Green Waters of the Sanctuary: the Role of Iron

The phytoplankton-rich green waters of the Monterey Bay National Marine Sanctuary support a remarkably rich ecosystem. Although the coastal waters off central California are highly productive, there is a great deal of variability, and some of the waters are not nearly as productive as one would expect them to be. One of the keys to understanding the variability in phytoplankton biomass and productivity lies in understanding the supply of the micronutrient iron.

Figure 1. Continental shelf 200-meter depth contour and annual average suspended sediment discharge (in thousand metric tons per year) by the various rivers along central California

Northwesterly winds along the coast of central California result in wind-driven coastal upwelling that brings colder, nutrient-rich water to the surface. This process is most intense during the spring and summer. The large flux of the essential plant macronutrients nitrate, phosphate, and silicic acid can allow extensive phytoplankton blooms to occur that may extend tens to hundreds of kilometers offshore. Phytoplankton blooms occur when nutrient-replete conditions promote rapid algal growth rates temporarily uncoupled from grazing pressure. Large diatoms tend to dominate the biomass in phytoplankton blooms that develop in these coastal upwelling regimes, and it has been argued that diatom-driven new production efficiently fuels the food chains that support coastal fisheries, seabirds, and marine mammals.

The potential productivity associated with upwelling centers, however, is not always realized. Recent studies have demonstrated that the supply of iron, a key micronutrient, plays a critical role in controlling phytoplankton blooms in these coastal upwelling regimes. Iron-rich upwelling regions experience extensive blooms of diatoms that deplete available macronutrients; while in iron-poor areas, the biomass of phytoplankton is greatly reduced and high concentrations of unutilized macronutrients persist. Thus, understanding the supply of iron is a key for understanding the variability in productivity along the California coast.

Diatoms are composed of soft organic tissue plus hard parts of biogenic opal. The ratio of elemental building blocks of coastal diatoms normalized to one iron atom is given by: Carbon : Nitrogen : Phosphorus : Silicon : Iron 20,000: 3,000: 190: 3,400: 1 This means that for every 20,000 atoms of carbon in the soft organic tissue of a coastal diatom, there is a requirement for 3,000 atoms of nitrogen and one atom of iron. The amount of these elements required to produce an extensive bloom of diatoms compared to the concentrations available in the upwelled water correspond to only 10 percent of the available inorganic carbon, versus 100 percent of the available nitrate, 85 percent of the phosphate, 95 percent of the silicic acid, and anywhere from 50 to 1,000 percent of the available iron. If adequate iron is available, then nitrate becomes the key nutrient that limits bloom development. If, however, only a small amount of iron is available, then iron can be the key nutrient that limits the bloom and can result in water low in phytoplankton biomass, but still rich in unutilized macronutrients such as nitrate.

Figure 2. Upwelling index off central California (light; NOAA/PFEL data) and the daily mean stream flow of the San Lorenzo River (dark; USGS data) from October 1995 to September 2001

The major source of iron to the central California upwelling regime originates from river discharge of suspended sediments, which is unevenly distributed spatially and temporally (Figures 1 and 2). It can be seen that the larger rivers are in the north where the shelf is also broader and that flows are dominated by episodic flood events during the winter. (Often greater than 90 percent of the water discharge can occur during a one-week period.) The suspended sediment discharge is even more episodic, with the rare, high-energy, and high-discharge events able to carry a tremendous amount of mud. In marked contrast, during the summer there is no significant discharge from coastal streams and rivers of central California.

The width of the continental shelf plays a role because when a sufficiently broad continental shelf is present, much of the winter fluvial (from rivers) discharge of suspended sediment is rapidly deposited on the shelf at depths of 40 to 100 meters; thus a relatively broad continental shelf can act as an “iron trap” for these fluvial inputs. This is important since in the winter, when fluvial input is the greatest, upwelling is at a minimum (Figure 2). When coastal upwelling of macronutrient-rich water takes place over these broad shelf regions, elevated iron concentrations can be entrained, resulting in water enriched with both macronutrients and iron (Figure 3, p. 13). This is the case in the regions to the north of Monterey Bay where extensive blooms of large diatoms are commonly observed. This extra source of iron is not provided along the Big Sur coast, with its narrow shelf and lack of rivers (Figure 3). This region along the Big Sur coast has been observed to be a low phytoplankton biomass, iron-limited regime.

Figure 3. Conceptual model depicting iron supply to surface waters during upwelling under different conditions: a) wide shelf with winter flood deposits; b) narrow shelf without winter flood deposits

The Monterey Bay area is a world center for iron research. Study on the role of iron in the oceans started with pioneering efforts by the late John Martin of Moss Landing Marine Laboratories (MLML) and our research group at the University of California Santa Cruz (UCSC). It has been continued by Kenneth Coale’s research group at MLML and Ken Johnson’s group at Monterey Bay Aquarium Research Institute (MBARI) along with our research group at UCSC, particularly Eden Rue. This expertise is also due in large part to the contributions of research technicians such as Mike Gordon of MLML and Geoffrey Smith at UCSC.

Questions that members of our research group at UCSC are still addressing in the sanctuary region include 1) gaining a better understanding of the mechanisms of delivery and entrainment of the benthic supply of iron during upwelling events; 2) determining the importance of particulate versus dissolved iron as a source of iron to the phytoplankton; 3) determining if there is an enhanced supply of iron to the system in the upwelling seasons following years with major flood events (see 1997 and 1998 in Figure 2) versus years without major flood events (2001 and 2002); 4) working with Raphe Kudela’s research group (UCSC) to examine the potential for optical data (e.g., backscattered light and fluorescence) as tools to map particulate iron distributions in coastal upwelling regions; and 5) examining the role of luxury uptake of iron by diatoms during a time of high availability and their ability to pass this stored iron on to their progeny for them to continue high rates of productivity even under low external iron availability.

A new research program that will allow us to continue to address some of these questions in the sanctuary is the recently funded interdisciplinary program entitled “From Wind to Whales: Understanding California’s Upwelling Ecosystems.” This is a collaborative research initiative that includes a variety of scientists from the Naval Postgraduate School, MBARI, and UCSC under the auspices of The Center for Integrated Marine Technologies with funding from NOAA.

Kenneth W. Bruland and Ana M. Aguilar-Islas
Institute of Marine Sciences and Department of Ocean Sciences,
University of California Santa Cruz