A comparison between the FlowCam 8100, microscopy, and sandwich hybridization assay for quantifying abundances of the saxitoxin-producing dinoflagellate, Alexandrium catenella.

Light microscopy, FlowCam, and sandwich hybridization assay (SHA) are three approaches that facilitate the monitoring of harmful algal bloom (HAB) forming phytoplankton. Yet, cross-comparisons among these techniques have not been conducted. This study addressed that gap using the saxitoxin-producing 'red tide' dinoflagellate Alexandrium catenella, a species responsible for blooms and paralytic shellfish poisoning worldwide. To achieve this goal, the dynamic ranges of each technique were compared using A. catenella cultures spanning low (pre-bloom), moderate (bloom), and high (dense bloom) levels. To assess field detection, water samples containing very low (<3 cells mL-1) A. catenella levels were collected from Long Island Sound, USA (Jun-Aug 2021) and evaluated using each method. Field samples were also spiked with A. catenella to high (160 cells mL-1) or low (40 cells mL-1) concentrations. In general, microscopy, FlowCam, and SHA returned comparable A. catenella cell concentrations for all tests. Mean cell concentrations from laboratory intercalibration experiments were not significantly different for any method or concentration (ANOVA, p > 0.05). However, relative to microscopy at times SHA produced non-detect signals <2 cells mL-1 in field samples and the FlowCam slightly underestimated cell concentrations when A. catenella abundances were high in laboratory and field samples. Mean cell concentrations of spike experiments were not significantly different for any test date, sampling location, or method, despite variability among methods within the high concentration treatment (ANOVA, p > 0.05 for all treatments). Findings are relevant to HAB researchers, managers, and public health officials because they help reconcile disparate cell abundance datasets that inform numerical models and enhance HAB monitoring and prediction. Results are also likely broadly applicable to several HAB species.

Transitions in nitrogen and organic matter form and concentration correspond to bacterial population dynamics in a hypoxic urban estuary.

Nitrogen (N) inputs to developed coastlines are linked with multiple ecosystem and socio-economic impacts worldwide such as algal blooms, habitat/resource deterioration, and hypoxia. This study investigated the microbial and biogeochemical processes associated with recurrent, seasonal bottom-water hypoxia in an urban estuary, western Long Island Sound (WLIS), that receives high N inputs. A 2-year (2020-2021) field study spanned two hypoxia events and entailed surface and bottom depth water sampling for dissolved nutrients as inorganic N (DIN; ammonia-N and nitrite + nitrate (N + N)), organic N, orthophosphate, organic carbon (DOC), as well as chlorophyll a and bacterial abundances. Physical water quality data were obtained from concurrent conductivity, temperature, and depth casts. Results showed that dissolved organic matter was highest at the most-hypoxic locations, DOC was negatively and significantly correlated with bottom-water dissolved oxygen (Pearson's r = -0.53, p = 0.05), and ammonia-N was the dominant DIN form pre-hypoxia before declining throughout hypoxia. N + N concentrations showed the reverse, being minimal pre-hypoxia then increasing during and following hypoxia, indicating that ammonia oxidation likely contributed to the switch in dominant DIN forms and is a key pathway in WLIS water column nitrification. Similarly, at the most hypoxic sampling site, bottom depth bacteria concentrations ranged ~ 1.8 × 104-1.1 × 105 cells ml-1 pre-hypoxia, declined throughout hypoxia, and were positively and significantly correlated (Pearson's r = 0.57; p = 0.03) with ammonia-N, confirming that hypoxia influences N-cycling within LIS. These findings provide novel insight to feedbacks between major biogeochemical (N and C) cycles and hypoxia in urban estuaries. Supplementary Information: The online version contains supplementary material available at 10.1007/s10533-023-01021-2.