Mati Kahru

Ocean color chlorophyll algorithms for SeaWiFS

A large data set containing coincident in situ chlorophyll and remote sensing reflectance measurements was used to evaluate the accuracy, precision, and suitability of a wide variety of ocean color chlorophyll algorithms for use by SeaWiFS (Sea-viewing Wide Field-of-view Sensor). The radiance-chlorophyll data were assembled from various sources during the SeaWiFS Bio-optical Algorithm Mini-Workshop (SeaBAM) and is composed of 919 stations encompassing chlorophyll concentrations between 0.019 and 32.79 μg L−1. Most of the observations are from Case I nonpolar waters, and ∼20 observations are from more turbid coastal waters. A variety of statistical and graphical criteria were used to evaluate the performances of 2 semianalytic and 15 empirical chlorophyll/pigment algorithms subjected to the SeaBAM data. The empirical algorithms generally performed better than the semianalytic. Cubic polynomial formulations were generally superior to other kinds of equations. Empirical algorithms with increasing complexity (number of coefficients and wavebands), were calibrated to the SeaBAM data, and evaluated to illustrate the relative merits of different formulations. The ocean chlorophyll 2 algorithm (OC2), a modified cubic polynomial (MCP) function which uses Rrs490/Rrs555, well simulates the sigmoidal pattern evident between log-transformed radiance ratios and chlorophyll, and has been chosen as the at-launch SeaWiFS operational chlorophyll a algorithm. Improved performance was obtained using the ocean chlorophyll 4 algorithm (OC4), a four-band (443, 490, 510, 555 nm), maximum band ratio formulation. This maximum band ratio (MBR) is a new approach in empirical ocean color algorithms and has the potential advantage of maintaining the highest possible satellite sensor signal: noise ratio over a 3-orders-of-magnitude range in chlorophyll concentration.

Seasonal and nonseasonal variability of satellite‐derived chlorophyll and colored dissolved organic matter concentration in the California Current

Time series of surface chlorophyll a concentration (Chl) and colored dissolved organic matter (CDOM) derived from the Ocean Color and Temperature Sensor and Sea-Viewing Wide Field-of-View Sensor were evaluated for the California Current area using regional algorithms. Satellite data composited for 8-day periods provide the ability to describe large-scale changes in surface parameters. These changes are difficult to detect based on in situ observations alone that suffer from undersampling the large temporal and spatial variability, especially in Chl. We detected no significant bias in satellite Chl estimates compared with ship-based measurements. The variability in CDOM concentration was significantly smaller than that in Chl, both spatially and temporally. While being subject to large interannual and short-term variations, offshore waters (100–1000 km from the shore) have an annual cycle of Chl and CDOM with a maximum in winter-spring (December-March) and a minimum in late summer. For inshore waters the maximum is more likely in spring (April-May). We detect significant increase in both Chl and CDOM off central and southern California during the La Nina year of 1999. The trend of increasing Chl and CDOM from October 1996 to June 2000 is statistically significant in many areas.

Influence of the 1997–98 El Niño on the surface chlorophyll in the California Current

Satellite-derived time series for the California Current System (CCS) showed marked changes in the surface chlorophyll a concentration (Chl, mg m−3) associated with the 1997–98 El Nino. In addition to the previously known decrease in Chl off Southern California (Fiedler, 1984), we also observed a significant increase of Chl off Baja California. Whereas the extent of eutrophic (Chl >1.0) areas decreased throughout the CCS, the extent of mesotrophic areas (0.2 <Chl <1.0) off Baja California approximately doubled. The reduced area of eutrophic waters is attributed to weakened upwelling but the increase in the offshore mesotrophic area off Baja may be caused by blooms of nitrogen-fixing cyanobacteria. Using revised Coastal Zone Color Scanner data we detected similar changes during the 1982–83 El Nino.

Empirical chlorophyll algorithm and preliminary SeaWiFS validation for the California Current

Anewempirical chlorophyll algorithmisproposedfor SeaWiFS(Sea- viewing WideField-of-viewSensor) and other ocean colour sensors. TheCAL-P6 algorithm uses a sixth-order polynomial of the ratio of normalized water leaving radiances (L W N) at 490nmand 555nmand is based on 348 measurements of L W N and chlorophyll-a in the California Current. Validation of the SeaWiFS-derived chlorophyll values with 27 concurrent in situ measurements showed high correla- tion (r 2 =0.93 in the log± log space) but signi® cant overestimation by SeaWiFS at high chlorophyll-a concentration. The problem was traced to signi® cant under- estimationof theSeaWiFS-derivedL W N (490) athighchlorophyll-a concentration (3± 5mgmO 3 ). Further re® nement of the atmospheric correction is needed for SeaWiFS to attain its goal of 35% accuracy for chlorophyll retrieval in the coastal zone.

Eddies enhance biological production in the Weddell‐Scotia Confluence of the Southern Ocean

Satellite data show that oceanic eddies generated in the Southern Antarctic Circumpolar Current Front (SACCF) are associated with increased phytoplankton biomass. Cyclonic eddies with high chlorophyll a concentration (Chl-a) retain phytoplankton within the eddy cores and increase the light available for photosynthesis in the upper mixed layer by limiting vertical mixing and lifting of the isopycnal surfaces. Anticyclonic eddies have low Chl-a in the core but increased Chl-a in the periphery. Cross-frontal mixing mediated by eddies transports nutrients (e.g., Fe and Si) to the north and contributes to the increased Chl-a in the frontal zone. Interannual variations in the cyclonic eddy activity are positively correlated with variations in Chl-a during the spring bloom in regions of the Antarctic Circumpolar Current around South Georgia.

