Introduction

The Attraction versus Production Debate

The tendency of fish and other motile animals to aggregate around structure, whether it is man-made, as in the case of artificial reefs, or natural, such as coral reefs, seagrass meadows, or a floating palm frond, has been well documented.  Artificial reefs create habitat complexity that can provide foraging and spawning sites for fish as well as refuge from predators. One important question about the value of artificial reefs is whether these structures merely concentrate existing fish populations rather than contribute to new fish production. Despite numerous studies, the question remains unresolved and has come to be known as the “attraction versus production debate”.  The attraction side of this debate argues that artificial reefs merely redistribute the extant fish standing stock without any increase in abundance.  In this case, artificial reefs will function as fish aggregating devices, and fish attraction to the reefs will be a result of behavioral preference. The production theory reasons that artificial reefs increase fish production by offering additional critical habitat and food that is limiting the abundance of the extant standing stock.  Fish that may be initially attracted to the reef as a result of instinctual behavior will find the necessary trophic resources they require for growth and reproduction, as well as additional refuge from predators. Artificial reefs intended for commercial and recreational fishing should result in enhancement of fish biomass through increased recruitment, decreased mortality, and/or increased growth in order to be considered successful. The focus of this study is on one aspect of the production debate, food availability, but it is important to remember that trophodynamics is only one factor influencing the production of fish species.

Trophodynamics

The success of artificial reefs in increasing fisheries production depends in part on the trophic resources required to support fish populations. The flow of energy between trophic levels begins with photosynthetic organisms that are able to synthesize organic carbon from inorganic carbon and nutrients using light energy. The study of primary production on and around artificial reefs is important since it will determine how much energy from in-situ carbon fixation is available to the reef fauna for survival, growth, and reproduction. Food webs can be benthic or plankton-based depending on whether the principle basal food source is benthic flora or phytoplankton in the water-column. Energy from primary production, whether benthic or planktonic, travels up the food web via grazer and/or detrital pathways. In the grazer pathway, the plant or algal material is consumed directly, while in the detrital pathway this material is colonized by bacteria and fungi, which break down the organic material making it easier for animals to digest as well as making it more nutritious.

In turbid waters having shallow photic zones, like the Mississippi Sound, benthic algae are limited by the quantity and quality of light.  Artificial reefs provide stable attachment sites for epilithic algae (i.e., growing on rocky substrates) and have surfaces higher up in the water column where more light may be available for photosynthesis. Marine periphyton (a complex of attached algae, bacteria, invertebrates, and detritus) forms quickly on submerged substrates and provides an additional food source in aquatic systems. Reef periphyton can provide a direct food source for grazing fish, crustaceans, mollusks, and worms, as well as an indirect energy source for higher level fish consumers through trophic transfer via grazing invertebrates and through their contribution of organic matter to the detrital chain. Primary production by phytoplankton also provides energy to higher trophic levels by supporting secondary production of filter feeding invertebrates or planktivorous prey fish, which make this energy available to their consumers. There are at least four abiotic factors that may limit primary productivity in the Sound. These include light quantity and quality, nutrient availability (N and P), temperature, and salinity. Seasonal and spatial variability in these factors influence the productivity and growth of autotrophs and can be used as indictors of community structure, abundance, and biomass at different times of the year and at different locations.

Purpose

The objective of this study is to estimate phycoperiphyton net primary production on four artificial reefs in Mississippi Sound, using settlement plate arrays as proxies for the reef community, in order to provide some insight into the value of these structures in increasing benthic primary production in this highly turbid, soft-bottomed lagoon. The specific goals of this project were to: (1) Determine seasonal, spatial, and vertical dynamics of community respiration, net productivity, chlorophyll-a, and organic biomass on the settlement plates over a one year study period. (2) Examine the seasonal and spatial patterns in water column respiration, net productivity, chlorophyll-a, and POM overlying the reefs. (3) Scale up the settlement plate and water column net productivity measurements to annual net habitat production on each reef by physically mapping the four study reefs and calculating their colonizable surface areas using ArcGIS. (4) Standardize the annual net benthic and water column productivity estimates for each reef, by area and volume respectively, in order to compare reefs and help resolve any differences due to reef elevation or location.