Strategies for pulsed nutrient supply to Gracilaria cultures in the Florida Keys: Interactions between concentration and frequency of nutrient pulses

A factorial design experiment, using in situ cage cultures, was used to investigate the effects of frequency and concentration of nutrient pulses on growth, nutrient uptake, and chemical composition (C, N, P) of Gracilaria tikvahiae McLachlan in nearshore waters of the Florida Keys. Both frequency and concentration of the nutrient pulses affected growth and chemical composition of G. tikvahiae, indicating nutrient limitation occurred during the study. Growth of G. tikvahiae increased with increasing pulse frequency up to the highest level used (2·wk−1) at all pulse concentrations; in contrast, growth increased with increasing pulse concentration to the highest concentration at the low pulse frequency but not at the higher pulse frequencies. Although the frequency of nutrient pulses appeared more important in regulating growth and pre-pulse levels of chemical constituents than pulse concentration, the effects of frequency were due to its effects on total nutrient loading (i.e. flux) and not to the effects of frequency of nutrient enrichment per se. Greater variation in percent P compared to percent N in G. tikvahiae tissue between pulses and an increased PO3−4 uptake rate in nutrient-limited G. tikvahiae suggests that P rather than N was the primary limiting nutrient during the study; however, N was an important secondary limiting nutrient indicating dual nutrient-limitation occurred. While the pulse medium used had a N:P ratio of 18 : 1, much higher uptake ratios, ranging from 27:1 to 80:1, actually occurred, supporting the contention of P-limitation. Thus, nutrient pulse strategies with G. tikvahiae in P-limited systems need to utilize excessively lowN:P ratios in the pulse medium to offset the differential uptake rates of NH+4 and PO3−4 at the high concentrations typically used in pulse-feeding strategies.