Pierre Larouche

Advanced recruitment and accelerated population development in Arctic calanoid copepods of the North Water

Abstract The timing of copepodite recruitment and population development of copepods in spring and early summer (April–July) were compared between the North Water polynya and Barrow Strait, a non-polynya region in the Canadian Archipelago. In the North Water, young copepodites (CI–CIII) of calanoid herbivores were concentrated in the cold and chlorophyll-rich water at the base of the Arctic surface layer, while later stages (CIV–CV) invaded the warmer surface layer. The phytoplankton bloom and the recruitment of the first cohort of copepodites of Calanus hyperboreus , C. glacialis , and Pseudocalanus spp started in May–June, some 1.5–3 months earlier than in Barrow Strait. Consistent with a precocious summer recruitment, population stage structure of these species in early spring (April–May) was more advanced in the North Water than in Barrow Strait. The recruitment in June of CI of the omnivore Metridia longa was advanced by at least 5 weeks in the polynya relative to Barrow Strait. We found no evidence for an acceleration of the population development of the small Microcalanus pygmaeus , Oithona similis or Oncaea borealis in the polynya. Once the recruitment of young copepodites had started, recruitment success (i.e. % of young copepodites in the population) increased primarily with Chl a concentration for C. hyperboreus , with both sea-surface temperature and Chl a for C. glacialis , and with temperature only for Pseudocalanus spp. Hence, depending on the species, both greater food availability and higher temperature resulting from reduced ice cover contributed to improve reproductive success in herbivorous copepods in the North Water relative to Barrow Strait. A climate-induced reduction of ice cover duration is predicted to favour the population growth of the predominant large calanoid copepods and Pseudocalanus on Arctic shelves.

Evaluation of ocean color algorithms in the southeastern Beaufort Sea, Canadian Arctic: New parameterization using SeaWiFS, MODIS, and MERIS spectral bands

With the increasing interest for Arctic Ocean resources and faced with its sensitivity to climate change, it is important to accurately monitor the chlorophyll-a (Chla) concentration that is a key indicator of phytoplankton biomass and marine productivity. The performances of three operational algorithms (OC4v6, OC3Mv6, OC4Mev6), two Arctic adapted algorithms (OC4L, OC4P), and one semi-analytical (GSM01) algorithm to estimate Chla were evaluated using in situ measurements collected in the southeastern Beaufort Sea. All evaluated algorithms clearly overestimated Chla by a factor of three to five. A high contribution of colored dissolved organic matter and nonalgal particles to the blue light absorption appeared as the source of that poor performance. It was also found that fluorometrically measured Chla were two times greater than those determined from high-performance liquid chromatography, contributing to the observed discrepancies between our findings and previous studies carried out in the Arctic. We propose regionally adapted and new algorithms allowing accurate estimation of Chla in the southeastern Beaufort Sea. Finally, a matchup analysis of coincident in situ data and satellite overpass showed that the normalized water-leaving reflectance and the blue-to-green ratio retrieval were more accurate for SeaWiFS than for MODIS and MERIS.

Phytoplankton pigment in the Gulf of St. Lawrence, Canada, as determined by the Coastal Zone Color Scanner—Part I: spatio-temporal variability

Abstract This study describes for the first time the spatial structure of surface phytoplankton pigments (chlorophyll a plus phaeophytin) in the Gulf of St. Lawrence (GSL), Canada, based on 80 Coastal Zone Color Scanner (CZCS) images taken between April 1979 and September 1981. Pigment concentrations estimated from the satellite images were validated using available concurrent in situ data. In general, broad spatial patterns in the production of the GSL inferred from shipboard measurements, were confirmed. The spatial distribution matched major physical features of the Gulf, with the highest pigment levels in the estuarine region in the northwestern Gulf and the lowest in the more marine eastern Gulf. The CZCS imagery revealed strong year to year variations in the abundance and distribution of pigments, comparable in magnitude to the seasonal variations. Furthermore, the seasonal cycle was dominated by pigment increases in late summer-early fall, not in the spring as is generally expected. Changes in the productivity of the Anticosti Gyre in the northwest Gulf, an area generally considered to be oligotrophic, seemed to dominate the seasonal and interannual signal. These variations could be related to changes in the balance between wind and buoyancy forcing on the circulation

