Marine particles in the Gulf of Alaska shelf system: Spatial patterns and size distributions from in situ optics

Continental Shelf Research 145 (2017) 13–20

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Marine particles in the Gulf of Alaska shelf system: Spatial patterns and size distributions from in situ optics

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Jessica S. Turnera,b, , Jessica L. Prettya, Andrew M.P. McDonnella a University of Alaska Fairbanks, School of Fisheries and Ocean Sciences, 905 N. Koyukuk Drive, 245 O’Neill Building, P.O. Box 757220, Fairbanks, AK 99775-7220, United States b Virginia Institute of Marine Science, 1375 Greate Road, P.O. Box 1346, Gloucester Point, VA 23062-1346, United States

A R T I C L E I N F O

A BS T RAC T

Keywords: Gulf of Alaska Marine particles, particle size distributions Suspended sediments Sediment transport In situ oceanographic optics

The Gulf of Alaska is a biologically productive ocean region surrounded by coastal mountains with high seasonal runoff from rivers and glaciers. In this complex environment, we measured the concentrations and size distributions of 2.5 µm–27 mm marine particles using the Laser in situ Scattering and Transmissometry device (LISST-DEEP) and the Underwater Vision Profiler 5 (UVP) during summer 2015. We analyzed the spatial distribution of particles across a wide range of size classes to determine the probable drivers. Spatially, total particle concentrations surpassed 1000 µl/l nearshore in the northeasternmost entrances and in the outflow of Cook Inlet, as well as offshore past the shelf break. These dual maxima suggest high lithogenic inputs nearshore and high biological production at and beyond the shelf break. Most large particles (> 0.5 mm) imaged by the UVP were detrital aggregates. In nearshore surface waters near river inputs, size distributions revealed small size classes (< 100 µm) to be most influential. At the shelf break, size distributions revealed a dual peak in both small (< 100 µm) and very large (> 2 mm) size classes. This study highlights the importance of lithogenic inputs from a mountainous margin to the coastal ocean and their potential to enhance sinking of biological material produced at the shelf break.

1. Introduction 1.1. Continental margins’ role in carbon cycling Continental margins play a crucial role in global ocean carbon cycling through the production, transport and export of particulate matter. High primary production combined with shallow depth enhances the export of biogenic particles to the seafloor in shelf environments, where an estimated 40% of all ocean carbon sequestration occurs (< 2000 m depths) (Muller-Karger et al., 2005; Premuzic et al., 1982). Terrigenous particles from rivers, which exceed 13.5 × 109 tons of suspended particulate matter annually, account for a majority of the lithogenic particles found in the global ocean (Dunne et al., 2007; Milliman and Meade, 1983). These biologically- and terrestriallyderived particles can be buried in shelf sediments or transported offshore. Globally 15–50% of particulate material initially deposited in the coastal ocean is resuspended and redeposited over continental shelves (Hedges and Keil, 1995). Particulate matter from the shelf can be transported long distances offshore, as evidenced by a bloom 900 km offshore in the Sub-Arctic Pacific triggered by continentally-

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sourced iron-rich particles (Lam et al., 2006; Tsunogai et al., 1999). Because the preservation of organic carbon in sediments is enhanced by the protective association with mineral surfaces (Keil et al., 1994) and correlated with sediment grain size and surface area (Hedges and Keil, 1995), high sedimentation rates can increase the burial of organic carbon on continental margins. Thus the biological carbon pump may be strengthened in continental shelf environments with both high seasonal primary production and high inputs of terrigenous material. The northern Gulf of Alaska (nGOA) system is an example of a shelf where multiple complex processes affect the distribution and fate, and influence of particulate matter. Particle dynamics are central to the functioning of the nGOA ecosystem, from the supply of sediments from rivers and glaciers to primary production and zooplankton activity offshore. Waves, tides, currents, and eddies actively transport sediments along and across the shelf. 1.2. Riverine and glacial inputs As a high-latitude, high precipitation region surrounded by glaciated coastal mountains, the nGOA sees a high supply of sediments

