Spatial variation in basal resources supporting benthic food webs revealed for the inner continental shelf

Abstract

We used stable isotopes to examine the relative importance of phytodetritus and microphytobenthos (MPB) in supporting benthic food webs on the Louisiana continental shelf. Primary producers and macroinfauna were collected from Ship Shoal (SS), a submerged, sandy, barrier island where sediment-associated algae are primarily MPB and from silty, off-shoal areas where sediment-associated algae are primarily phytodetritus or a mixture of the two resources. Macroinfauna, as individual taxa and trophic guilds, were significantly more enriched in 13 C on SS indicating greater dependence on MPB, which is enriched in 13 C compared to phytoplankton. Using d 13 C in a two-source mixing model, the estimated MPB dietary contribution for SS macroinfauna averaged across trophic guilds, ranged from 53.4% to 83.0%, depending on the mixing-model end-members used, and 5 of 14 taxa had lower 95% confidence intervals (CI) . 20%, indicating that MPB was a primary resource for most SS macroinfauna. Off-shoal, the dietary contribution of MPB ranged from14.5% to 47.7% and most taxa had a lower CI near 0%. Settled phytoplankton are the primary microalgal food source in muddy sediments, but the importance of MPB increases in more sandy sediments where MPB are the predominant microalgal resource. Sandy shoals play a unique food-web role in deltaic shelf systems and support benthic food webs on the continental shelf.

What Is the Relative Importance of Phytoplankton and Attached Macroalgae and Epiphytes to Food Webs on Offshore Oil Platforms?

“…For example, δ 13 C of phytoplankton and red algae differed by a relatively large and statistically significant amount, and phytoplankton and epiphytes differed significantly in both δ 15 N and δ 13 C. In addition, temporal changes in isotopic composition were detected, and September δ 15 N values for some producers and consumers differed from those observed in other collections. Temporal shifts facilitate isotope tracking and can be powerful tools in resolving the relative importance of basal resources (McCutchan and Lewis 2002;Perga and Gerdeaux 2005;Grippo et al 2011) even when mixing models indicate high levels of overlap or when gut content data are rare (as is the case for many taxa here such as amphipods, copepods, and crabs). Here we use both lines of evidence to support the contention that phytoplankton rather than platform in situ primary production was the most important resource for platform-dwelling consumers both at the time of our September collection and over the succeeding 7 months.…”

Alternative approaches for solving underdetermined isotope mixing problems

“…Shifting the mixing axis from specific food sources to aggregated sources indicated by certain consumers (e.g. benthic versus pelagic consumers) is often a useful way to aggregate source data and find useful source information in food web data (Demo poulos et al 2008, Grippo et al 2011. Where food sources differ strongly in C, N or S element concentrations, alternative concentration-dependent modelling provides important alternative estimates of source contributions for food web problems (Phillips & Koch 2002, Bouillon et al 2008epa.gov/ wed/pages/models/stableIsotopes/ isoconc/isoconc1_01.htm).…”

“…Work with both the original and TEF-corrected graphs in the following, so that you know the effect trophic corrections have on your interpretations. If TEF effects are large for especially nitrogen, you may want to consider using carbon isotopes alone in mixing models (Sierszen et al 2006, Grippo et al 2011. In any event, you may want to publish one or both of these plots along with any output from IsoSource or SIAR, so that it is clearer how data are being interpreted.…”

Seasonal microphytobenthos on the hypoxic ­northern Gulf of Mexico continental shelf

“…The digestive tracts of some surface deposit-feeding polychaetes from one of these stations contained pennate diatoms (M. M. Baustian unpubl. data), as did those from nearby sites (Grippo et al 2011), and the density of these polychaetes decreases with hypoxia (Baustian & Rabalais 2009). Alternatively, with excess primary production from the water column, the benthic grazing pressure on microphytobenthos may be reduced (Grippo et al 2011).…”

“…data), as did those from nearby sites (Grippo et al 2011), and the density of these polychaetes decreases with hypoxia (Baustian & Rabalais 2009). Alternatively, with excess primary production from the water column, the benthic grazing pressure on microphytobenthos may be reduced (Grippo et al 2011). Suitable conditions for growth of benthic diatoms coupled with decreased grazing pressure would support a healthy microphytobenthic community, and especially larger pennate forms.…”