Is there a decline in marine phytoplankton

Arising from D. G. Boyce, M. R. Lewis & B. Worm 466, 591–596 (2010)10.1038/nature09268; Boyce et al. replyPhytoplankton account for approximately 50% of global primary production, form the trophic base of nearly all marine ecosystems, are fundamental in trophic energy transfer and have key roles in climate regulation, carbon sequestration and oxygen production. Boyce et al. compiled a chlorophyll index by combining in situ chlorophyll and Secchi disk depth measurements that spanned a more than 100-year time period and showed a decrease in marine phytoplankton biomass of approximately 1% of the global median per year over the past century. Eight decades of data on phytoplankton biomass collected in the North Atlantic by the Continuous Plankton Recorder (CPR) survey, however, show an increase in an index of chlorophyll (Phytoplankton Colour Index) in both the Northeast and Northwest Atlantic basins (Fig. 1), and other long-term time series, including the Hawaii Ocean Time-series (HOT), the Bermuda Atlantic Time Series (BATS) and the California Cooperative Oceanic Fisheries Investigations (CalCOFI) also indicate increased phytoplankton biomass over the last 20–50 years. These findings, which were not discussed by Boyce et al., are not in accordance with their conclusions and illustrate the importance of using consistent observations when estimating long-term trends.

Spectral reflectance and absorption of a massive red tide off southern California

Spectral reflectance and absorption of a massive Lingulodinium (Gonyaulax) polyedra red tide in March 1995 off southern California are compared to a “baseline” of biooptical measurements from the California Cooperative Oceanic Fisheries Investigations. The red tide was characterized by increased absorption and therefore reduced remote sensing reflectance (Rrs) in the 340–400 nm spectral range. The increased ultraviolet absorption was probably caused by mycosporine-like amino acids in the particulate fraction as well as increased absorption by dissolved organic matter. The chlorophyll a (chl a) specific particulate absorption of the L. polyedra bloom in the visible spectral range remained relatively constant for the chl a range 1–150 mg m−3 indicating accumulation of cells with similar optical characteristics. The difference in the Rrs versus chl a relationship of the red tide and “normal” California Current phytoplankton diminished with increasing wavelength from 340 nm and disappeared at 412 nm. Ratios of Rrs at 340 nm (or 380 nm) and 412 nm (or 443 nm) provided differentiation of the red tide starting at chl a concentration of 1–2 mg m−3. The forthcoming Japanese Global Imager (GLI) satellite sensor has, among others, the 380 nm band. If the signal to noise ratio and atmospheric correction for the 380 nm band are sufficient to retrieve the dynamic range of the water leaving radiance, then it might be possible to differentiate red tides from other phytoplankton blooms with the algorithm described here.

Human activities and climate variability drive fast-paced change across the world's estuarine-coastal ecosystems

Time series of environmental measurements are essential for detecting, measuring and understanding changes in the Earth system and its biological communities. Observational series have accumulated over the past 2-5 decades from measurements across the worlds estuaries, bays, lagoons, inland seas and shelf waters influenced by runoff. We synthesize information contained in these time series to develop a global view of changes occurring in marine systems influenced by connectivity to land. Our review is organized around four themes: (i) human activities as drivers of change; (ii) variability of the climate system as a driver of change; (iii) successes, disappointments and challenges of managing change at the sea-land interface; and (iv) discoveries made from observations over time. Multidecadal time series reveal that many of the worlds estuarine-coastal ecosystems are in a continuing state of change, and the pace of change is faster than we could have imagined a decade ago. Some have been transformed into novel ecosystems with habitats, biogeochemistry and biological communities outside the natural range of variability. Change takes many forms including linear and nonlinear trends, abrupt state changes and oscillations. The challenge of managing change is daunting in the coastal zone where diverse human pressures are concentrated and intersect with different responses to climate variability over land and over ocean basins. The pace of change in estuarine-coastal ecosystems will likely accelerate as the human population and economies continue to grow and as global climate change accelerates. Wise stewardship of the resources upon which we depend is critically dependent upon a continuing flow of information from observations to measure, understand and anticipate future changes along the worlds coastlines.

MODIS detects a devastating algal bloom in Paracas Bay, Peru

The medium-resolution bands on NASAs Moderate-Resolution Imaging Spectroradiometer (MODIS) were successfully used to detect and map the distribution of a harmful phyto-plankton bloom in the Paracas Bay Peru, that caused economic losses worth millions of dollars. Routine application of MODIS data can be a valuable and cost-effective way to monitor harmful blooms and other turbid water plumes that cause disruption to the fishery and aquaculture operations of many coastal areas.

Deep ocean communities impacted by changing climate over 24 y in the abyssal northeast Pacific Ocean

Significance Global warming is now a well-documented phenomenon that is influencing every aspect of our world, from increased storm intensity to melting of polar ice sheets and rising sea level. The impact of such changes in climate is least known for the deep ocean, which covers over 60% of the earth’s surface. An unprecedented 24-y time series measuring changes in food supply and utilization by benthic communities at 4,000-m depth in the deep northeast Pacific reveal strong connectivity with changing surface ocean conditions, which have broad implications for the global carbon cycle. The deep ocean, covering a vast expanse of the globe, relies almost exclusively on a food supply originating from primary production in surface waters. With well-documented warming of oceanic surface waters and conflicting reports of increasing and decreasing primary production trends, questions persist about how such changes impact deep ocean communities. A 24-y time-series study of sinking particulate organic carbon (food) supply and its utilization by the benthic community was conducted in the abyssal northeast Pacific (∼4,000-m depth). Here we show that previous findings of food deficits are now punctuated by large episodic surpluses of particulate organic carbon reaching the sea floor, which meet utilization. Changing surface ocean conditions are translated to the deep ocean, where decadal peaks in supply, remineralization, and sequestration of organic carbon have broad implications for global carbon budget projections.