Widespread anatoxin-a detection in benthic cyanobacterial mats throughout a river network

Abstract

Benthic algae fuel summer food webs in many sunlit rivers, and are hotspots for primary and secondary production and biogeochemical cycling. Concerningly, riverine benthic algal assemblages can become dominated by toxic cyanobacteria, threatening water quality and public health. In the Eel River in Northern California, over a dozen dog deaths have been attributed to cyanotoxin poisonings since 2000. During the summers of 2013-2015, we documented spatial and temporal patterns of cyanotoxin concentrations in the watershed, showing widespread distribution of anatoxin-a in benthic cyanobacterial mats. Solid phase adsorption toxin tracking (SPATT) samplers were deployed weekly to record dissolved microcystin and anatoxin-a levels at 10 sites throughout the watershed, and 187 Anabaena-dominated or Phormidium-dominated cyanobacterial mat samples were collected from 27 locations to measure intracellular anatoxin-a (ATX) and microcystins (MCY). Anatoxin-a levels were higher than microcystin for both SPATT (mean MCY = 0.8 and ATX = 4.8 ng g resin-1 day-1) and cyanobacterial mat samples (mean MCY = 0.074 and ATX = 1.89 μg g-1 DW). Of the benthic mats sampled, 58.9% had detectable anatoxin-a (max = 70.93 μg g-1 DW), while 37.6% had detectable microcystins (max = 2.29 μg g-1 DW). SPATT cyanotoxin levels peaked in mid-summer in warm mainstem reaches of the watershed. This is one of the first documentations of widespread anatoxin-a occurrence in benthic cyanobacterial mats in a North American watershed.

Fig 1. Map of cyanotoxin monitoring locations in the Eel River.

Site names indicate sub-watershed (letters) and drainage area in km² (numbers). Three sites with white circles were only monitored in 2014, while sites with black circles were monitored in 2013 and 2014. Stars represent sites that were monitored monthly in 2015. (SF = South Fork, MS = Mainstem, LE = Lower Eel, VD = Van Duzen).

Fig 2. Anabaena in the Eel River.

A-B) Micrographs of Anabaena cells (400x); C-E) Dark-green Anabaena “spires” growing on top of senescing macro-algae Cladophora glomerata; F-G) Anabaena-dominated mats on riverbed at bottom of shallow pools; H) SPATT sampler deployed in the Eel River.

Fig 3. Phormidium in the Eel River.

A-B) Micrograph of Phormidium cells (400x); C-E) Underwater photographs of Phormidium growing on cobbles; F-G) Looking down on brown or orange patches of Phormidium mats in the river (blue thermometer is 15 cm long).

Fig 4. Anatoxin-a (ATX) and microcystin (MCY) accumulations on SPATT samplers in the Eel River for 2013 and 2014.

Each point represents the retrieval date of a single sampler, and the line extends back to the day the sampler was deployed. Note the different scales of the y-axis for anatoxin-a and microcystin.

Fig 5. Anatoxin-a (ATX) accumulated each month from SPATT monitoring locations in 2013 and 2014.

Red points indicate sites where cyanotoxins were no accumulated anatoxin-a was detected (ND) on SPATT samplers. The diameter of the black points categorize the range of cyanotoxins accumulated on the SPATT samplers for a given month.

Fig 6. Detection of anatoxin-a and microcystin accumulated on SPATT samplers deployed in 2015 in locations throughout the Eel River watershed.

Fig 7. Boxplots of intracellular anatoxin-a and microcystin concentrations from cyanobacterial mats dominated by Anabaena (n = 116) or Phormidium (n = 62) collected in 2014 and 2015.

Only samples with cyanotoxin concentrations above the detection limit are included in the boxplots. For anatoxin-a, there were 69 Anabaena-dominated and 36 Phormidium-dominated samples above the detection limit, and for microcystin there were 31 Anabaena-dominated and 36 Phormidium-dominated samples above the detection limit. The boxplots are plotted on a log₁₀ scale, and the box represents the 25th percentile, median, and 75th percentile of the data, the whiskers extend up to 1.5x the interquartile range, and points beyond the whiskers are plotted individually.

Fig 8. Total anatoxin-a and microcystin concentrations (dissolved plus lysed suspended cells) in unfiltered water samples collected in summer 2015 in the Eel River.