Spatial distribution of Meroplanktonic larvae. A Canadian Healthy Oceans Network Population Connectivity project, PC-06

Larval abundance (count m-3) was sampled at 11 sites on 7-8, and 11-12 Aug 2008 and at 16 sites on Aug 2-4, 2009 (Table 1), with a 200-μm plankton ring net (0.75-m diameter) towed for 5 min at each of 3 m and 12 m depth. These depths were designed to sample: 1) the surface mixed layer and 2) within the pycnocline, at or near the fluorescence maximum. The net was towed at ~1.7 m s-1 and the volume of filtered water was quantified using a General Oceanics flow meter. Using a net of this mesh size may under-estimate abundance of small larvae (< 200 μm). However, it is a necessary compromise in this multi-species study to allow capture of a wide range of larval types at sufficient numbers (e.g. very abundant but small gastropods to larger but rare decapods). All plankton samples were preserved in 95% ethanol and larvae were identified and enumerated under a Nikon SMZ 1500, as described in Lloyd et al. (2012). Samples were split into subsamples using a Folsom plankton splitter. For n = 8, samples were split to 1/64 of the original volume and all subsamples were processed. Based on those samples, we determined that at least 20 individuals of each species were required to obtain an estimate of abundance that was within 5% of the true sample abundance. The remainder of the samples were split to between 1/128 and 1 to ensure that ≥20 individuals of the most abundant species (Margarites spp., Astyris lunata, Mytilus spp., Electra pilosa. and Cancer irroratus) were enumerated. In addition to these 5 species that met the above criteria, we used some less abundant species in some data analyses (see below); however, the validity of the results should not be affected because there was no bias in the estimated abundances and there is often spatial and/or temporal replication.