Phytoplankton at the Shoreline
Coastal productivity and the density of plankton at the base of the marine food web
Phytoplankton are microscopic, single-celled photosynthetic organisms that form the base of marine food webs together with larger marine "plants" including seaweeds and seagrasses. The major groups of phytoplankton include diatoms, dinoflagellates, coccolithophores, and cyanobacteria. This evolutionarily diverse group of organisms is named to reflect their common lifestyle: they drift together with the water they inhabit, largely at the mercy of ocean currents. Like all photosynthetic organisms they require light, nutrients and water to grow, but as they live in the ocean, the availability of nutrients and light are the primary factors that control their abundance. Their access to light and nutrients is strongly influenced by physical factors such as winds and the salinity and temperature of the water. When conditions are especially conducive to their growth they can become so abundant that the water appears colored. These events are called “blooms” and can occasionally be dominated by species that produce toxins (harmful algal blooms or "red" tides). Although they are very small, phytoplankton are found over the entire surface of the ocean and are responsible for ~ 50% of the Earth’s primary production. Furthermore, because they form the base of marine food webs (even whales, sharks, seals and seabirds ultimately depend on them), and their abundance is very responsive to changes in ocean climate, they have a profound influence on the abundance, health and wellbeing of most marine life.
The abundance of phytoplankton in the ocean can be measured in a variety of ways. Direct counts and identification of species can be made from plankton net samples. The overall abundance of phytoplankton can be measured by the concentration of chlorophyll a (the primary photosynthetic pigment found in all phytoplankton and a correlate of abundance) in a water sample. However, if you are interested in understanding how the abundance of these organisms changes over time or in different places, this means collecting many, many samples and is not very practical. Technological advances such as ocean color images from satellites and instruments that measure the fluorescence of chlorophyll a (chl a) have made automated sampling possible, but these methods also have limitations. For example, the highly productive ribbon of ocean that hugs the shoreline and supports a diversity of concentrated marine life (including kelp forests, intertidal zones and a variety of commercial and recreational fisheries) is very poorly represented (if at all) in satellite imagery.
To help fill this critical gap and provide information on the primary productivity of the waters in the Hopkins Marine Life Refuge, the Marine Life Observatory has initiated long-term monitoring of chl a fluorescence from a shore-based fluorometer. The fluorometer is affixed to a rocky bench in the intertidal zone at approximately 1 foot above mean lower low water (MLLW) and records chl a fluorescence every 15 minutes. We collect bottle samples from the surf zone adjacent to the fluorometer every week to calibrate the fluorescence readings to the true concentration of chl a in the water (fluorescence yield per unit chl a can vary depending on several factors including the physiological state and taxonomic composition of the phytoplankton in the sample measured, thus regular calibration is important). We measure the concentration of chl a in the bottle samples using standard extraction methods on a benchtop fluorometer. Fluorescence data are downloaded periodically, then processed to remove out of water periods and calibrated to the actual concentration of chl a measured in the bottle samples. The installation at HMLR represents one node in a network of 13 intertidal zone fluorometers in Oregon and California (maintained in collaboration with the Partnership for Interdisciplinary Studies of Coastal Oceans [PISCO] and the Central and Northern California Ocean Observing System [CeNCOOS]). The network provides important information on regional and local dynamics of phytoplankton abundance and provides one metric of an important ecosystem function, primary production, to inform ecological studies throughout the region. Recent monitoring at HMLR clearly shows several blooms as well as periods of relatively low chl a concentration. The causes and consequences of these short term fluctuations in phytoplankton abundance are not well understood and are the subject of current investigations.