First (bottom) layer has gaps
You can, in most slicers, set a separate extrusion multiplier for the first layer. This doesn't appear to be possible in slic3r, but there are a couple of ways to work around this:


Change the Z offset (either in software or by adjusting the endstop). Bring the nozzle closer to the bed for the first layer. You can combine this with an increased first layer height. This is the most appropriate fix for your issue.
Use M92 EX.XXX to increase the extruder steps/mm in the start G-code, then set it back to normal in the layer change code. This emulates increasing the extrusion multiplier for the first layer.