Kelp detritus: Unutilized productivity or an unacknowledged trophic resource?

Kelp beds are one of the most productive marine systems and, while little of this production is directly consumed, there is growing evidence that kelp detritus is an essential food source for many detrital and suspension feeders, and forms an important component of offshore sedimentary carbon pools. However, the extent of the contribution of kelp detritus to the nutrition of coastal fauna is not well resolved. In this study, we compare the contribution of phytoplankton, kelp detritus, and waste from fish cages to the diet of a sentinel suspension feeder, the blue mussel (Mytilus edulis) using stable isotopes. We found a significant depletion in both 13C and 15N in kelp tissue with age (distance from stipe to the deteriorating distal end of the kelp frond) which may have biased dietary estimates in previous studies which have applied isotopic source values derived from fresh kelp. Our mixing models indicate that macroalgal detritus formed 59% of the diet of the mussels in Berehaven, Bantry Bay, Ireland. We support the isotopic mixing model results by modelling the relative production of phytoplankton, kelp, and salmon farm waste, and found the supply of C and N from kelp and phytoplankton far exceeded the requirements of the mussels with much less coming from the nearby fish cages. Monthly chlorophyll measurements indicated there was only sufficient phytoplankton density to support mussel growth during the spring and autumn, explaining our observation of patterns in the relative importance of utilization of kelp detritus. Where there is pressure to harvest kelp beds, this study highlights the supporting ecosystem service they provide as an important dietary source in coastal food webs and emphasises the need for appropriate management measures for this resource.

Faunal mediated carbon export from mangroves in an arid area.

The outwelling paradigm argues that mangrove and saltmarsh wetlands export much excess production to downstream marine systems. However, outwelling is difficult to quantify and currently 40-50% of fixed carbon is unaccounted for. Some carbon is thought outwelled through mobile fauna, including fish, which visit and feed on mangrove produce during tidal inundation or early life stages before moving offshore, yet this pathway for carbon outwelling has never been quantified. We studied faunal carbon outwelling in three arid mangroves, where sharp isotopic gradients across the boundary between mangroves and down-stream systems permitted spatial differentiation of source of carbon in animal tissue. Stable isotope analysis (C, N, S) revealed 22-56% of the tissue of tidally migrating fauna was mangrove derived. Estimated consumption rates showed that 1.4% (38 kg C ha-1 yr-1) of annual mangrove litter production was directly consumed by migratory fauna, with <1% potentially exported. We predict that the amount of faunally-outwelled carbon is likely to be highly correlated with biomass of migratory fauna. While this may vary globally, the measured migratory fauna biomass in these arid mangroves was within the range of observations for mangroves across diverse biogeographic ranges and environmental settings. Hence, this study provides a generalized prediction of the relatively weak contribution of faunal migration to carbon outwelling from mangroves and the current proposition, that the unaccounted-for 40-50% of mangrove C is exported as dissolved inorganic carbon, remains plausible.

Growth and nitrogen uptake by Salicornia europaea and Aster tripolium in nutrient conditions typical of aquaculture wastewater.

The increasing need for environmentally sound aquaculture development can, in part, be addressed by using halophytic plants in integrated multitrophic aquaculture systems (IMTA) to remove waste dissolved nitrogen (N). However, knowledge of plant ability to take up nitrogen is of foremost importance to predict plants performance in such systems. Two species, Salicornia europaea and Aster tripolium, have been identified as potential candidates for IMTA due to their salt tolerance, potential N removal capabilities and their high commercial value as an additional crop. This study investigated the growth and N uptake rates of these two species under different N supply (NH4(+), NO3(-), NH4NO3). S. europaea plants produced a lower biomass when grown in NH4(+) compared to NO3(-) or NH4NO3, while A. tripolium biomass was not affected by the form in which N was supplied. N uptake in plants incubated at different concentrations of (15)N enriched solution (up to 2 mmol l(-1)) fitted the Michaelis-Menten model. While S. europaea NH4-N maximum uptake did not differ between starved and non-starved plants, A. tripolium NH4-N uptake was higher in starved plants when supplied alone. When NO3(-) was supplied alone, NO3-N maximum uptake was lower, for both species, when the plants were not starved. Comparison of starved and non-starved plants N uptake demonstrates the need for cautious interpretation of N uptake rates across different conditions. According to the observed results, both S. europaea and A. tripolium are capable of significantly high biomass production and N removal making them potential species for inclusion in efficient IMTA.

Flow regime in a restored wetland determines trophic links and species composition in the aquatic macroinvertebrate community.

