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1 Lafayette College, Easton, PA
2 Yale University, New Haven, CT
3 Boston University Marine Program, Woods Hole, MA
* Corresponding author: mirta{at}bu.edu
Growth of macroalgae in coastal environments is, to a large extent, limited by the available supply of nitrogen (1, 2). Macroalgal growth rates may be influenced by their internal pool of nitrogen, and also by the supply of new nitrogen provided by ambient water (3). Few studies have investigated the relative role of internal and external nitrogen supply on growth of macroalgae. In this study we investigate the net growth of Ulva lactuca, a widespread opportunistic species of macroalga found in the estuaries of Waquoit Bay, Cape Cod, Massachusetts, in response to internal and external nitrogen supply by using a field transplantation experiment and isotopic measurements (4).
Fronds of U. lactuca were collected from Sage Lot Pond, Quashnet River, and Childs River, subestuaries in the Waquoit Bay estuarine system. The land-use patterns on the watershed of these estuaries differ enough to lead to substantially different nitrogen loads of 14, 350, and 601 kg N ha-1 y-1, respectively (2). Fronds collected from these three estuaries will therefore have grown under different nitrogen regimes and have entered the experiment with different internal nitrogen contents. Two fronds from the same originating estuary were weighed (blotted wet weight) and placed inside cages constructed out of transparent disposable containers (GladWare) with two sides covered by mesh, allowing for water flow.
To assess the effect of ambient nitrogen supply on growth of U. lactuca, the algae collected from each estuary and placed in the cages were then transplanted into either Sage Lot Pond, which has the lowest nitrogen load (and hence the lowest nitrogen supply for fronds (5)), or Childs River, which has the highest nitrogen load (and highest supply of nitrogen), with 30 cages per estuary. The cages were randomly set 1 m apart within the existing macroalgal canopy, and 0.2 m above the bottom in locations with salinities between 25 and 30 ppt. This experimental design was deployed from 18 to 27 June 2003 and was repeated from 17 to 24 July 2003.
We measured the net growth response of U. lactuca by determining the initial and final wet weights of each algal frond within each cage. Growth data from both runs of the experiment were pooled to span possible differences across the months. Fronds were dried at 60 °C, ground, and sent to the Stable Isotope Facility, University of California, Davis, for analysis of carbon and nitrogen content and stable isotope signatures.
Net growth rate of U. lactuca depended on both the nitrogen pool within the fronds, obtained from the estuary from which the fronds were collected, and the nitrogen supply provided by the estuary to which the fronds were transplanted (Fig. 1A). Growth rates of U. lactuca collected from Sage Lot Pond were significantly lower than those achieved by fronds collected from Childs and Quashnet rivers when transplanted into the Childs River (ANOVA, F = 13.66, P = 0.000017). An ad hoc Duncan’s test showed no differences between growth rates of algae collected from the Childs and Quashnet rivers when transplanted into Childs River (ANOVA, F = 2.64, P = 0.11). Growth rates of fronds from all three estuaries transplanted into Sage Lot Pond were not significantly different (ANOVA, F = 0.71, P = 0.40). These results suggest that, first, fronds of U. lactuca grown in an estuary with nutrient-poor water grew slowly if at all, even when transplanted in nutrient-rich estuaries. Second, U. lactuca fronds from nutrient-rich estuaries grew faster, and more so when transplanted in nutrient-rich estuaries. Third, fronds from nutrient-poor estuaries lag considerably, even when transplanted into nutrient-rich water; this is evidence of some other impairment of growth.
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The relative importance of external nitrogen supply and internal nitrogen content were corroborated by the isotopic signatures (Fig. 1C). Fronds initially differed markedly in carbon and nitrogen isotopic signatures; signatures in fronds from Childs River and transplanted into Childs River were notably different from those of Sage Lot Pond fronds transplanted into Sage Lot Pond (Fig. 1C). Transplanted fronds soon reflected the signature of the nitrogen in the estuaries in which they were incubated, and showed less influence from the estuary of origin. Although the initial percent nitrogen may have slowed growth of Sage Lot Pond fronds (Fig. 1A, 1B), the nitrogen pools of the fronds from Childs River and from Quashnet River showed fast responses to the ambient nitrogen supply (Fig. 1C). Some factor other than internal pool size must be responsible for the lag in growth of Sage Lot Pond fronds.
The range of 
15N between Childs River and Sage Lot Pond has been reported (7), and is associated with different land-derived nitrogen loads, bearing different 15N signatures, arriving from the watershed to Childs River and Sage Lot Pond. The range of 13C values measured for U. lactuca in our study (-12
to -7) fall within the heavier part of the range (-35 to -5) reported in a compilation of such values for macroalgae (8). Various mechanisms, such as the use of HCO3- in photosynthesis (8, 9), have been proposed to explain the range of values, but further work is needed to understand the processes that control the values found in U. lactuca.
The isotopic conversion evident in Figure 1C suggests that there is relatively rapid turnover of the internal N pools of U. lactuca. For the 8-day duration of the transplant, and assuming linear growth, we roughly calculate that the nitrogen pool turns over completely every 12–15 d. This relatively rapid turnover seems consistent with our conclusion that external nitrogen supply plays a larger role than initial nitrogen content in the growth of this macroalgal species.
This research was supported by internships to A.B.A. from the Woods Hole Marine Science Consortium and to J.A.M. from a grant from the National Science Foundation’s Research Experience for Undergraduates (# OCE-0097498). This work was also supported by a grant from the National Oceanic and Atmospheric Association/NOS (ECOHAB award # NA16OP2728), and is ECOHAB publication #76.
Literature Cited
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