## 2 Methods
### Measurements and Analysis of Natural Seawater Samples  
Optical measurements with the LISST-VSF and ancillary analyses of natural particle assemblages were performed on seawater samples collected between summer 2016 and spring 2017 in contrasting marine environments, namely, in open ocean waters off the coast of Southern California, nearshore ocean waters at the pier of the Scripps Institution of Oceanography (SIO Pier) in La Jolla, and the tidal estuary of the San Diego River. Overall 17 samples representing a broad range of natural particle assemblages were analyzed. Most samples (number of samples \(N=11\)) were collected at the SIO Pier. These samples were collected during typical dry weather conditions, phytoplankton bloom events, and after heavy rain. The tidal estuary samples (\(N=3\)) include three tidal states between low and high tide. The offshore samples (\(N=3\)) were collected in the Santa Barbara Channel, about 8 km off San Diego Bay, and about 2 km off SIO Pier. Seawater samples were collected just beneath the sea surface using either Niskin bottles or a bucket, except for one offshore sample (off San Diego Bay) that was collected at the subsurface chlorophyll-\(a\) maximum at a depth of 18 m. All samples were analyzed in the laboratory within 24 h of sampling. To characterize the concentration and composition of particulate matter for each sample, we determined the dry mass concentration of total suspended particulate matter, SPM [g m\({}^{-3}\)], mass concentration of particulate organic carbon, POC [mg m\({}^{-3}\)], and mass concentration of the pigment chlorophyll-\(a\), Chla [mg m\({}^{-3}\)]. For these determinations, the particles were collected on glass-fiber filters (GF/F Whatman) by filtration of appropriate volumes of seawater (150-2100 mL depending on the sample). SPM was determined following a gravimetric method using pre-washed and pre-weighted filters [7; 70]. The determinations of POC were made on precombusted filters with a standard CHN analysis involving high temperature combustion of sample filters [7; 71; 72]. Chla was determined spectrophotometrically using a Lambda 18 spectrophotometer and placing 1-cm cuvettes containing acetone extracts of the samples inside the integrating sphere. The measured absorbance values at 630, 647, 665, and 691 nm (after subtraction of acetone baseline values) were used in the calculation of Chla [73]. For each seawater sample, replicate determinations of SPM and POC were made on separate sample filters. The final SPM and POC are average values of replicate determinations. The replicates for SPM and POC agreed generally to within 15% and 10%, respectively. No replicates were taken for Chla. In addition to information about particle concentration, SPM, POC, and Chla provide useful proxies of bulk composition of particulate matter. The organic and inorganic fractions of SPM can be characterized using the ratio POC/SPM, and the contribution of phytoplankton to SPM using Chla/SPM [65]. These ratios are expressed on a [g/g] basis. The measurements of particle size distribution (PSD) were made with a Coulter Multisizer 3 (Beckman Coulter, Brea, CA, USA) equipped with a 100 \(\upmu\)m aperture, which allows particle counting and sizing in the range of volume-equivalent spherical diameter from 2 \(\upmu\)m to 60 \(\upmu\)m. Within this size range we used 300 log-spaced size bins to provide high resolution PSDs. For each experiment, 0.2 \(\upmu\)m filtered seawater was used as a blank that was subtracted from sample measurements. Approximately 10 to 15 replicate measurements of 2 mL subsamples of each seawater sample were collected. After removing outliers, the remaining measurements were summed and divided by the total analyzed volume to produce an average density function of PSD in particle number per unit volume per width of size bin.