Corresponding author at: Virginia Institute of Marine Science, 1375 Greate Road, P.O. Box 1346, Gloucester Point, VA 23062-1346, United States. E-mail addresses: [email protected] (J.S. Turner), [email protected] (J.L. Pretty), [email protected] (A.M.P. McDonnell).

http://dx.doi.org/10.1016/j.csr.2017.07.002 Received 3 June 2016; Received in revised form 4 July 2017; Accepted 5 July 2017 Available online 14 July 2017 0278-4343/ © 2017 Elsevier Ltd. All rights reserved.

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extremely variable in their composition and behavior. For example, in Glacier Bay, flocs sized 0.63–5 mm had highly variable densities and sinking rates, as well as highly variable proportions of organic and inorganic material (Hill et al., 1998). Due to hydrodynamic sorting within these estuarine environments, only the finer flocs are transported out onto the shelf, where they can aggregate with other particles in the more saline environment (Bianchi et al., 2002). These flocs, composed of both terrigenous and biogenous substances, are dispersed by currents and other turbulent forces when they reach the coastal ocean (Sholkovitz, 1976). When aggregates are dispersed, some of the organic material bound as particles is preserved within the estuary and some is transported offshore. For example, in North Dawes Inlet in Southeast Alaska, about half of all total organic carbon was deposited within the glacial estuary while the other half was transported to the coastal ocean (Loder and Hood, 1972). Over the GOA shelf, sediment organic carbon concentrations are relatively low (1% by mass), and stable isotope data suggest that inner-shelf sediment organic carbon is terrestrially-sourced while outer-shelf sediment organic carbon is marine in composition (Jaeger and Nittrouer 2006; Ding et al. in revision).

from rivers due to fast erosion rates and glacial scour. Glacial scouring leads to fine grained sediment input from both inland and tidewater glaciers (Burbank, 1974; Powell and Molnia, 1989). In glacial fjords, high sediment inputs create hyperpycnal flows due to the density of the saturated meltwater (Mulder et al., 2003), such as in the Columbia Glacier fjord in Prince William Sound (Boldt, 2014). The Copper River, the largest river flowing into the nGOA, drains a large, mountainous watershed area of 62,678 km2, producing a discharge of 56 km3 annually, 69% of which comes from glacial sources (Neal et al., 2010). Sediment deposition rates offshore of the Copper River are greater than 20 mm/year (Jaeger et al., 1998; Jaeger and Nittrouer 2006). Aside from the Copper River, the nGOA has few large point source inputs of freshwater and terrigenous particles, which are instead supplied via diffuse small mountain streams (Royer, 1982). These types of inputs are less predictable in the timing of their discharge than large river inputs, due to the small buffering capacities of the steep, mountainous watersheds (Milliman and Syvitski, 1992; Wheatcroft et al., 2010). A strong relationship between salinity and turbidity has been measured in coastal Alaskan environments and attributed to glacial silt-laden river runoff (Curran et al., 2004; Pickard, 1967). Compositionally, higher concentrations of aluminosilicates than carbon in suspended particulate matter in the nGOA indicate coastal rivers as the source of most particles (Feely et al., 1981, 1979). This fresh, sediment laden water is characteristic of the ACC, which stays very close to the coast when driven by inputs of freshwater (Royer, 1982). This pattern is most pronounced in the summer during times of high freshwater discharge, as seen in the extreme offshore-to-nearshore gradient in dissolved aluminum (Brown et al., 2010).

