Nitrogen uptake and internal recycling in Zostera marina exposed to oyster farming: eelgrass potential as a natural biofilter

Oyster farming in estuaries and coastal lagoons frequently overlaps with the distribution of seagrass meadows, yet there are few studies on how this aquaculture practice affects seagrass physiology. We compared in situ nitrogen uptake and the productivity of Zostera marina shoots growing near off-bottom longlines and at a site not affected by oyster farming in San Quintin Bay, a coastal lagoon in Baja California, Mexico. We used benthic chambers to measure leaf NH4 (+) uptake capacities by pulse labeling with (NH4)-N-15 (+) and plant photosynthesis and respiration. The internal N-15 resorption/recycling was measured in shoots 2 weeks after incubations. The natural isotopic composition of eelgrass tissues and vegetative descriptors were also examined. Plants growing at the oyster farming site showed a higher leaf NH4 (+) uptake rate (33.1 mmol NH4 (+) m(-2) day(-1)) relative to those not exposed to oyster cultures (25.6 mmol NH4 (+) m(-2) day(-1)). We calculated that an eelgrass meadow of 15-16 ha (which represents only about 3-4 % of the subtidal eelgrass meadow cover in the western arm of the lagoon) can potentially incorporate the total amount of NH4 (+) excreted by oysters (similar to 5.2 x 10(6) mmol NH4 (+) day(-1)). This highlights the potential of eelgrass to act as a natural biofilter for the NH4 (+) produced by oyster farming. Shoots exposed to oysters were more efficient in re-utilizing the internal N-15 into the growth of new leaf tissues or to translocate it to belowground tissues. Photosynthetic rates were greater in shoots exposed to oysters, which is consistent with higher NH4 (+) uptake and less negative delta C-13 values. Vegetative production (shoot size, leaf growth) was also higher in these shoots. Aboveground/belowground biomass ratio was lower in eelgrass beds not directly influenced by oyster farms, likely related to the higher investment in belowground biomass to incorporate sedimentary nutrients

Inorganic carbon physiology underpins macroalgal responses to elevated CO2

Beneficial effects of CO2 on photosynthetic organisms will be a key driver of ecosystem change under ocean acidification. Predicting the responses of macroalgal species to ocean acidification is complex, but we demonstrate that the response of assemblages to elevated CO2 are correlated with inorganic carbon physiology. We assessed abundance patterns and a proxy for CO2:HCO3- use (\u3b413C values) of macroalgae along a gradient of CO2 at a volcanic seep, and examined how shifts in species abundance at other Mediterranean seeps are related to macroalgal inorganic carbon physiology. Five macroalgal species capable of using both HCO3- and CO2 had greater CO2 use as concentrations increased. These species (and one unable to use HCO3-) increased in abundance with elevated CO2 whereas obligate calcifying species, and non-calcareous macroalgae whose CO2 use did not increase consistently with concentration, declined in abundance. Physiological groupings provide a mechanistic understanding that will aid us in determining which species will benefit from ocean acidification and why

Epiphyte response to in situ manipulation of nutrient availability and fish presence in a Posidonia oceanica (L.) Delile meadow

Epiphytes are important components of the biomass and productivity of Posidonia oceanica meadows. Bottom-up mechanisms may promote epiphyte biomass through a bottom-up mechanism. At the same time, epiphytes represent an essential resource for higher trophic levels of seagrass food webs. Posidonia oceanica meadows host a diverse assemblage of fish that feed directly on the leaves, on the epiphytes, and on the mesograzers inhabiting the meadows. In this study, we experimentally evaluate the overall effect of fish community and increased water column nutrient availability on seagrass and the associated epiphytes. Our results show a large increase of epiphyte biomass in P. oceanica shoots after 2 months of nutrient addition. Increased nutrient concentrations also resulted in a reduction of shoot size. Fish exclusion did not affect epiphyte biomass under ambient or increased nutrient availability. Although herbivorous fish bites were present in 6% of the shoots, the percentage of shoots with fish bites did not respond to the higher leaf nutrient content found in the increased nutrient treatments. Consumption marks of gastropod herbivores in the leaves were present in 78% of the shoots; however, grazer activity did not modify the response of epiphytic biomass to nutrient addition. These results highlight the importance of nutrient impact in oligotrophic meadows, where bottom-up processes seem to be more relevant in the control of epiphyte and seagrass growth than top-down processes. © 2012 Springer Science+Business Media B.V.Peer Reviewe