Nutrient inputs to coastal waters have increased in coastal environments worldwide as a direct consequence of the growing human population and increased settlement and use of coastal areas (Nixon et al., 1986; Valiela, 2006). These changes in nutrient availability lead to increased eutrophication, a growing threat facing coastal ecosystems (National Research Council (NRC), 2000; Bricker et al., 2008). One common symptom of eutrophication is profuse blooms of marine seaweeds, or macroalgae (Lavery et al., 1991; Sfriso et al., 1992; Valiela et al., 1997; Morand & Merceron, 2005; Fox et al., 2008; Fig. 1a), a feature that has received wide press and public notice (e.g. New York Times, July 1, 2008; Naples Daily News, March 15, 2006; Boston Globe, September 27, 2001), and is widespread along the coasts of the world (Table 1; Raffaelli et al., 1998; Morand & Merceron, 2005).
|Site||Seaweed taxa*||Some effects||References|
|Nahant Bay, USA||Pilayella||Noxious odor, accumulated on beaches, nuisance to swimming and fishing||Wilce et al. (1982), Pregnall & Miller (1988)|
|Waquoit Bay, USA||Cladophora, Gracilaria, Ulva||Replaced seagrasses, anoxia, shell- and fin-fish kills||Valiela et al. (1997), Hauxwell et al. (2001), Fox et al. (2008)|
|Hog Island Bay, USA||Ulva, Gracilaria, Codium||Loss of species diversity||Thomsen et al. (2006)|
|San Francisco Bay, United States||Ulva||Anoxia, replaced benthic fauna||Fong et al. (1996)|
|Kanehoe Bay, Hawaii||Dictyosphaeria||Replaced corals||Smith (1981)|
|Southeast Florida, USA||Codium||Impact coral reefs||Lapointe et al. (2005)|
|Bermuda||Cladophora, Laurencia, Codium||Anoxia, reduced benthic diversity and commercial fisheries||Lapointe & O'Connell (1989)|
|Laholm Bay, Sweden NW Baltic Sea||Ulva, Cladophora||Replaced seagrasses, nuisance to swimming fishing and boating||Baden et al. (1990), Rosenberg et al. (1990)|
|Maasholm Bay, Germany||Ulva, Pilayella||Replaced native macroalgae, lowered benthic diversity and fishery yield, nuisance to swimming, fishing and boating||Lotze et al. (2000), Worm et al. (1999)|
|Mondego Estuary, Portugal||Ulva||Replaced seagrasses, reduced benthic diversity||Martins et al. (2001), Cardoso et al. (2004)|
|Venice Lagoon, Italy||Ulva, Gracilaria, Dictyota||Anoxia, fish kills, nutrient release, phytoplankton blooms||Sfriso et al. (1992), Sfriso & Marcomini (1997)|
|Gulf of California, Mexico||Ulva, Gracilaria, Cladophora||Anoxia, loss of species diversity||Ochoa-Izaguirre et al. (2002), Piñon-Gimate et al. (2008)|
|Nuevo Gulf, Patagonia||Ulva, Undaria||Accumulated on beaches, interferes with recreational uses||Díaz et al. (2002), Piriz et al. (2003)|
|Quingdao, China||Ulva||Loss of species diversity, accumulated on beaches and nuisance for recreational activities||Liu et al. (2007, 2009)|
|Seto Inland Sea, Japan||Ulva||Replaced seagrasses||Sugimoto et al. (2007)|
|Peel-Harvey Estuary, Western Australia||Cladophora, Ulva, Chaetomorpha||Accumulated on beaches||Lavery et al. (1991)|
|Tuggerah Lakes Estuary, New South Wales||Ulva||Replaced seagrasses, reduced benthic diversity||Cummins et al. (2004)|
Macroalgal blooms have many detrimental effects. Seaweed wrack accumulates along shorelines and produces foul odors (Wilce et al., 1982), deep canopies of seaweeds physically obliterate other coastal life (Hauxwell et al., 2001), and decay of algal organic matter fosters anoxic conditions that lead to fish and shellfish kills (Baden et al., 1990; Valiela et al., 1992; D'Avanzo et al., 1996; Worm et al., 1999; Diaz, 2001). Macroalgal blooms not only make coastal environments increasingly undesirable for human uses and threaten commercial harvests, but also drastically restructure natural communities and ecosystem function of affected environments (Duarte, 1995; Valiela et al., 1997; Raffaelli et al., 1998; Oesterling & Pihl, 2001; Fox et al., 2009, in press).
It has been argued that N supply is a main control on macroalgal growth in temperate coastal areas (Nixon & Pilson, 1983; Oviatt et al., 1995; Howarth et al., 2000). In tropical latitudes carbonate sediments derived from coral reefs may sequester phosphate (PO43−) and may lead to P limitation of macroalgal growth (Lapointe et al., 1992; McGlathery et al., 1994), but other studies show exceptions to this general pattern (Larned, 1998; Fong et al., 2001; Elser et al., 2007), and much research is still needed to better understand the processes driving coastal eutrophication and strategies for water quality management (NRC, 2000; Smith & Schindler, 2009).
External N supply interacts with internal N pools in macroalgae (Fujita, 1985; Bjornsater & Wheeler, 1990; Fong et al., 2003; Teichberg et al., 2008) to create different growth responses (Fujita, 1985; Pedersen & Borum, 1996; Teichberg et al., 2008). The sources of internal N pools have been assessed with stable isotopic methods (McClelland & Valiela, 1998; Aguiar et al., 2003; Savage & Elmgren, 2004; Cole et al., 2005; Teichberg et al., 2007, 2008). Some macroalgae reflect N isotopic signatures of their source with little fractionation, making them potential indicators of anthropogenic nutrient inputs (Savage & Elmgren, 2004; Deutsch & Voss, 2006; Thornber et al., 2008).
No comprehensive study of algal blooms and algal physiological responses has been done on a global scale across latitudes and oligotrophic to eutrophic conditions to understand nutrient limitation and the potential for macroalgal blooms worldwide. This is particularly important at this time as nutrient additions to estuaries and coasts increase, and eutrophication becomes one of the greatest threats to our coasts and estuaries (NRC, 2000; Howarth, 2008). Macroalgal blooms involve relatively few taxa (Valiela et al., 1997; Morand & Merceron, 2005) that are widely distributed throughout the coasts of the world. In particular, species of Ulva can be found in many coastal waters (Table 1), thus providing a useful biological model to make geographical comparisons as to the nutrient controls on growth of bloom-forming macroalgae.
In this study we examined the relationship of macroalgal growth, internal nutrient pools, and natural stable isotopes of Ulva spp. to different ambient nutrient supplies and tested whether additional N or P above ambient nutrient regimes increased macroalgal growth, increasing likelihood of blooms. We measured growth responses and nutrient content of fronds of local Ulva spp. in each of seven coastal systems, including three subestuaries within one estuarine system, for a total of nine sites where ambient nutrient supplies differed. From these experiments, we compared the relative growth responses across latitudes under oligotrophic to eutrophic conditions and under additional N and P enrichment.