1.4. Mechanisms for particle transport Particle transport in the nGOA is controlled in part by sinking fluxes, resuspension, and lateral advection. Particle sinking fluxes drive the global biological carbon pump, which sequesters atmospheric carbon dioxide and regulates climate (Broecker, 1982; Siegenthaler and Wenk, 1984; Volk and Hoffert, 1985), Sinking rates of depend on particle concentration, diameter, density, and morphology. Inorganic constituents such as silicate and calcium carbonate planktonic skeletons, as well as lithogenic clays, silts and sands have a ballasting effect on organic particles, accelerating their descent and limiting time available for remineralization (Armstrong et al., 2001; Klaas and Archer, 2002; Ittekkot, 1993; Iversen and Ploug, 2010). Resuspension of bottom sediments in the nGOA is mediated by waves, currents and tides. In past studies, turbid plumes were found at bottom depths over the nGOA shelf with trace element distributions matching bottom sediments caused by resuspension (Feely et al., 1979, 1981; Hampton et al., 1987). Large storm waves can penetrate to bottom depths over the shelf, as evidenced by eroded sediments on shallow banks of the Kodiak Shelf (Evans et al., 2000) Suspended particulate matter is advected laterally along the shelf by the Alaska Coastal Current (ACC) and Alaskan Stream, and advected across the shelf by anticyclonic eddies and tidal mixing. Tides account for up 20–80% of all energy on the nGOA shelf, and energy is highest in constrained locations such as narrow channels and inlets (Burbank, 1974; Ladd et al., 2005). For example, Cook Inlet is home to some of the largest tide ranges in the world, reaching up to 11.4 m fluctuations at spring tide (Bartsch-Winkler and Ovenshine, 1984), and tidal velocities in Cross Sound can reach 2 m/s (Weingartner et al., 2009). The nGOA shelf is riddled with submarine canyons (Carlson et al., 1982), in which tidal mixing is enhanced by the shelf topography (Ladd et al., 2005). Past studies have investigated marine particles in the nGOA through geological surveys, water sampling, and sediment traps (Boyd et al., 2005; Burbank, 1974; Feely et al., 1981, 1979; Feely and Massoth, 1982; Landing and Feely, 1981; Takahashi et al., 2002; Wong et al., 1999). It is clear from this past work in the nGOA that abiotic and biotic factors both drive particle dynamics in this region. In this study, we utilize modern optical instrumentation to examine the validity of the hypothesis that physical processes drive particle dynamics nearshore while biological processes drive particle dynamics offshore. We use observations to qualitatively examine the patterns of particle size distributions with respect to depth and distance from shore.

1.3. Primary production and particle aggregation The GOA is characterized by iron limitation in the offshore environment contrasting with nitrate limitation over the shelf (Martin and Gordon, 1988). Most of the reactive iron, the fraction considered potentially available to biology, is ultimately sourced from glacial streams (Schroth et al., 2014, 2011). The particulate fraction of this micronutrient, along with the dissolved fraction, is an important component of the iron available to phytoplankton in the GOA, and the particulate fraction is especially important in the summer season (Aguilar-Islas et al., 2015; Lippiatt et al., 2010; Wu et al., 2009). The nGOA experiences seasonal primary production in the form of a high-magnitude spring bloom of large celled phytoplankton and a smaller-magnitude fall bloom (Napp, 1996; Sambrotto and Lorenzen, 1987). The highest primary productivity (200–600 mg C/m2/d) occurs over the shelf break, with lower productivity nearshore due to nitrate limitation and light-limiting turbidity in freshwater runoff (Parsons, 1987; Larrance et al., 1977; Stabeno et al., 2004; Jaeger and Nittrouer, 2006). Phytoplankton production drives the creation and export of organic particles, as those cells aggregate, sink, and are grazed by zooplankton. Primary production over the shelf and in the offshore environment supports a large biomass of zooplankton (Coyle and Pinchuk, 2005, 2003). Zooplankton fragment large particles through grazing and sloppy feeding, and repackage organic material into dense, quickly sinking fecal pellets (Turner and Ferrante, 1979). For example, along the Seward Line, large Neocalanus spp. copepods selectively graze large particles and large-celled phytoplankton in the upper water column (Dagg et al., 2009), and microzooplankton also consume a large biomass of the annual phytoplankton bloom (Strom et al., 2001). Flocculation occurs in the estuaries and fjords where rivers and tidewater glaciers meet the coast. Small particles aggregate into larger particles, by way of physical shear or differential sinking velocities, aided in some cases by biological exopolymeric substances and moderate turbulence (Burd and Jackson, 2009; Elimelech et al., 1995; Freidlander, 2000; McCave, 1984). In the nGOA, aggregates likely include a combination of terrigenous clays and silts, phytoplankton cells, fecal pellets, and other organic detritus. These flocs are 14

