures and grain sizes that may strengthen predictive models. For instance, soft bottoms with high levels of fine-grained sediment accumulation (e.g. as a result of input from river runoff) may have large effects on the distribution of macroinvertebrates, due to e.g. altered sediment composition. Substrate is commonly measured using well-established mapping techniques such as echo-sounders, sidescan sonar, acoustic mapping, and remote sensing (aerial photography or satellite imagery).

Hydrographical variables
Among environmental predictor variables potentially determining benthic organisms, those classified as hydrographic variables, e.g. salinity, water temperature, oxygen, pH and secchi depth, are well-studied and particularly useful in species distribution modeling of large or strong gradients. For example, strong salinity and temperature gradients can profoundly influence the abundance and distribution of many fish and invertebrate species in nearshore coastal waters. The measure of the physical properties of ocean is a scale-dependent procedure and to estimate natural variability requires different techniques depending on the scale. To acquire direct and indirect measurements of water characteristics, including salinity and water temperature, at relatively small scales (km) a CTD (conductivity–temperature–depth) recorder is an appropriate instrument, while at larger scales (100-1000s km) other techniques (e.g. satellite sensors) are more useful.

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