Publications

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2015
Jackson, GA, Checkley DM, Dagg M.  2015.  Settling of particles in the upper 100 m of the ocean detected with autonomous profiling floats off California. Deep-Sea Research Part I-Oceanographic Research Papers. 99:75-86.   10.1016/j.dsr.2015.02.001   AbstractWebsite

We have deployed an autonomous profiling float, the SOLOPC, to sample the concentration of particles larger than 100 mu m off the California coast at approximately hourly intervals down to at least 100 m for periods as long as 12 d. We used the data to estimate total aggregate concentrations hourly at 2-m depth intervals, studying the dynamics of particle sedimentation in this difficult-to-sample region. We find that even over time scales of a week, sedimentation is highly variable, with detectable sedimentation events on about one quarter of the days. Most of these observations were along the southwest coast of the United States, a region known for its coastal upwelling and not necessarily representative of more oligotrophic regions. The aggregate settling rates that we estimate, on the order of 50 m d(-1), are consistent with in situ measurements and with rates calculated from coagulation models. The time interval between observations and their vertical resolution constrain the velocities that can be measured. To capture particle settling with velocities less than the 100 m d(-1) that is usually reported for near surface aggregates requires a sampling interval no more than about 0.25 d with a 2 m vertical resolution. This technique provides a powerful new tool to study the dynamics of particles and their sedimentation near the ocean surface, where export starts. (C) 2015 The Authors. Published by Elsevier Ltd.

2013
Petrik, CM, Jackson GA, Checkley JDM.  2013.  Aggregates and their distributions determined from LOPC observations made using an autonomous profiling float. Deep Sea Research Part I: Oceanographic Research Papers.   10.1016/j.dsr.2012.12.009   AbstractWebsite

The vertical flux of particles in the ocean drives the movement of organic carbon to the deep ocean. We have been studying the distribution and flux of these particles using the SOLOPC, a profiling Lagrangian (SOLO) float with a Laser Optical Particle Counter (LOPC). We have been able to distinguish between aggregate-like and zooplankton-like particles with diameters > 2 mm but needed a way to separate the smaller particles into aggregates and zooplankton. Observations included a lognormal-shaped fraction in the normalized volume distribution similar to that observed in results for simulations of particles in the euphotic zone. By fitting a lognormal distribution to the volume spectrum of particles with diameters ≤ 2 mm , we have been successful at making a separation of marine snow material from other, presumably living, particles. The particle volumes derived using the separations are positively correlated with fluorescence, particulate organic carbon, and the volume of larger particles classified as aggregate-like, which supports the conclusion that these particles are truly aggregates, in some cases derived from phytoplankton. The residual volumes (total less the above fit) are highly correlated with the volumes of large, zooplankton-like particles. Downward velocities of the aggregate fraction calculated from time series of particle profiles are consistent with previous estimates of particle settling rates ( 20 – 70 m d − 1 ) . We now have a tool to estimate aggregate distributions, properties, and vertical fluxes in the euphotic zone, including when and where they change.

2008
Checkley, DM, Davis RE, Herman AW, Jackson GA, Beanlands B, Regier LA.  2008.  Assessing plankton and other particles in situ with the SOLOPC. Limnology and Oceanography. 53:2123-2136.   10.4319/lo.2008.53.5_part_2.2123   AbstractWebsite

