Some Discussions

Synopsis of Findings

            This study revealed a seasonal trend in mean settlement plate net productivity where rates were greatest during cooler months, i.e., fall and winter, when water clarity was also highest. Mean chl-a concentrations on the settlement plates were also greatest in the fall; however, winter chl-a levels on the LRR (low relief reef) settlement plates did not follow the same trend, having equally low chl-a concentrations as those seen during the warmer, more turbid months. Unfortunately, winter data was not available for HRRs (high relief reef) and therefore it was not possible to determine if low winter chl-a concentrations occurred at those reefs as well.

            Net productivity was found to be significantly greater on the HRR settlement plates located further from shore compared to the nearshore LRR plates. This trend was not observed for chl-a concentrations, which were greater on centrally located reefs compared to western reefs, although Square Handkerchief reef did not have significantly lower chl-a concentrations than central reefs. However, patterns due to reef location and distance from shore are difficult to determine as they are confounded by the fact that one pair of LRR and HRR reefs are located in the central Sound and the other pair are located in the western Sound.

            There was a significant trend of decreasing Pn and chl-a with depth on the settlement plates, as expected. However, when the LRR reefs were analyzed separately, the decline in chl-a with depth was not significant. The vertical distribution of Pn and chl-a can be attributed to the rapid attenuation of light with depth due to the highly turbid waters of the Sound.

            Water column Pn and chl-a were greatest in the warmer months and lowest in the winter, exhibiting typical seasonal phytoplankton productivity and biomass patterns. POM concentrations exhibited an inverse seasonal pattern to that observed for chl-a, which is likely caused by changing composition of the organic biomass (i.e., autotrophs vs. heterotrophs). There were no significant differences in Pn, chl-a, or POM among reefs, but water column respiration rates were significantly higher at centrally located reefs compared to western reefs.

            As expected, annual net benthic production per unit area was greatest on HRRs compared to LRRs, while annual net water column production per unit volume was fairly similar among all four reefs. Annual reef net production, which accounted for both the benthic and water column sources, was greatest on HRRs and lowest on LRRS.

The Attraction versus Production Debate revisited

The intention of this study was not to attempt to provide an answer to the production versus attraction question, but rather to describe and estimate potential net autotrophic production associated with artificial reefs in the Sound. The study provides information that will be useful to researchers and fisheries managers investigating the effects of these reefs on populations of economically and recreationally important fish species. Nonetheless, some assumptions can be made based on the findings of this study regarding the function of Mississippi Sound artificial reefs as either fish aggregating devices or as critical habitat which provides food and shelter and thereby enhances fisheries production.

Simplified conceptual diagram of primary production and trophic transfer at HRRs (a) and LRRs (b). Green arrows indicate incorporation of phytoplankton primary production into the reef food-web by benthic filter feeders.

Artificial reefs in the Sound do not appear to augment benthic primary production in any considerable way; however the hard substrata provided by these structures offer settlement sites, which are naturally limiting in the Sound, for larvae of various benthic invertebrate species. Observations during this study indicate that the artificial reefs augment secondary production of habitat-limited, sessile filter-feeding invertebrates. These organisms act as a trophic link between phytoplankton production and higher-level consumers, such as the target fish species with which artificial reef construction is concerned. Although food does not appear to be limiting, artificial reefs are able to concentrate a variety of prey items for fishes, thus potentially increasing feeding efficiency. In addition, artificial reefs provide shelter, which can be considered a limiting resource in the interior of the Sound. Shelter is probably not limiting along the periphery of the Sound where seagrass beds bordering the barrier islands and salt marshes near the mainland provide essential habitat for many important marine organisms. Based on these observations, artificial reefs in the Mississippi Sound have the potential to increase fisheries production, but more detailed studies are needed to determine which scenario, attraction, production, or a combination of the two, best characterizes these reefs.

Conclusions

In conclusion, the results of this study suggest that artificial reefs in the Mississippi Sound do not substantially increase benthic primary production as a result of the poor water clarity. However, because these structures offer refuge in an otherwise barren, mud sediment landscape, they may still be beneficial to the enhancement of fish stocks. Furthermore, primary production in the water column appears to be sufficient to support a diversity of prey items on which higher level consumers can subsist. Although food is likely not a limiting factor for target fish species, they will find a forage base during their visits to the reefs. The mechanism(s) by which artificial reefs can enhance fish stocks is difficult to tease apart, especially for fishes with wide foraging ranges. The assessment of fish biomass enhancement as a result of artificial reef deployment must be considered at the species level and include food web and behavioral studies to determine what a particular species is feeding on, what the primary carbon sources are (e.g., phytoplankton, benthic algae on the reef, or terrestrial plant detritus), and where that species spends most of its time foraging and reproducing.