I believe you can use some map algebra (raster > raster calculator) before you can preform your volume measurements in grass.
Assuming that your bathymetric data use positive values to represent the sea depth, and using your example for the range as 50 the min_depth and 200 the max_depth. For each of the raster cells you need to "remove" anything below the 200 and above the 50. You have 3 possible situations:
your cell depth is higher than the max_depth
(@bathymetry > 200)
your cell depth in the min_depth - max_depth range
(@bathymetry <= 200) * (@bathymetry >= 50)
your cell depth is lower that your min_depth
(@bathymetry < 50)
In case 1 the size of the water column will be the entire size of our range:
(@bathymetry > 200.0) * (200.0 - 50.0)
In case 2 the "water column" is from your bathimetry depth until the minimum value of the range:
(@bathymetry <= 200.0) * (@bathymetry > 50.0)*(@bathymetry - 50.0)
In case 3 there is no useful "water column" for your specie therefore it would be:
(@bathymetry <= 50)*0.0
Since the 3 cases never occurs at the same time, all we need is to sum the expressions:
(@bathymetry > 200.0) * (200.0 - 50.0) + (@bathymetry <= 200.0) * (@bathymetry > 50.0)*(@bathymetry - 50.0) + (@bathymetry <= 50)*0.0
Since the last expression is always zero, the is no point in including it:
(@bathymetry > 200.0) * (200.0 - 50.0) + (@bathymetry <= 200.0) * (@bathymetry > 50.0)*(@bathymetry - 50.0)
And the generic version of the expression would be:
(@bathymetry > max_depth) * (max_depth - min_depth) + (@bathymetry <= max_depth) * (@bathymetry > min_depth)*(@bathymetry - min_depth)
After this, you can use the output raster in r.volume to sum all "water columns" values
You can use GRASS GIS r.stats as a plugin in QGIS or in stand alone GRASS:
r.stats calculates the area present in each of the categories of user-selected raster map layer(s). Area statistics are given in units of square meters and/or cell Counts
If you need more options like different units use r.report:
r.report allows the user to set up a series of report parameters to be applied to a raster map layer, and creates a report.
Best Answer
This note you've quoted is devoted especially to ArcGIS users - Float to Raster tool is dedicated to convert float files (float reffers to file format not values type) to ArcGIS raster dataset suitable to perform further analisys in ESRI's ecosystem.
Using QGIS you can skip this step. Of course if you wish to convert float file to any other grid format - gdal_translate will do the work.
To derive volume try GRASS module
r.volumeof course combine it with some clipping and/or other analisis.