Research in Estuarine Ecosystems: Various Harmful Species in the Neuse Estuary

A recent Ph.D. dissertation completed at the CAAE used a continuous, 13- year record of environmental data and phytoplankton taxa in the mesohaline estuary (1994-2006; biweekly, April-October; monthly, November-March) to evaluate phytoplankton assemblage responses to changing environmental conditions. Ordination techniques including non-metric multidimensional scaling (NMDS), indicator species analysis, and BIOENV software were used to investigate potential environmental predictors of phytoplankton assemblage patterns under chronic eutrophication (see Rothenberger et al., Limnol. Oceanogr. 54: 2107-2127).

Phytoplankton assemblages were strongly related to temperature and total nitrogen : total phosphorus (TN:TP) ratios, with expected seasonal changes in species composition. Inter-annual changes in river discharge influenced whether phytoplankton assemblages were dominated by diatoms and phototrophic flagellates, or by mixotrophic and heterotrophic dinoflagellates. From this analysis, increasing NH4 + concentrations also emerged as an important influence on the phytoplankton assemblages. Raphidophytes (including potentially toxic Heterosigma akashiwo), haptophytes, chlorophytes, and the bloom-forming dinoflagellate Heterocapsa rotundata increased in more recent years (2000- 2006), concomitant with increasing NH4 + concentrations (e.g., 105 cells of H. rotundata mL-1 at NH4 + > 14 μM). Abundance of the toxigenic dinoflagellate Prorocentrum minimum was positively related to dissolved organic nitrogen (DON), whereas the highest abundance of the grouping Pfiesteria spp., ‘pfiesteria-like’ dinoflagellates, and Karlodinium veneficum occurred during summer and fall and was related to high TP concentrations, temperature, and salinity. The data suggested an increasingly important role of NH4 + in controlling phytoplankton assemblage structure, including increased abundance of some harmful species, in this eutrophic estuary. The field study was supported by other field research and by laboratory experiments (e.g., NH4 + supports more rapid growth of H. akashiwo than nitrate or urea).

The CAAE has also characterized blooms in the Neuse Estuary that have been dominated or co-dominated by Prorocentrum minimum (e.g. see Springer et al. 2005, Harmful Algae 4: 533-551). The developing bloom was first detected from a web-based alert provided by the Center’s real-time remote monitoring (RTRM) platforms, indicating elevated dissolved oxygen (DO) and pH levels in the upper estuary. These data were used to augment shipboard sampling in characterizing bloom initiation, development, movement, and dissipation over a 7-month period (October 2001-April 2002).

Chlorophyll a (chla) within bloom areas averaged 106 +13 μg L-1 (mean + 1 SE), with a maximum of 803 μg L-1, in comparison to 20 μg L-1 outside the bloom. There were significant positive relationships between dinoflagellate abundance and TN and TP. NH4 +, NO3 -+NO2 -, and soluble reactive phosphorus (SRP) concentrations did not decrease within the main bloom, suggesting that upstream inputs and other sources provided nutrientreplete conditions. In addition, PAM fluorometric measurements (at 0900-1300 hours) of maximal PSII quantum yield (Fv/Fm) were consistently 0.6-0.8 within the bloom until late March, providing little evidence of photophysiological stress as would have been expected under nutrient-limiting conditions. An analysis of N uptake kinetics during the period when P. minimum was dominant (October- December) suggested that NH4 + was the major N species that supported the bloom. Bloom displacement (January-February) coincided with higher diversity of heterotrophic dinoflagellate species as P. minimum abundance decreased.