Appendix A. Nitrogen loading rates in coastal systems and experimental manipulations in which we based are loading rates experimental design. Experimental design and calculation of the current fertilizer release rate.
TABLE A1. Table showing nitrogen loading rates in coastal systems and experimental manipulations. The mean (and range) N-loading rate for our experiment was 121 (24–234) µmol·m-2·d-1.
|Loading rates in coastal systems|
|French Mediterranean, Thau lagoon||0.59||De Casabianca 1996|
|RI coastal lagoon, Ninigret||0.6||Lee and Olsen 1985|
|RI coastal lagoon, Point Judith||0.67||Lee and Olsen 1985|
|HI, Kaneohe Bay||0.9||Smith 1979|
|HI, Kaneohe Bay||0.9||Smith 1979|
|RI coastal lagoon, Trustom||0.94||Lee and Olsen 1985|
|Spain, Alfacs Bay||1.6||Romero et al 1996|
|RI coastal lagoon, Potter||1.65||Lee and Olsen 1985|
|Apalachicola||1.9||Estimated from Nixon 1986|
|SW Australia, Peel-Harvey estuary, 1984||1.9||McComb and Humpries 1992, Lavery et al. 1991|
|SW Australia, Peel-Harvey estuary, 1986||1.94||McComb and Humpries 1992, Lavery et al. 1991|
|RI coastal lagoon, Green Hill||2.41||Lee and Olsen 1985|
|SW Australia, Peel-Harvey estuary, 1985||2.52||McComb and Humpries 1992, Lavery et al. 1991|
|SW Australia, Peel-Harvey estuary, 1982||2.61||McComb and Humpries 1992, Lavery et al. 1991|
|Narragansett Bay||2.7||Estimated from Nixon 1986|
|SW Australia, Peel-Harvey estuary, 1988||3.28||McComb and Humpries 1992, Lavery et al. 1991|
|SW Australia, Peel-Harvey estuary, 1981||3.78||McComb and Humpries 1992, Lavery et al. 1991|
|Denmark, Roskilde Fjord, Mollekrogen Bay||4.01||Flindt et al. 1997, Salomonsen et al. 1997|
|French Mediterranean, Prevost lagoon||4.5||De Casabianca 1996|
|Portugal, Mondego Estuary||7.25||Flindt et al. 1997|
|Italy, Venice lagoon||7.6||Sfriso et al. 2003a,b|
|Netherlands, intertidal and subtidal Lake Veere, 1992||7.82||De Vries et al. 1996, Malta and Verschuuure 1997|
|Netherlands, intertidal and subtidal Lake Veere, 1994||7.82||De Vries et al. 1996, Malta and Verschuuure 1997|
|SW Australia, Peel-Harvey estuary, 1987||9.2||McComb and Humpries 1992, Lavery et al. 1991|
|Greece, Gulf of Thessaloniki||10.27||Haritonidis 1996|
|UK, intertidal basin||14.76||Lowthion et al. 1985, Fletcher 1996|
|Chesapeake Bay||19.2||Estimated from Nixon (1986)|
|Thames Estuary||87.7||Estimated from Nixon (1986)|
|Loading rates in experimental manipulations|
|St Joseph Bay, USA||77–123||Heck et al. 2000|
|Bailey’s Bay, Bermuda||300||McGlathery 1995|
|Perdido Bay, USA||576–886||Heck et al. 2006|
|Coastal lagoon, Rhode Island||1971–3857||Harlin and ThorneMiller 1981|
|The present study||121 (24–234)|
Experimental design and fertilizer release rate
Fifteen PVC tubes, each containing 1000g DW of Osmocote TM (N: P molar ratio = 19:1) slow release fertilizer, were anchored to the bottom (i.e., within the seagrass canopy) and spaced evenly within each fertilized plot. Fertilization was carried out when seagrass growth was expected to be high (April 20th to October 15th 2005). Tubes were replaced three times during the experiment (June 6th, July 5th, and August 15th) to allow for continuous release throughout the experiment. Empty tubes were placed in the control plots to account for experimental artifacts (for instance, fish attraction to the bright PVC tubes). Other experiments have also used similar PVC tubes to fertilize seagrass beds (Heck et al. 2000).
To quantify the rates of nutrient release in our experiment, additional tubes with 1000 g DW fertilizer each were deployed 20 m away from the fertilized plots at the start of the experiment and with the first two fertilizer replacements. Three of these extra tubes were collected every 7–14 days from April through August 2005. The remaining fertilizer inside the tubes was dried to a constant mass, ground, dissolved, and diluted to analyze nutrient (nitrate, ammonium, and phosphate) concentrations per gram of fertilizer following the standard wet chemical technique of Strickland and Parsons (1972) modified for the Skalar San+ Autoanalyzer. This allowed us to calculate the mean amount of nitrogen and phosphorus released per tube per sampling interval, which was converted to an areal and daily basis (i.e., moles of nitrate, ammonium, phosphate released per square meter per day) by multiplying times the number of tubes per plot (15) and dividing by the plot area (30 m2) and number of days in the sampling interval. To confirm that the release rates obtained with the extra tubes corresponded to rates within the plots, we compared the mean dry mass of fertilizer remaining in four tubes haphazardly chosen in each plot at the time of fertilizer change with the mean dry mass of fertilizer remaining in the three extra tubes collected on that same date. Those means were never significantly different.
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