Factors affecting fast‐ice consolidation in Southeastern Hudson Bay, Canada

Abstract Analysis of satellite images of southeastern Hudson Bay taken over aperiod of 13 years led to the classification of ice distribution into three categories. The first category is for complete fast‐ice cover of the area, the second for fast ice covering only half the area and the third for the absence of fast ice extending away from the coast. Of the three factors considered‐ wind, water circulation and air temperature—the occurrence of strong southwesterly winds during the freezing period is probably the main factor regulating the extent of the fast‐ice cover for the first two categories. Through melting action, above‐freezing air temperatures appear to prevent the consolidation of ice into a solid cover giving rise to the rare third category of ice distribution.

Phytoplankton pigment in the Gulf of St. Lawrence, Canada, as determined by the Coastal Zone Color Scanner—Part II: multivariate analysis

Abstract We report here on a statistical study of physical-biological interactions in the Gulf of St. Lawrence that uses for the first time ocean color images (phytoplankton pigments) and data on runoff and wind. Based on a Monte Carlo test for statistical significance, we extracted four orthogonal (independent) spatial patterns (Empirical Orthogonal Functions, EOF) in pigments that explain 64% of the total variance. We also computed four EOFs from the wind data that explain 90% of the total variance. Based on multiple correlations among these EOFs and runoff anomalies, we derived two modes of physical-biological variability. The first mode is the dual regulation of production by runoff in the Lower St. Lawrence Estuary (LSLE) and the western Gulf and by alongshore wind stress in the northeastern Gulf. The second mode incorporates mesoscale circulation features (eddies, meanders) that respond to low frequency fluctuations in runoff (in the lower estuary) or wind (Gaspe Current, northwestern Gulf, northern Esquiman Channel). The third mode reflects the impact of seasonal wind regimes on pigment levels in the LSLE, the Gaspe Current, and the southwestern Gulf. The fourth mode is dominated by one coccolithophore bloom event in August 1979. The analysis also gives some insight into sources of interannual variability in pigment levels and distributions. The spring bloom does not dominate the seasonal pigment cycle; only the third pigment EOF (5% of the variation) displays a spring peak stronger than the fall peak. The seasonal cycle of pigments is in part linked to that of runoff, driven mostly by year to year differences in the spring freshet. However, year to year differences in the summer wind and runoff regimes also play a role. This suggests that late summer and fall blooms and the physical factors that regulate them deserve more study.

Characterization and variability of particle size distributions in Hudson Bay, Canada

Particle size distribution (PSD) plays a significant role in many aspects of aquatic ecosystems, including phytoplankton dynamics, sediment fluxes, and optical scattering from particulates. As of yet, little is known on the variability of particle size distribution in marine ecosystems. In this study, we investigated the PSD properties and variability in Hudson Bay based on measurements from a laser diffractometer (LISST-100X Type-B) in concert with biogeochemical parameters collected during summer 2010. Results show that most power-law fitted PSD slopes ranged from 2.5 to 4.5, covering nearly the entire range observed for natural waters. Offshore waters showed a predominance of smaller particles while near the coast, the effect of riverine inputs on PSD were apparent. Particulate inorganic matter contributed more to total suspended matter in coastal waters leading to lower PSD slopes than offshore. The depth distribution of PSD slopes shows that larger particles were associated with the pycnocline. Below the pycnocline, smaller particles dominated the spectra. A comparison between a PSD slope-based method to derive phytoplankton size class (PSC) and pigment-based derived PSC showed the two methods agreed relatively well. This study provides valuable baseline information on particle size properties and phytoplankton composition estimates in a sub-arctic environment subject to rapid environmental change.

