Spatial and temporal variation in indicator microbe sampling is influential in beach management decisions

Abstract

Fecal indicator microbes, such as enterococci, are often used to assess potential health risks caused by pathogens at recreational beaches. Microbe levels often vary based on collection time and sampling location. The primary goal of this study was to assess how spatial and temporal variations in sample collection, which are driven by environmental parameters, impact enterococci measurements and beach management decisions. A secondary goal was to assess whether enterococci levels can be predictive of the presence of Staphylococcus aureus, a skin pathogen. Over a ten-day period, hydrometeorologic data, hydrodynamic data, bather densities, enterococci levels, and S. aureus levels including methicillin-resistant S. aureus (MRSA) were measured in both water and sand. Samples were collected hourly for both water and sediment at knee-depth, and every 6 h for water at waist-depth, supratidal sand, intertidal sand, and waterline sand. Results showed that solar radiation, tides, and rainfall events were major environmental factors that impacted enterococci levels. S. aureus levels were associated with bathing load, but did not correlate with enterococci levels or any other measured parameters. The results imply that frequencies of advisories depend heavily upon sample collection policies due to spatial and temporal variation of enterococci levels in response to environmental parameters. Thus, sampling at different times of the day and at different depths can significantly impact beach management decisions. Additionally, the lack of correlation between S. aureus and enterococci suggests that use of fecal indicators may not accurately assess risk for some pathogens.

Figure 1

Comparison of physical parameters knee depth water enterococci knee depth water S. aureus and bathing load. The top graph displays tidal height (dark grey line) solar shortwave (SW) radiation averaged over the previous hour (dotted line with data points) and times of rainfall (grey rectangles). The middle graph displays knee depth enterococci levels with dashed lines aligning peak enterococci events (>1000 CFU/100mL) to corresponding physical parameters. The bottom graph displays knee depth water S. aureus levels and number of bathers present at each hour.

Figure 2

Box plot of knee-depth and waist-depth water enterococci levels (n = 238 for knee-depth samples n = 40 for waist-depth samples). The center line in the boxes indicates the median value. Whiskers indicate 10th and 90th percentiles. Outliers (dots) indicate 5th and 95th percentiles. Ninety percent of the waist-depth samples had lower values than 50 percent of knee depth values.

Figure 3

Knee-depth water enterococci levels grouped by hour. Black squares indicate night samples (9PM-5AM) white squares indicate morning samples (6AM-12PM) and grey squares indicate afternoon samples (1PM-8PM). The dotted line indicates the percentage of samples each hour above the swim advisory single sample guideline of 104 CFU/100 mL.

Figure 4

Daytime and nighttime mean enterococci levels grouped by number of hours before and after high tide.