Inlets of Prince Edward Island (PEI) provide an important source of income, offered by the rapidly growing molluscan shellfish (mostly blue mussels, Mytilus edulis) aquaculture industry. At the same time, some of those inlets have been described as being subject to eutrophication. Nutrient enrichment may be an advantage to the aquaculture industry by stimulating the growth of phytoplankton eaten ... by aquacultured mussels. However, this may occur only if nutrient concentrations are at levels that may limit phytoplankton biomass and other factors are not limiting. As well, the concentrations and ratios of nutrients may select for certain groups of phytoplankton, not all of which may be desirable food for mussels, and some of which are harmful or toxic, producing harmful algal blooms (HABs).
This study was carried out to compare the results of nutrient and phytoplankton measurements in PEI inlets, in order to look for relationships between species composition and nutrient levels. We also considered the likelihood of nutrient limitation within the phytoplankton community by comparing ambient nutrient concentrations with the inherent ability of the phytoplankton to take up the nutrients at low concentrations.
Samples for nutrients and phytoplankton were collected from 14 PEI inlets, in the late summer and fall of 2001, 2002 and 2003. Detailed time-series phytoplankton community composition and species enumeration data were obtained every week from four inlets (Cardigan River, Tracadie Bay, New London Bay, and Lennox Channel). The inorganic nitrogen (N) in the 14 inlets was dominated by ammonia in the late summer and fall: the mean ammonia concentration (2.23 μM) was ~3.5 times higher than that of nitrate (0.64 μM) and ~19 times greater than that of nitrite (0.12 μM). The mean total inorganic N was only 3.22 μM, a value well below that thought to indicate eutrophication. Silicate (Si) and phosphate (P) levels were also low most of the time, with mean values of 1.20 and 0.45 μM, respectively. Except for P, these concentrations were generally below those believed to limit phytoplankton nutrient uptake or growth rates. Nutrient conditions in which two or three nutrients are simultaneously at limiting levels occur frequently in the fall in PEI inlets. Such “nutrient co-limitation” has not been considered extensively in the marine phytoplankton literature, but was explored here using an interactive model for multiple nutrient limitation of diatoms. Nutrient limitation of carrying capacity, determined from Redfield ratios, was also evaluated for both Bacillariophyceae (diatoms) and Dinophyceae (dinoflagellates). Both diatom growth and carrying capacity were limited by Si in about ~75% of the samples; dinoflagellate carrying capacity was limited by N in 72% of the samples and by P in 26% of the samples. Comparisons among the different inlets show that limitation varies, both between inlets in the same year and between years in the same inlet. Several multivariate statistical analyses identified relationships between the phytoplankton and nutrient levels. However, it was difficult to find clear-cut relationships that might help define the specific nutrient conditions that select for individual phytoplankton species or groups.
Of the 124 distinct species of phytoplankton identified for all inlets studied during 2001-2003, there were 49 centric diatoms (Bacillariophyceae), 27 pennate diatoms (Bacillariophyceae), 36 dinoflagellates (Dinophyceae), 3 Dictyochophyceae, 2 Chlorophyceae, 2 Cyanophyceae, 1 Haptophyceae, 1 Chrysophyceae, 1 Euglenophyceae, 1 Litostomatoea, and 1 protist. Of these, 11 have apparently never before been reported in the Gulf of St. Lawrence. The diatom Skeletonema costatum was the most abundant species. Nineteen potentially toxic or harmful species were recorded, but their numbers were generally too low to have caused any shellfish harvesting closures or environmental harm during the sampling period. The toxic dinoflagellate Karenia mikimotoi was abundant in Cardigan River during October 2001 and 2003, but no harmful effects were observed. The domoic-acid-producing diatom Pseudo-nitzschia multiseries was infrequently found, and only in low numbers. Other species of Pseudo-nitzschia, including the non-toxic P. calliantha, P. pungens, and P. delicatissima, bloomed in its place. Two new species of Pseudo-nitzschia (P. americana and the tentatively identified P. subpacifica) were reported for the first time in the Gulf of St. Lawrence. Pseudo-nitzschia fraudulenta was identified for the first time in PEI inlets.
During all three years, Tracadie Bay exhibited an order of magnitude lower number of total phytoplankton cells than did Cardigan River; Lennox Channel and New London Bay showed intermediate values. More data are required to explain this finding, but we calculated that Tracadie Bay had a greater percentage of harvesting lease area coverage (37%) than did Cardigan River (20%). This study points out the importance of continuing to compile phytoplankton species lists over time, in order to look for species that are toxic / harmful or that have never before appeared in our waters, as well as to better understand the dynamics of phytoplankton blooms.