Effects of Temperature on Reproduction in an Atlantic Kelp, Laminaria longicruris, in the North Atlantic Ocean

Laminaria longicruris, a North Atlantic kelp with rapid growth rates and large biomass, is of both economic and ecological importance in subtidal coastal ecosystems, and is a key component of a variety of food webs. This species is known to suffer sporophyte deterioration at a temperature of 23°C, and summer temperature is known to constrain its southern limit of distribution. Our research shows that reproductive output in kelp is partially regulated by temperature as well. The role of temperature in determining the allocation of blade surface to meiospore production was quantified for populations of Laminaria longicruris at three study sites along a temperature gradient in the Northwestern Atlantic. Using a stratified random sampling technique, adult sporophytes were collected at sites in Long Island Sound (41° 17' N), Portsmouth, New Hampshire (43° 03'N) and Halifax, Nova Scotia (44° 38'N) in the spring of 1992. Photoperiod and insolation were roughly equivalent for all three sites. The ratio of sorusarea (reproductive tissue) to blade (vegetative tissue). was determined by morphometric analysis and used asa measure of reproductive effort. Nonparametric statistical· analysis revealed significant variation between sites with least reproductive effort occurring near the southern limit of the·geographical distribution range (Figure 1).With increasing temperature and decreasing latitude, fewer resources were allocated to reproduction. A regression line fit to the data for the three latitudes shows a positive correlation between reproductive output and latitude, with an X-intercept occurring at 37.8° N, roughly equivalent to the known observed limit of the species (Figure 2). A second regression line fit to the data (Figure 3) shows that, theoretically, reproduction does not occur at spring temperatures above 12°C,corresponding roughly to a summer temperature of 23°. A global temperature warming of Ito 2°C could eliminate the species from the Long Island Sound estuary by causing its geographical range to retreat northward.