Trophic importance of diatoms in an intertidal Zostera noltii seagrass bed: Evidence from stable isotope and fatty acid analyses

Abstract

A current predominant paradigm emphasizes the role of epiphytic algae for invertebrates in most seagrass food webs. However, in some intertidal Zostera noltii beds, epiphyte biomass is very low compared to microphytobenthos and seagrass biomasses. We assessed the role of microphytobenthos in a temperate intertidal Z. noltii bed by combining stable isotope and fatty acid (FA) analyses on primary producers, composite sources-suspended particulate organic matter (SPOM) and sediment surface organic matter (SSOM)-and the main macrofaunal consumers. Z. noltii showed high δ 13 C (−9.9‰) and high 18:2(n-6) and 18:3(n-3) contents. Microphytobenthos was slightly more 13 C-depleted (−15.4‰) and had high levels of diatom markers: 14:0, 16:1(n-7)c, 20:5(n-3). Low mean δ 13 C (−22.0‰) and large amounts of diatom and bacteria (18:1(n-7)c) markers indicated that SPOM was mainly composed of a mixture of fresh and decayed pelagic diatoms. Higher mean δ 13 C (−17.9‰) and high amounts of diatom FAs were found in SSOM, showing that microphytobenthic diatoms dominate. Very low percentages of 18:2(n-6) and 18:3(n-3) in consumers indicated a low contribution of Z. noltii material to their diets. Grazers, deposit and suspension-deposit feeders had δ 13 C close to microphytobenthos and high levels of diatom FAs, confirming that microphytobenthos represented the main part of their diet. Lower δ 13 C and higher amounts of flagellate FAs-22:6(n-3) and 16:4(n-3)-in suspension feeders indicated that their diet resulted from a mixture of SPOM and microphytobenthos. These results demonstrate that invertebrates do not consume high amounts of seagrass and highlight the main role of benthic diatoms in this intertidal seagrass bed.

Dissolved inorganic nitrogen uptake kinetics and δ15N of Zostera marina L. (eelgrass) in a coastal lagoon with oyster aquaculture and upwelling influence

“…In addition to differences in δ 15 N among plants from different sites, a clear intra-plant variation was detected, with belowground tissues showing consistently an enriched isotopic signal, by~1-2‰ with respect to leaf tissues; a similar enrichment has been observed for seagrasses in other studies (Lepoint et al, 2003;Lebreton et al, 2011). Although this can be partially explained by differences in the isotopic signal of the external DIN to which each vegetative compartment is exposed, other processes at plant physiological level may also be involved.…”

Assessing the dietary sources of two cichlid species in River Nile sub-branches: Stomach contents, fatty acids and stable isotopes analyses

“…On the other side, the low percentages of bacterial FAs and LCFAs (2.02%) and FA 6:0 (mean < 0.5%) suggest that detritus of M. spicatum was partially utilized via bacteria mediation, which corroborates the previous study of Lebreton et al (2011). When M. spicatum decomposed, LCFAs remain unchanged in planktonic and deposition detritus for more than 4 months, or even for years, whereas PUFAs including 18:3x3 are rapidly decomposed (Mfilinge et al, 2003).…”

“…Living diatom biomarkers were highly contributed to the cichlids' diets (total mean = 16.9% of both consumers profile), whereas decaying diatom biomarker was lower contributor (mean = 3.77% of both consumers profile). These results showed that diatoms are the main trophic resource in tilapias diets due to their high biomass, constant availability and their high nutritional quality (Lebreton et al, 2011). Nevertheless fatty acid profile of epiphytic diatoms was similar to that of cichlids but it was excluded as a potential food source because the carbon signature of epiphytic diatoms (d 13 C) was much enriched (mean = À21.95‰) compared to d 13 C of cichlids (mean = À26.68‰).…”

Resource utilization and trophic position of nematodes and harpacticoid copepods in and adjacent to &lt;i&gt;Zostera noltii&lt;/i&gt; beds

“…Several studies during the last decade have used natural stable isotope ratios to elucidate the principal food sources of macrobenthos in seagrass beds, stressing the importance of seagrass-associated sources and/or microphytobenthos (MPB) (Lepoint et al, 2000;Kharlamenko et al, 2001;Moncreiff and Sullivan, 2001;Baeta et al, 2009;Carlier et al, 2009;Lebreton et al, 2011;Ouisse et al, 2012;Vafeiadou et al, 2013a). Less information is available for meiobenthos resource utilization in seagrass beds (Vizzini et al, 2000b(Vizzini et al, , 2002aBaeta et al, 2009;Leduc et al, 2009;Lebreton et al, 2011Lebreton et al, , 2012, with none of the studies including meiofauna at the level of feeding types, families, genera or species.…”

“…Less information is available for meiobenthos resource utilization in seagrass beds (Vizzini et al, 2000b(Vizzini et al, , 2002aBaeta et al, 2009;Leduc et al, 2009;Lebreton et al, 2011Lebreton et al, , 2012, with none of the studies including meiofauna at the level of feeding types, families, genera or species. The few studies using natural isotope abundances to unravel food resources of coastal meiofauna at this level (Carman and Fry, 2002;Moens et al, , 2005Moens et al, , 2013Rzeznik-Orignac et al, 2008) do not examine seagrass habitats.…”