Drift algae-epiphyte-seagrass interactions in a subtropical Thalassia testudinum meadow

Abstract

Determination of the percentage cover and residence time of drift algae over permanently marked 0.25 m 2 plots at 8 study sites indicated that drift algal cover was relatively low (< 20% at most sites) and did not remain in the same location longer than 2 to 3 mo. Experimental maintenance of high drift algal cover for 2 to 3 mo produced a ca. 25% reduction in above-ground biomass compared to plots free of drift algae, but the magnitude of the decline in biomass did not increase with an additional 3 mo of algal cover. Below-ground biomass and shoot density, however, were not affected by the presence of drift algae, suggesting that a change in plant morphometrics contributed to reduced above-ground biomass and limited long-term negative impacts as root reserves were not depleted. We investigated the effects of drift algal cover on short-term growth of Thalassia testudinum at 2 different sites in 2 different seasons. In neither instance did we observe any decrease in growth rates of T. testudinum in the presence of drift algae. The lack of a negative effect of drift algae on seagrass growth is likely due to the clonal nature of the seagrass plants, their ability to share resources among shaded and unshaded short shoots, and the availability of stored reserves in robust roots and rhizomes. We also conducted growth experiments assessing the effects of epiphytes on growth of T. testudinum. Unlike drift algal cover, the presence of encrusting calcareous epiphytes was shown to have a negative impact on growth of T. testudinum. This is likely due to a combination of decreased light levels as well as a reduction in exchange of materials across the boundary layer at the leaf surface. Collections of seagrass leaves from under drift algae and from areas free of drift algae indicated that filamentous epiphyte loads are decreased in the presence of macroalgae. Our results suggest that temporary, moderate cover of macroalgae may benefit seagrass by reducing epiphyte loads if the epiphyte cover negatively impacts the seagrass. KEY WORDS: Plant/plant interactions · Macroalgae · Seagrass · Epiphytes · Thalassia testudinum Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 279: [81][82][83][84][85][86][87][88][89][90][91] 2004 community (Fry 1984, Virnstein & Carbonara 1985, Heijs 1987, and provide additional structure to seagrass meadows and important habitat for numerous estuarine species (Kulczycki et al. 1981, Herrnkind & Butler 1986, Virnstein & Howard 1987, Schneider & Mann 1991, Holmquist 1994.Seagrass, epiphytes, and macroalgae occur together with their abundances influenced both directly and indirectly by top-down (grazing) and bottom-up (nutrient) controls (e.g. Neckles et al. 1993, Williams & Ruckelshaus 1993, McGlathery 1995, Hauxwell et al. 1998, Gacia et al. 1999, Heck et al. 2000. Under enhanced nutrient inputs typical of many coastal estuaries, blooms of epiphytes (Tomasko & Lapointe 1991, Neckles et al. 1993, Williams & Ruckelshaus 1993, Coleman & Burkholder...

Macroalgae blooms and their effects on seagrass ecosystems

“…The macroalgae blooms leading to light reduction for seagrasses are correlated to the duration of the macroalgal cover. Shoot growth does not increase over 10 days after the macroalgae are removed from the seagrass bed, and growth of Thalassia testudinum does not decrease over 10 days when macroalgae are present, suggesting that improving and declining light availability by drift algae does not affect the short-term growth of seagrasses (Irlandi et al, 2004). It may be because the shaded and unshaded shoots of seagrass can utilize the resources together.…”

“…Warmer species increase photosynthesis and respiration with higher temperature, but temperate species reach their photosynthesis optimum below the highest seasonal temperature (Short and Neckles, 1999). Sea level rise may decrease the light level for seagrasses, therefore negatively impacting seagrass productivity and functional values (Short and Neckles, 1999;Hauxwell et al, 2001;Irlandi et al, 2004). Sea level rise may increase water flow or enhance tidal circulation, which could accelerate flushing of the macroalgae, and therefore reduce the negative effects of macroalgae bloom on seagrass ecosystems (Flindt et al, 1997;Lenzi et al, 2003).…”

“…But in a long period, the macroalgae can significantly decrease the seagrass biomass. For example, under the conditions of 100% macroalgae canopy for 23 months, 25% above-ground biomass of seagrass is reduced (Irlandi et al, 2004).…”

Combined nutrient and macroalgae loads lead to response in seagrass indicator properties

“…5B). Nutrient enrichment of sediment drives plants to increase photosynthetic C fixation to meet the increased demand for C. However, if photosynthetic C fixation in leaf and rhizome tissues cannot meet the increased C demand, stored C in plant leaf tissues may be used to support metabolic processes (Burkholder et al, 1992;Irlandi et al, 2004;Lee and Dunton, 1999). This may have led to the shift in balance between aboveground and belowground biomass in our study.…”

Individual and population indicators of Zostera japonica respond quickly to experimental addition of sediment-nutrient and organic matter

“…Nitrogen fixation in seagrass sediments is dependent on photosynthates exuded by seagrass roots (Welsh et al, 1996(Welsh et al, , 1997. Nutrient enrichment increased seagrasses demand for carbon to support metabolic processes in anaerobic sediments with organic matter addition (Burkholder et al, 1992;Irlandi et al, 2004;Lee and Dunton, 1999), which may explain the negative effects of organic matter loading on seagrass growth during sediment-nutrient enrichment.…”