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2. Methods We used two instruments that targeted overlapping size spectra to measure particulate matter belonging to a wide range of size classes (2.5 µm–27 mm). Data from both the UVP and the LISSTDEEP were binned into 5-m depth bins at all stations. The UVP imaged 0.88 L water parcels as it descended through the water column, with each image capturing all the particles 0.06–27 mm in equivalent spherical diameter (ESD) within each parcel. All cruises used mixed processing mode, which counts and sizes particles while also saving JPEG images of the largest particles > 0.5 mm ESD (Picheral et al., 2010). This instrument had both a fine enough resolution to detect small particles and a large enough sampling volume and frequency to count larger objects with statistical confidence (Picheral et al., 2010; Stemmann et al., 2008). UVP numerical particle data were downloaded and processed using Zooprocess software, a macro plugin for Java-based image-processing program ImageJ. Only images from downcasts were used in order to minimize the hydrodynamic influences on the imaged particles. The projected area of each particle was calculated by converting the area in pixels to the area in mm2 using the power law relationship described by Picheral et al. (2010). Thus mean particle size was initially calculated as a surface area (mm2), which was converted to ESD assuming spheres. The UVP generated particle size distributions in terms of a count per volume of water photographed (#/l). These values were then converted to a volumetric concentration (µl/l) using the median ESD (mm) of each logarithmically-spaced size bin and assuming spherical shape using the following equation:

V = N*

Fig. 1. Stations sampled with both the UVP and LISST-DEEP in 2015. Names given to transects are used throughout the text to refer to those respective groups of stations. [2column figure].

3. Results and discussion 3.1. Particle concentrations Together, the UVP and LISST-DEEP data revealed strong spatial variability in particle concentration and size distributions throughout the Gulf of Alaska. Overall, particle concentrations were highest (> 1000 µl/l) at stations closest to large inputs of freshwater and glacial discharge and just offshore of the shelf break, intermediate (100– 1000 µl/l) at some nearshore stations and at the shelf break, and lowest (< 100 µl/l) over the mid-shelf (Fig. 2). Particle concentrations were highest (> 1000 µl/l) in Cross Sound, Hinchinbrook Entrance, and western Cook Inlet at all depths (Fig. 3B). These stations are located close to large glacial and fluvial inputs of freshwater from land, implying that the high concentrations are driven by lithogenic inputs. Particle concentrations were similarly high offshore of the shelf break on the Seward Line from the surface down to 200 m depths (Fig. 3D). Since the highest concentrations on the Seward Line were found > 200 km from shore, isolated from nearshore terrestrial inputs, the high particle concentrations here can be attributed to biological production. Moderately high particle concentrations (100–1000 µl/l) were measured at stations closest to shore and at the shelf break on the Shelikof Strait line, the Seward Line, and the Glacier Bay line down to ~ 40 m depths (Fig. 3A, D, G). A nearshore station of the Glacier Bay line also showed moderately high particle concentrations at depths below 100 m (Fig. 3G), possible evidence of lithogenic inputs sinking to depth in a hyperpycnal plume. At all stations near the outflow of the Copper River, a surface plume of moderately high particle concentration was measured down to ~ 20 m depths (Fig. 3F).