In a restored wetland (South of Spain), where different flow regimes control water exchange with the adjacent Guadalquivir estuary, the native Palaemon varians coexists with an exotic counterpart species Palaemon macrodactylus. This controlled m\acrocosm offers an excellent opportunity to investigate how the effects of water management, through different flow regimes, and the presence of a non-native species affect the aquatic community and the trophic niche (by gut contents and C-N isotopic composition) of the native shrimp Palaemon varians. We found that increased water exchange rate (5% day(-1) in mixed ponds vs. 0.1% day(-1) in extensive ponds) modified the aquatic community of this wetland; while extensive ponds are dominated by isopods and amphipods with low presence of P. macrodactylus, mixed ponds presented high biomass of mysids, corixids, copepods and both shrimp species. An estuarine origin of nutrients and primary production might explain seasonal and spatial differences found among ponds of this wetland. A combined analysis of gut contents and isotopic composition of the native and the exotic species showed that: (1) native P. varians is mainly omnivorous (2) while the non-native P. macrodactylus is more zooplanktivorous and (3) a dietary overlap occurred when both species coexist at mixed ponds where a higher water exchange and high abundance of mysids and copepods diversifies the native species' diet. Thus differences in the trophic ecology of both species are clearly explained by water management. This experimental study is a valuable tool for integrated management between river basin and wetlands since it allows quantification of wetland community changes in response to the flow regime.

A multiple stressor approach to study the toxicity and sub-lethal effects of pharmaceutical compounds on the larval development of a marine invertebrate.

We studied the effects of three common pharmaceutical compounds on growth, development and body mass of larval stages of the marine shrimp Palaemon serratus at different temperatures and salinities. The pharmaceuticals compounds tested were the anti-inflammatory and analgesic diclofenac sodium, the lipid regulator clofibric acid and the fungicide clotrimazole. Neither diclofenac nor clofibric acid had any effect on growth, development or survival, although the maximum concentrations tested were 40 times higher than those observed in European coastal waters. Clotrimazole had significant effects at the higher concentration (2.78 µg L(-1)) when larvae were reared in full salinity sea water (32 PSU) and at the lower concentration (0.14 µg L(-1)) when larvae were reared at 20PSU. Changes in body mass at larval stage resulted from effects of these compounds on growth and developmental rates, specifically the changes in intermoult duration and in the number of larval instars required to reach the juvenile stage. The results demonstrate that the effects of emergent compounds on growth and development may be stronger when organisms are under some additional stress.

Halophyte filter beds for treatment of saline wastewater from aquaculture.

The expansion of aquaculture and the recent development of more intensive land-based marine farms require efficient and cost-effective systems for treatment of highly nutrient-rich saline wastewater. Constructed wetlands with halophytic plants offer the potential for waste-stream treatment combined with production of valuable secondary plant crops. Pilot wetland filter beds, constructed in triplicate and planted with the saltmarsh plant Salicornia europaea, were evaluated over 88 days under commercial operating conditions on a marine fish and shrimp farm. Nitrogen waste was primarily in the form of dissolved inorganic nitrogen (TDIN) and was removed by 98.2 ± 2.2% under ambient loadings of 109-383 µmol l(-1). There was a linear relationship between TDIN uptake and loading over the range of inputs tested. At peak loadings of up to 8185 ± 590 µmol l(-1) (equivalent to 600 mmol N m(-2) d(-1)), the filter beds removed between 30 and 58% (250 mmol N m(-2) d(-1)) of influent TDIN. Influent dissolved inorganic phosphorus levels ranged from 34 to 90 µmol l(-1), with 36-89% reduction under routine operations. Dissolved organic nitrogen (DON) loadings were lower (11-144 µmol l(-1)), and between 23 and 69% of influent DON was removed during routine operation, with no significant removal of DON under high TDIN loading. Over the 88-day study, cumulative nitrogen removal was 1.28 mol m(-2), of which 1.09 mol m(-2) was retained in plant tissue, with plant uptake ranging from 2.4 to 27.0 mmol N g(-1) dry weight d(-1). The results demonstrate the effectiveness of N and P removal from wastewater from land-based intensive marine aquaculture farms by constructed wetlands planted with S. europaea.

Digestion in relation to feeding strategies exhibited by crustacean larvae.

Decapod crustaceans have adopted a full range of reproductive strategies from the release of large numbers of small eggs (Penaeoidea) to the release of relatively low numbers of large advanced larvae (Nephropidae). As larval size determines trophic position in planktonic food webs, all food sources from phyto- to zooplankton are exploited, with many species changing trophic level during ontogenetic development. Comparative studies on digestive enzymes, levels of activity and changes during ontogeny, together with measurements of gastroevacuation rates and food energy values appear to reveal a general pattern. While herbivorous decapod larvae adapt to low food energy values with high enzyme activity levels, rapid food turnover and low assimilation efficiency, carnivorous larvae exhibit low levels of enzyme activity but compensate by extending retention time of high-energy food to maximise assimilation efficiency. New studies on digestive enzyme levels during development in the penaeid Litopenaeus vannamei, the caridean Lysmata debelius and the cirriped Elminius modestus, appear to agree with previous observations.