Report Number: 110
Year: 2005

Nutrient Status of Tumon Bay in Relation to Intertidal Blooms of the Filamentous Green Alga Enteromorpha clathrata

In Guam, intertidal blooms of the filamentous green alga, Enteromorpha clathrata, typically occur on beaches under the influence of groundwater intrusion. This particular species is especially abundant in Tumon Bay, Guam's premier tourist location, on the northwest side of the island. Local hoteliers in this area consider the algal blooms unsightly and a very real threat to tourism. The blooms are commonly believed to be associated with high levels of nitrate (NO3-) that occur naturally in Guam's groundwater (2-3 mg/L). However, the distribution and abundance of E. clathrata in Tumon Bay seems to have paralleled commercial development in the area, which suggests other factors are also important. Previously, reactive P (RP) levels approaching 500 μg/L were detected in surface runoff from the gardens of one of Guam's leading hotels overlooking Agana Bay. Since P is normally limiting in Guam's nearshore waters, it was hypothesized that similar releases from hotel gardens along the waterfront, in Tumon Bay, account for the green algal problem as it is today.

In the current study N, P and silica (SiO2) levels were determined in emergent groundwater seeps and springs from 9 intertidal sites (mostly 50-100 m apart) in Agana Bay and 70 intertidal sites (~50 m apart) in Tumon Bay. RP levels in Agana Bay ranged from 12.7-30.6 μg/L with the highest level occurring closest to the hotel mentioned above, at the northern end of the bay. The highest levels of NO3-N (1.3-4.0 mg/L) and SiO2(2.7-5.5 mg/L) were also found here. In Tumon Bay, RP levels ranged from 1.3-31.9 μg/L while NO3-N and SiO2were <0.01-7.9 mg/L and 0.42-3.8 mg/L respectively. RP accounted for >90% of total P in all samples while NO3-N was the predominant form of dissolved inorganic nitrogen (DIN = NO3-N + NO2-N + NH4-N). Levels of all three nutrients were far more variable in seeps than springs. NO3-N concentrations generally decreased with increased salinity while the reverse was true for SiO2. No significant correlation was found between RP and salinity.

Levels of all three nutrients in Tumon Bay seeps and springs were compared with those found in groundwater from 96 drinking water production wells located further inland. A comparison of frequency distribution histograms between beach and well data sets showed the measure of central tendency for RP in seep and spring samples was displaced to the right of that for the wells implying that the aquifer was not the only source of P into the bay. The reverse was found to be the case for NO3-N (as a result of conservative mixing in the transition zone) while measures of central tendency for Si were the same for both data sets.

Tumon Bay nearshore waters were collected daily from 9 sites over a three-month period. NO3-N and RP behaved conservatively in the surf zone and were rapidly diluted and dispersed. Close to 70% of all samples taken ~50 m offshore contained RP levels below the threshold concentration (~3 μg/L) required to promote macroalgae blooms, whereas only 20% were below the threshold concentration for DIN (~70 μg/L). Thus, while N was generally present in oversupply in this region of the bay, P levels were frequently limiting. Such findings highlight the dramatic effect relatively small anthropogenic inputs of RP could have on the abundance and distribution of E. clathrata in the bay. Hotel managers are, therefore, advised to pay close attention to the landscaping activities that go on in their grounds in order to eliminate, or at least minimize excess fertilizer and water applications to their lawns and gardens.

Gary R. W. Denton
Carmen M. Sian-Denton
Lucrina P. Concepcion
Rick H. Wood