3 4 ⎛d ⎞ π⎜ ⎟ 3 ⎝2⎠

For each UVP size class, V is the particle volume concentration (µl/ l), N is the number of particles counted in each liter of water (#/l), and d is the median diameter of the respective size bin. The LISST-DEEP enabled the assessment of the particle size distribution between 2.5 and 500 µm using the laser diffraction method based on the scattering intensity over multiple angles with a ring-type detector (Agrawal and Pottsmith, 2000). The multi-angle scattering is converted to a size distribution using a mathematical inversion, as per Hirleman (1987). LISST-DEEP data were downloaded and processed using software developed by Sequoia Scientific, Inc., generating a particle volume concentration for each size class (µl/l). UVP and LISST-DEEP datasets were combined into one unified size distribution to encompass all size classes measured. Initially, raw data from the LISST-DEEP showed anomalously high concentrations of particles > 100 µm in diameter. LISST type-c instruments are known to be less accurate for particles greater than 250 µm in diameter because particles larger than the detection limit (500 µm) may alias in the largest size ranges, overestimating the abundance of the largest detectable particles (100–500 µm) (Davies et al., 2012). To correct for this artifact, size bins > 100 µm were removed from the LISST dataset. The three remaining overlapping size bins from each instrument between 60 and 100 µm were averaged to unify the datasets. The mean difference in volume concentration for the three size bins before averaging was 0.08 µl/l (+/- 0.57), two orders of magnitude below the scale of variance visible in our results with regard to depth and distance from shore. Despite inherent differences in the two optical methods used, the unified dataset is robust in its range of detailed size distributions of particles from 2.5 µm −27 mm in diameter. For deployment, the UVP and LISST-DEEP were incorporated directly into the CTD rosette package. Particle concentrations and size distributions were collected in July-August 2015 throughout the coastal GOA on the NOAA Ocean Acidification (GOA-OA) cruise (Fig. 1).

Fig. 2. Total particle volume concentration (PVC) (µl/l) as the sum of concentrations in all depth bins at each station measured with both instruments. [2-column figure]. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article).

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Fig. 3. Particle volume concentrations(PVC) (µl/l) across seven transects over the GOA shelf. Cross Sound and Hinchinbrook Entrance are excluded due to concentrations higher than the range shown and little variation with depth. White dotted lines indicate locations of rosette casts. Red markers on map indicate locations of the start of each transect, which correspond to the left side of each subplot (0 km). [2-column figure]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).

pycnal plume of low-density freshwater containing fine eroded material from the Copper River watershed. Offshore, past the shelf break on the Seward Line, small size classes (10–100 µm) contributed greatly to volume concentration, but here the pattern extended down to 200 m depths (Fig. 5C). This feature of small particle size isolated from land inputs likely indicates phytoplankton production. Seward Line shelf break size distributions also showed increased concentrations of the very largest particle sizes (> 2 mm) (Fig. 4D). Lastly, in Cross Sound the size classes contributing most to volume concentrations were relatively large (50 µm–1 mm) and highly concentrated deeper in the water column (15–150 m) (Figs. 4I, 5D). Though lower in magnitude, this trend was seen in the Glacier Bay line as well (Fig. 4H). The shape of the particle size distribution in Cross Sound and the Glacier Bay line was unique to the dataset and may represent a region of particle aggregation and flocculation. A possibility exists that Cross Sound size distributions were a product of the limitations of the UVP in turbid waters (see Methodological Limitations). Images of large (> 0.5 mm) particles photographed by the UVP revealed that the vast majority of large objects were floc-like amorphous detrital aggregates, especially nearshore (Fig. 6). The

3.2. Particle size distributions Overall, large size classes > 100 µm made the greatest contribution to particle volume concentrations (Fig. 4). A decrease in the contributions of size classes 10–80 µm was prevalent at stations where total concentrations were low, such, in Kennedy Entrance and Montague Strait (Fig. 4C, E), and over the mid-shelf portion of the cross-shelf lines such as the Seward Line (Fig. 4D). The areas with the highest particle concentrations also showed anomalous slopes in their size distributions. For example, western Cook Inlet was characterized by a much different slope in the size distribution, with small particles < 100 µm contributing more to particle volume concentrations than at any other station. (Figs. 4B, 5A). This can be attributed to the physical circulation of Cook Inlet and the riverine inputs and tidal resuspension of terrigenous particles within the Inlet. Coastal waters enter the inlet on the east side of the entrance, move in a counter-clockwise manner through the inlet, and exit into the Gulf of Alaska on the western side of the entrance. Further to the east, the size distributions measured near the mouth of the Copper River also showed enhanced concentrations of small particles (< 200 µm) at near-surface depths (0–20 m) (Fig. 5B). This is evidence of a hypo16