We combined a Sounding Oceanographic Lagrangian Observer float with a Laser Optical Plankton Counter (LOPC) and a fluorometer to make an autonomous biological profiler, the SOLOPC. The instrument senses plankton and other particles over a size range of 100 mm to 1 cm in profiles to 300 m in depth and sends data ashore via satellite. Objects sensed by the LOPC include aggregates and zooplankton, the larger of which can be distinguished from one another by their transparency. We hypothesized that the diel production of particles and their loss by sinking and grazing are reflected in the change of the particle distribution. We present data from four deployments of the SOLOPC off California. Particle volume was maximal at the base of the surface mixed layer and correlated with chlorophyll a fluorescence. In a 3-d deployment in 2005, particle volume was greatest in the early evening and smallest in the morning, and average particle size increased with depth. Eigenvector analysis of the particle volume distribution as a function of diameter for each of the deployments yielded size peaks characteristic of planktonic crustaceans. Ship-based measurements showed that the abundance of opaque particles of 1.1-1.7 mm equivalent spherical diameter was positively correlated with copepods of this size and simultaneously collected in nets. This relationship was used with SOLOPC data to estimate the distribution of large copepods, which were most abundant beneath the depth of maximal particle flux, estimated from particle size and published sinking rates. Our data are consistent with a model with diel production of particles and their loss by sinking and grazing.

2006
Gonzalez-Quiros, R, Checkley DM.  2006.  Occurrence of fragile particles inferred from optical plankton counters used in situ and to analyze net samples collected simultaneously. Journal of Geophysical Research-Oceans. 111   10.1029/2005jc003084   AbstractWebsite

[1] We hypothesized that the optical plankton counter (OPC) senses particles in situ that are not collected by nets and analyzed in the laboratory. An OPC was deployed in situ between 1998 and 2004 in the mouth of a bongo net with 505-mu m-mesh nets in the upper 210 m at stations in the California Current Region. Here we compare paired data sets from the OPC in situ and the OPC analysis in the laboratory of the simultaneously collected net samples for four seasons of 2 years. We restricted our analysis to particle sizes 1.26 - 6.35 mm equivalent spherical diameter (ESD), a size class shown from the lab OPC data to be retained efficiently by the net. On average, 4 (3) times more particles by number (volume) were sensed in situ by the OPC than sensed in net collections by the OPC in the lab. These values varied an order of magnitude among the eight cruises examined. Time of day, distance offshore, season, year, chlorophyll a concentration, and Brunt-Vaisala frequency each explained significant variation in these differences. The excess of particles sensed in situ over that measured in the net samples was due primarily to smaller particles in the 1.26 - 6.35 mm ESD range. We infer that the particles measured by the OPC in situ but not in the lab were fragile and thus not collected by the net. We hypothesize that these fragile particles are primarily aggregates and abandoned houses of larvaceans.

2004
Johnson, CL, Checkley DM.  2004.  Vertical distribution of diapausing Calanus pacificus (Copepoda) and implications for transport in the California undercurrent. Progress in Oceanography. 62:1-13.   10.1016/j.pocean.2004.08.001   AbstractWebsite

Migration to deep water during diapause may contribute to the retention of several dominant oceanic calanoid copepod populations in eastern boundary current systems, where the mean flow of poleward undercurrents is in opposition to mean equatorward surface flow. The vertical distributions of Calanus pacificus late copepodid stages were measured at a 1200-m deep, open-ocean station in the Southern California Bight on 13 dates between April 2000 and March 2001 using a MOCNESS (multiple opening and closing net and environmental sensing system). Copepod vertical distribution was compared to the vertical position of the California Undercurrent. Diapausing C pacificus were primarily found between 300 and 400 m at the beginning of the diapause season, in June and July, and between 250 and 350 at the end of the diapause season, in November and January. Depth distributions were broader from August to October, ranging from about 350 m to the maximum depth sampled, 1100 m, and the median depth of diapausing C pacificus was deeper, up to 800-900 m, during this period. Maximal depths of diapausing C. pacificus, 1100-1000 m, were greater than have previously been reported. The mean depth of the California Undercurrent was 250 m, and its approximate depth range was 110-430 m. Diapausing C pacificus CV were abundant in the California Undercurrent at the beginning and end of the diapause season, in June to July and late-November to January, suggesting that poleward transport of diapausing copepods in the California Undercurrent contributes to C pacificus Population retention in the California Current System. (C) 2004 Elsevier Ltd. All rights reserved.

