Recent Advances and Future Visions: Temporal Variability of Optical and Bio-optical Properties of the Ocean

Introduction Ocean optics concerns studies of light and its propagation through the ocean, and bio-optics connotes biological effects on optical properties and vice versa. These two terms are so intertwined that they are often used interchangeably. Knowledge of the variability of optical and bio-optical properties of the ocean is important for many scientific and practical problems (e.g., Dickey and Falkowski, 2001). A few examples follow. Light and its utilization is key to life in the ocean beginning with phytoplankton and primary productivity, the ecology of the upper ocean, and biogeochemical cycling. The thermal structure and heat budget of the upper ocean are driven by the penetration of solar radiation as modulated by the absorption and scattering of light by water and its particulate and dissolved constituents. Both of the aforementioned examples illustrate key factors that affect and are affected by global climate change. Optical properties are also important indicators of the health of the ocean in the form of changing turbidity, diversity and distributions of species (indigenous and non-indigenous), and harmful algal blooms (e.g., red tides) and associated toxins. Underwater visibility, which has industrial and military relevance, also depends on optical properties. Importantly, in situ optical sensors and systems allow us to study a host of oceanographic processes, which bear on these and other problems as well as a variety of interests including quantification and interpretation of satelliteand aircraft-based observations of ocean color and studies of sediment resuspension, pollution, and bathymetry. Readers interested in learning more about the subdisciplines of ocean optics and bio-optics are directed to other papers in this volume and books by Kirk (1994), Spinrad et al. (1989), and Mobley (1994). Other papers focus on new technologies applied to ocean optics and bio-optics (e.g. Dickey, 1991, 2001; Dickey et al., 1998a, 2001; Maffione, this issue) and biogeochemistry (e.g. Tokar and Dickey, 2000; Varney, 2000; Dickey et al., 2000, 2001). Time series observations have proven valuable for many environmental problems. Perhaps the most famous and one of the more important of these is the Mauna Loa, Hawaii time series, which has shown the dramatic increase of the concentration of atmospheric carbon dioxide (CO2) since the beginning of the Industrial Revolution. There is no direct equivalent for optical properties of the ocean, in part because of the earher lack of adequate optical instrumentation and the high degree of natural optical variability in the ocean. Nonetheless, there are growing numbers of scientifically important optical time series data sets. For example, a remarkable time series of Secchi disk 1 depth, a rough and somewhat subjective measure of optical clarity or turbidity, has been described for the Black Sea by Mankovsky et al. (1998). This record shows the dramatic decrease in the Secchi disk depth (increase of turbidity) in the Black Sea from about 15-16 m in the mid1980s to 6-10 m between 1990 and 1993 with some observations as low as 2-4 m in 1992 (Figure 1). There was some horizontal variability in Secchi disk depths (e.g. due to river inflow), however, the basIn-scale variations have been extraordinary. One of the explanations for these changes is that coccolithophores, phytoplankton with highly reflective disks or coccoliths that can cause the sea to appear whitish, increased by up to two orders in magnitude. Some of the variability also may have been caused by increasing colored dissolved organic matter (CDOM). The Black Sea optical time series likely reflects a major change in the community structure and ecology of the Black Sea. Several other examples of optical and bio-optical time series are described below. The availability of instrumentation and appropriate platforms to measure optical properties has