Document: TES-3-93-4

El Nio impacts on rocky intertidal algae: A nutrient hypothesis.

FREIDENBURG, T.L.*, G.W.ALLISON and B.A.MENGE

Oregon State University, Corvallis, OR 97331 USA ¹

Abstract:
Rocky intertidal community structure and dynamics vary with changes in nearshore oceanographic patterns. Two sites on the central Oregon coast show dramatically different patterns in the low intertidal zone. At Boiler Bay (BB) the low zone is dominated by macrophytes while at Strawberry Hill (SH), 80 km to the south, the low zone is dominated by invertebrates. One possible explanation for this pattern is that high nutrient availability fueled faster growth rates of algae at Boiler Bay, but field studies showed that nutrients and growth rates of macrophytes did not differ between sites. Research in southern California has shown that by interfering with the upwelling of nutrients, El Nios can cause sharp decreases in kelp biomass through the mechanism of nutrient depletion. During the 1997/98 El Nio, intertidal kelps appeared to be adversely affected by nutrient depletion at BB, SH and other sites along the central Oregon coast, but were seemingly unaffected at Cape Blanco, ~250 km to the south of SH. The sharp contrast between the unhealthy and dying plants on the central Oregon coast and the healthy robust plants at Cape Blanco during the El Nio suggested that nutrients might have a much larger characteristic scale of variation and influence on macroalgae than previously considered. SST imagery revealed that upwelling persisted at Cape Blanco throughout the 97/98 El Nio even at its height. To initiate studies of these large-scale patterns, we quantified kelp length, density and percent cover in permanent transects at sites on the central and southern coasts of Oregon at the end of the El Nio (summer '98) and one year later (summer '99). In general, the annual kelp Postelsia palmaeformis were larger at Cape Blanco than those on the central coast and were larger in '99 than in '98. Abundance of the kelp Hedophyllum sessile in larger (10 by 50 meter) permanently marked grids dropped sharply during the El Nio, but had recovered in 1999. Evidence suggests that intertidal algae can be severely affected during El Nios, but that these effects can be alleviated in coastal areas experiencing persistent upwelling.

Keywords: El Nio, rocky intertidal, macroalgae, nutrients, Oregon

This abstract is being presented at: 10:30 AM in session:
Oral Session #37: Phytoplankton.