1997
Checkley, DM, Ortner PB, Settle LR, Cummings SR.  1997.  A continuous, underway fish egg sampler. Fisheries Oceanography. 6:58-73.   10.1046/j.1365-2419.1997.00030.x   AbstractWebsite

We describe a method to sample the highly contagious distribution of pelagic fish eggs. CUFES, the continuous, underway fish egg sampler, consists of a submersible pump, concentrator, electronics and sample collector. This system operates continuously and under nearly all sea conditions, providing a real-time estimate of the volumetric abundance of pelagic fish eggs at pump depth, usually 3 m. CUFES-derived estimates of volumetric abundance agree well with those from nets towed at pump depth and with areal abundance estimated from vertically integrated plankton tows. CUFES has been used successfully to sample the eggs of menhaden, pinfish, sardine, and anchovy off the coasts of the eastern and western United States and South Africa. Two large patches of eggs of the Atlantic menhaden were sampled off North Carolina in winter 1993-94, had a linear scale of 5-10 km, and were found in waters between the Gulf Stream and mid-shelf front. Spawning location may he related to bathymetry. CUFES is now being used to estimate spawner biomass by the daily egg production method. An optical plankton counter provided accurate estimates of the number of Atlantic menhaden eggs sample by CUFES.

Osgood, KE, Checkley DM.  1997.  Observations of a deep aggregation of Calanus pacificus in the Santa Barbara Basin. Limnology and Oceanography. 42:997-1001. AbstractWebsite

An optical plankton counter/CTD package was used with zooplankton net samples to map the distribution of fifth copepodid (C5) Calanus pacificus in the Santa Barbara Basin region during two autumn cruises. Diapausing C5 C. pacificus were aggregated in a layer just above the basin's oxygen-deficient bottom waters and below its sill depth. The maximal concentration measured was 6,900 ind. m(-3) from a net sample spanning a depth range double the thickness of the C5 layer Although the C5 concentration varied, the layer was found at all stations of sufficient bottom depth within the basin. During November 1994, C5 C. pacificus accounted for 95-97% of all zooplankton caught in net samples from the layer. Relatively low concentrations of deep-dwelling C5 C. pacificus were observed at nearby stations outside the basin. We hypothesize that C5 C. pacificus descend into the Santa Barbara Basin at diapause, are trapped, and accumulate in a region of relatively low predator abundance. The resultant aggregation is estimated to contain a significant fraction of the regional C. pacificus population and thus assumes an important role in its dynamics.

1992
Checkley, DM, Dagg MJ, Uye S.  1992.  Feeding, excretion and egg production by individuals and populations of the marine, planktonic copepods, Acartia spp. and Centropages furcatus. Journal of Plankton Research. 14:71-96.   10.1093/plankt/14.1.71   AbstractWebsite

Diel variations in vertical distribution, gut pigment content, ammonium excretion and egg production were investigated for adult females of Acartia erythraea and A. pacifica in the vertically mixed Inland Sea of Japan and Centropages furcatus in the stratified, neritic Gulf of Mexico. Gut pigment content and egg production rate were maximal at night and ammonium excretion was maximal during the daytime. Neither A. erythraea nor A. pacifica adult females showed an apparent diel migration, but the former were highly concentrated in the sur-face layer during the afternoon. In contrast, C. furcatus adult females showed a clear diel migration, residing immediately above the bottom during the daytime and being concentrated between 10 and 25 m depth during the night-time. Individual-based data on gut content and excretion and egg production rates were combined with vertical-distribution data to calculate population values. In the Inland Sea of Japan, the resultant pattern for Acartia spp. reflected the diel variation in physiological rates and even distribution of adult females, except for the afternoon, surface aggregation of A. erythraea. In the Gulf of Mexico, the pattern for C. furcatus reflected largely the diel variation in each rate process and the heterogeneous distribution of adult females in the water column. Elevated nocturnal feeding activity of these copepods may be due to an endogenous rhythm. The daytime maximum in ammonium excretion and night-time maximum in egg production rate indicated approximate half-day and day time lags, respectively, after the intake of food until its conversion into dissolved excreta and released eggs.