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Fig. 4. Size distributions of particle volume concentrations (PVC) of each size class in equivalent spherical diameter (ESD) (mm) as the sum of concentrations at all depths at each station. The volume concentration of each size class of particles was normalized to the width of the respective size bin (mm), resulting in a normalized volumetric size distribution (µl/l/ mm). Line color scheme indicates distance from the start of each transect (km). Red markers on map indicate locations of the start of each transect, which correspond to the red line (0 km) in each subplot. [2-column figure]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).

Flocculation in the coastal ocean is mainly driven by organic matter (Eisma, 1986). Sticky organic substances such as EPS mediate the aggregation of mineral grains, increasing particle size and resistance to fragmentation (Decho, 1990; Fettweis et al., 2014). In the coastal ocean, where high concentrations of mineral particles coincide with the EPS created by phytoplankton, sinking velocities of particles can be greatly increased (Maerz et al., 2016).

abundance of this type of image suggests that even particles too small to be photographed in detail (< 0.5 mm) would likely be dominated by detrital aggregates. In contrast, shelf-break Seward Line stations contained the highest abundance of large zooplankton images in the dataset. Because these zooplankton are so much larger in diameter than the other particles imaged by the camera, the resulting volume concentrations for size classes > 2 mm can be easily biased by a few (1–10) large organisms. This bias may have contributed to the high volume concentrations of the largest (> 2 mm) size classes measured past the shelf break on the Seward Line (Figs. 4D, 5C). Detrital aggregates still dominated the UVP images of large objects, with zooplankton only making a minor (~ 5%) contribution. Detrital aggregates photographed by the UVP varied in shape, size, and greyscale, visually containing diverse primary particles (Fig. 6). Photographed particles’ large size (1–6 mm) was likely the result of flocculation via the presence of extracellular polymeric substances (EPS) which act as a biological glue.

3.3. Implications Two separate maxima in particle concentrations nearshore and at the shelf break suggest that carbon cycling in this region is complex. Nearshore, high particle concentrations are most likely sourced from freshwater runoff from rivers and glaciers. At the shelf break, high particle concentrations far from land are most likely biologically derived from phytoplankton production, zooplankton activity, and subsequent sinking. 17

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Fig. 5. Size distributions of particle volume concentration (PVC) (µl/l/mm) with depth (m) for all size classes (mm) at four stations with high total particle concentrations, including western Cook Inlet (A), near the mouth of the Copper River (B), past the shelf break on the Seward Line (C), and mid-channel in Cross Sound (D). [2-column figure]. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article).

could augment sinking rates of organic matter in the nGOA. At the same time, high energy transport mechanisms in this environment can advect particles before they settle, allowing lithogenic particles from the continental margin to transport particulate iron offshore. In the subarctic Pacific, particulate iron is sourced from land as resuspended shelf sediments, eddies travelling offshore, and dust plumes from large uncovered braided rivers (Crusius et al., 2011; Lippiatt et al., 2010; Lam and Bishop, 2008; Lam et al., 2006). I Iron-rich terrigenous particles have been known to travel hundreds of kilometers to the central GOA (Lam and Bishop, 2008; Lam et al., 2006). This longdistance transport would decrease the amount of organic carbon settling to the inner shelf while enhancing the high biological production seen offshore in our results. 3.4. Methodological limitations The two optical instruments used in this study are fundamentally different in their measurement properties. The data generated by the UVP, a camera system that photographically counts individual particles, are therefore fundamentally different than the data generated by the LISST-DEEP, a laser that calculates turbidity based on the backscattering of light within seawater. Inherent error may result from combining data from instruments that “see” particles in such different ways. This study is also limited by the sole use of optics. Without compositional data, it is not possible to determine the origin or biogeochemical implications for the particles measured using optical instruments. We recommend that future studies pair optical measurements with sediment traps or filtration systems to physically collect particles. Larger uncertainties may be present in highly turbid waters. Fine particulate matter driving the elevated concentrations in the northeastern gulf was seen in the background of some of the large-object

