Experimental evaluation of nutrient limitation of phytoplankton communities in the Gulf of Riga

Phytoplankton nutrient limitation was studied in the Gulf of Riga during sp ring bloom (April 1995), early summer stage (June 1994), cyanobacterial blo om (July 1994) and post cyanobacterial bloom (August 1993). Each year six f actorial nutrient enrichment experiments were carried out in various locati ons in the Gulf; including outer Irbe Strait, northern Gulf and southern Gu lf The responses of natural phytoplankton communities to the nutrient addit ions (80 mu g NH4-N l(-1), 20 mu g PO4-P l(-1) and two levels of combined a dditions) were followed for 3 days using 6 1 experimental units. To evaluat e the nutrient limitation patterns, time series of chlorophyll a were analy sed using polynomial regression models and ranking method, taking advantage of the relatively constant experimental error. Apparent nutrient depletion rates and ratios were estimated, and compared with the changes in particul ate nutrient ratios. During the spring diatom bloom in 1995, ambient inorga nic nutrient concentrations were still high, and thus phytoplankton biomass did not respond to additions of nutrients. Chlorophyll a specific nutrient depletion rates were low (0.01-0.12 mu g N (mu g chi a)(-1) h(-1) and 0.00 2-0.016 mu g P (mu g chl a)(-1) h(-1)) and linear over time, thus also reve aling that phytoplankton was not limited by these nutrients in that time. I n June 1994, there was an areal shift from N limitation in the outer Irbe S trait towards co-limitation in the southern Gulf. Later in July 1994, durin g the bloom of N-fixing Aphanizomenon flos-aquae, the N limitation was obvi ous for the whole study area. For this period chlorophyll a specific nutrie nt depletion rates were high (0.36-0.67 mu g N (mu g chi a)(-1) h(-1) and 0 .089-0.135 mu g P (mu g chi a)(-1) h(-1)), and added nutrients were almost totally depleted during the first light period. After the collapse of cyano bacterial bloom in August 1993, the experiment carried out in the southern Gulf indicated P limitation of phytoplankton. The central Gulf was obviousl y co-limited, while the area between northern Gulf and outer Irbe Strait wa s N-limited. Our results indicate that phytoplankton in the Gulf of Riga, e arlier considered strictly as P-limited, is at least until late-summer peri od N- or co-limited. It seems also obvious that there exists a spatial tend ency in the phytoplankton limitation patterns, generally from mon P- or co- limited southern Gulf towards more N-limited northern basin. (C) 1999 Elsev ier Science B.V. All rights reserved.