Changes in the under‐ice characteristics of La Grande Rivière plume due to discharge variations*

Abstract As a result of hydroelectric development on La Grande Riviere (James Bay), the areal extent of the under‐ice plume is now over three times larger than for natural runoff levels. Reduced turbulence under the continuous landfast ice zone allows for plume dilution of the James Bay surface waters over 100 km north of the river at February 1984 discharge levels. Offshore of the 20–25 km wide landfast ice zone, the upper water column is generally well mixed. This occurs because of wind‐generated turbulence and salt rejection during ice formation in the open lead areas formed intermittently by the offshore transport of ice floes. Further increases in the midwinter discharge will lead to dilution of the near‐shore surface waters in southeast Hudson Bay.

Beam attenuation, scattering and backscattering of marine particles in relation to particle size distribution and composition in Hudson Bay (Canada)

This study investigated the relationships between the concentration of biogeochemical parameters and particulate beam attenuation (cp), scattering (bp), and backscattering (bbp) in Hudson Bay. Results showed that most of the variability resulted from the presence of a deep chlorophyll maximum. cp, bp, and bbp were all adequate proxies to estimate total suspended matter (TSM) but were mostly sensitive to particulate inorganic matter (PIM) in the surface layer, and particulate organic matter (POM) at the chlorophyll maximum depth. The backscattering ratio b∼bp varied in the range of 0.005–0.05 and was inversely related to the POM : TSM ratio. According to the Twardowski et al. (2001) model, the PSD slope ξ well represented b∼bp and bulk refractive index n¯p in relation to particulate composition. For inorganic particulate dominated waters, both b∼bp and n¯p had a larger range and a higher mean value than at organic particulate dominated waters. This knowledge on the optical properties related to the PSD and particulate composition provides valuable information for further investigation and broadens our understanding of ocean optics in high latitude waters leading to potential improvements of regional scale remote sensing algorithms.

Thermal Fronts Atlas of Canadian Coastal Waters

Abstract Oceanic fronts are often associated with enhanced biological activity. Depending on their generation mechanism, they are often linked to specific geographical areas. Here we use 25 years of high-resolution satellite sea surface temperature (SST) daily images to generate maps of SST fronts over Canadian coastal waters. Results show that fronts are ubiquitous features, but some fronts are more persistent than others. We confirmed the location of previously known major fronts, but some new persistent frontal areas were also detected as a result of the use of high-resolution (1.1 km) data and a methodology adapted to detect smaller-scale frontal features. Results also show that some of the frontal areas are associated with enhanced phytoplankton biomass or higher trophic level organisms (whales and birds) confirming the ecological importance of this physical process.

Regional algorithms for remote-sensing estimates of total suspended matter in the Beaufort Sea

The large and variable riverine inflow to Arctic continental shelves strongly influences their chemical, biological, and optical properties. The Beaufort Sea receives the largest amount of suspended sediments amongst all Arctic shelves, with sediment-laden Mackenzie river waters strongly influencing bio-optical properties on the shelf. Here, we developed two regional algorithms for the estimation of total suspended matter (TSM) concentration using Medium Resolution Imaging Spectrometer (MERIS) spectral bands, based on in situ optical and suspended particulate data collected in the summer during the Canadian Arctic Shelf Exchange Study (CASES) in 2004 and during the Arctic Coastal Ecosystem Study (ACES) in 2010. The band ratio (where R rs is remote-sensing reflectance) R rs,560/R rs,490 was best correlated with low TSM concentrations (less than 3.0 g m−3), while higher TSM concentrations were well correlated to R rs,681/R rs,560. An empirical piecewise algorithm is thus proposed with the switch between the ratios being triggered by R rs,681/R rs,560 at a threshold value of 0.6. The second algorithm made use of support vector machines (SVMs) as a nonlinear transfer function between TSM concentrations and remote-sensing reflectance ratios R rs,681/R rs,560, R rs,665/R rs,560, and R rs,560/R rs,490. Results show that both algorithms perform better (31% and 25%, respectively) than other published TSM algorithms including the MERIS Case 2 water processor (C2R) neural network algorithm in the study area.