Fig. 6. Examples of aggregates photographed by the UVP at stations nearshore on the Seward Line (A-D), in Hinchinbrook Entrance (E), and near Glacier Bay (F). Scale bar indicates 5 mm. [1-column figure].

Large terrigenous sediment loads may increase sinking fluxes as lithogenic particles increase the density overall of aggregates (Hamm, 2002; Ittekkot, 1993; Ittekkot et al., 1992). Based on quantity alone, glacially-sourced particles and other suspended lithogenic particles 18

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images (> 0.5 mm) generated by the UVP. Images from nearshore stations at depths of maximum particle concentration had noisy backgrounds full of small particles, showing the extreme turbidity of these waters (Fig. 6E, F). The fact that turbidity is visible in background of the saved large-object images highlights a possible caveat of using the UVP in environments with high suspended loads. With the UVP, particle concentrations could actually be underestimated in these environments if multiple particles overlap one another in the field of view. The UVP could also overestimate particle diameters by counting smaller disaggregated particles as one larger particle if the camera field of view is supersaturated. With LISST instruments, extreme turbidity can cause an overestimation of concentration, due to the extreme scattering of light caused by the suspended matter with high refractive index mineral particles (Stramski et al., 2004). This overestimation was present in our raw data in LISST size classes > 100 µm, which were removed from the dataset (see Section 2). In turbid environments, particle size distributions should be interpreted with care. In these extremely turbid waters, separating out the biological signals from the physical signals in the water column was challenging given the high concentrations from rivers and resuspended bottom sediments. Although primary production was likely contributing to the particle dynamics over the shelf at the time of sampling, our data and methodology do not allow us to separate out biological contributions. 4. Conclusions We have mapped particle concentrations and size distributions using optical instrumentation, revealing detailed size distributions of terrestrial sediment inputs nearshore and biological production at the shelf break. Questions remain after this study about the ultimate supply, transport, and fate of particles in this region. Our results highlight the need to use compositional data to investigate the origin and biogeochemical implications of the particulate matter. A shortage of data still exists for many of the physical processes surrounding the nGOA. Most of the rivers that drain into the nGOA have undocumented sediment loads due to their small size and inaccessibility. Our results show that the numerous small rivers and glaciers may be important drivers of nearshore particle dynamics, and that these inputs can be studied from the marine perspective. Finally, this work illustrates the importance of the processes that drive particle distributions in a system affected by large-scale climatic change in the present and future. Rapidly melting glaciers could contribute even greater loads of sediments to the nGOA and the rivers flowing into it. Earlier spring melt seasons could change the timing of particle inputs from land and the timing of biological production across the shelf. Overall, climate warming could increase the magnitude of suspended sediment loads and change the relative contribution of the driving processes of particle dynamics in years to come. Acknowledgements We thank the Captain and crew of the NOAA ship Ronald H. Brown for their help collecting data in the Gulf of Alaska. We thank the anonymous reviewers of this manuscript for their thoughtful suggestions. We also thank Dr. Ana Aguilar Islas and Dr. Mark Johnson for their helpful editing of this document in its early stages. The instrumentation for this project was funded in part by the M.J. Murdock Charitable Trust. This project was partially funded through the Northern Gulf of Alaska Applied Research Award and the NSF OCE Award Numbers 1459835 and 1421118. References Agrawal, Y.C., Pottsmith, H.C., 2000. Instruments for particle size and settling velocity observations in sediment transport. Mar. Geol. 168, 89–114. http://dx.doi.org/ 10.1016/S0025-3227(00)00044-X.

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