ANTHROPOGENIC ENRICHMENT ALTERS A MARINE BENTHIC FOOD WEB

The identification and understanding of shifts in resource availability and community structure caused by a variety of anthropogenic perturbations is essential to future rehabilitation efforts and recovery of essential ecosystem processes. We focus here on the contribution of nutrient enrichment to the overgrazing of macrophyte-dominated systems, which has often been linked to predator and competition release of urchins due to overfishing. However, the contribution of nutrient loading to the progression and per- sistence of this phenomenon should also be considered. In an effort to understand the contribution of nutrient loading to the overgrazing phenomenon and associated simplified community in a Caribbean seagrass bed, a detailed isotopic (d 13 C, d 15 N) assessment of the food web structure was conducted at both a nutrient-enriched and a control site. The general pattern at the enriched site indicated significantly lighter d 13 C signatures (i.e., phytoplankton influenced) for non-generalist primary consumers (i.e., specialist grazers, subsurface deposit feeders) and for the sediment organic material (SOM) when compared to the control site. The contribution of phytoplankton and associated particulate organic material to the SOM was also different, 7% vs. 44% at the enriched and control site, respectively. The loss of the autochthonous seagrass detritus pool, in the wake of high densities of generalist urchins (.66 000 individuals/ha) and low overall consumer diversity, appears to have been partially replaced by opportunistic ''alternatives,'' epiphytes but mainly phytoplankton, that benefit directly from elevated input of anthropogenic, allochthonous nutrients. The availability of such alternative, allochthonous resources to generalist urchins could potentially allow for the persistence of simplified seagrass communities. Here, elevated densities of urchins enable a persistent suppression of autochthonous benthic-macrophyte production through grazing and the consumption of newly recruited competitors and predators.

[1]  D. Phillips,et al.  Source partitioning using stable isotopes: coping with too many sources , 2003, Oecologia.

[2]  S. Bouillon,et al.  Carbon and Nitrogen Stable Isotope Ratios of Subtidal Benthic Invertebrates in an Estuarine Mangrove Ecosystem (Andhra Pradesh, India) , 2002 .

[3]  B. Lapointe.  Nutrient thresholds for bottom‐up control of macroalgal blooms on coral reefs in Jamaica and southeast Florida , 1997 .

[4]  S. Williams.  Competition between the seagrasses Thalassia testudinum and Syringodium filiforme in a Caribbean lagoon , 1987 .

[5]  A. Mazumder,et al.  Patterns of Algal Biomass in Dominant Odd‐ vs. Even‐Link Lake Ecosystems , 1994 .

[6]  M. Ray,et al.  Effects of a large herbivorous gastropod on macrofauna communities in tropical seagrass meadows , 1995 .

[7]  S. Nixon,et al.  “A strikingly rich zone”—Nutrient enrichment and secondary production in coastal marine ecosystems , 2002 .

[8]  T. S. Adams,et al.  The effect of dietary nitrogen content on trophic level 15N enrichment , 2000 .

[9]  Simone Mariani,et al.  Effects of sea urchin grazing on seagrass (Thalassodendron ciliatum) beds of a Kenyan lagoon , 2002 .

[10]  J. Rasmussen,et al.  QUANTIFYING ASSIMILATION OF SEWAGE‐DERIVED ORGANIC MATTER BY RIVERINE BENTHOS , 2002 .

[11]  Orlando Sarnelle,et al.  Nutrient Enrichment and Grazer Effects on Phytoplankton in Lakes , 1992 .

[12]  G. Polis,et al.  Food webs at the landscape level , 2004 .

[13]  J. Jackson.  What was natural in the coastal oceans? , 2001, Proceedings of the National Academy of Sciences of the United States of America.

[14]  I. Valiela,et al.  Changes in food web structure under the influence of increased anthropogenic nitrogen inputs to estuaries , 1998 .

[15]  J. Ogden,et al.  Ecological studies on the sea urchin, Lytechinus variegatus, and the algal-seagrass communities of the Miskito Cays, Nicaragua , 1982 .

[16]  D. K. Camp,et al.  Overgrazing of Seagrasses by a Regular Urchin, Lytechinus variegatus , 1973 .

[17]  I. Valiela,et al.  Linking nitrogen in estuarine producers to land‐derived sources , 1998 .

[18]  D. Phillips.  Mixing models in analyses of diet using multiple stable isotopes: a critique , 2001, Oecologia.

[19]  B. Menge,et al.  Top-down and bottom-up community regulation in marine rocky intertidal habitats. , 2000, Journal of experimental marine biology and ecology.

[20]  J. Jackson,et al.  Reefs since Columbus , 1997, Coral Reefs.

[21]  Frederick T. Short,et al.  Natural and human-induced disturbance of seagrasses , 1996, Environmental Conservation.

[22]  A. Mazumder,et al.  Sedimentation of algae: relationships with biomass and size distribution , 1996 .

[23]  D. Post,et al.  Detritus, trophic dynamics and biodiversity , 2004 .

[24]  T. Brey,et al.  Food web structure of the benthic community at the Porcupine Abyssal Plain (NE Atlantic): a stable isotope analysis , 2001 .

[25]  Robert J. Miller.  Seaweeds, Sea Urchins, and Lobsters: A Reappraisal , 1985 .

[26]  K. Bjorndal,et al.  Historical Overfishing and the Recent Collapse of Coastal Ecosystems , 2001, Science.

[27]  K. McGlathery,et al.  Nutrient and grazing influences on a subtropical seagrass community , 1995 .

[28]  C. Duarte,et al.  Patterns in leaf herbivory on seagrasses , 1998 .

[29]  Robert S. Steneck,et al.  Kelp forest ecosystems: biodiversity, stability, resilience and future , 2002, Environmental Conservation.

[30]  J. Cloern.  Our evolving conceptual model of the coastal eutrophication problem , 2001 .

[31]  D. Duggins.  Kelp Beds and Sea Otters: An Experimental Approach , 1980 .

[32]  A. Stoner,et al.  TROPHIC BIOLOGY OF STROMBUS GIGAS IN NURSERY HABITATS: DIETS AND FOOD SOURCES IN SEAGRASS MEADOWS , 1991 .

[33]  J. Lawrence,et al.  On the relationship between marine plants and sea urchins , 1975 .

[34]  M. Sullivan,et al.  Trophic importance of epiphytic algae in subtropical seagrass beds: evidence from multiple stable isotope analyses , 2001 .

[35]  C. Duarte,et al.  Diet and association of Pontonia pinnophylax occurring in Pinna nobilis: insights from stable isotope analysis , 2001, Journal of the Marine Biological Association of the United Kingdom.

[36]  K. McCann,et al.  Food Web Stability: The Influence of Trophic Flows across Habitats , 1998, The American Naturalist.

[37]  R. Michener,et al.  The role and contribution of the seagrass Posidonia oceanica (L.) Delile organic matter for secondary consumers as revealed by carbon and nitrogen stable isotope analysis , 2002 .

[38]  A. Siung-Chang.  A review of marine pollution issues in the Caribbean , 1997 .

[39]  Jefferies.  Allochthonous inputs: integrating population changes and food-web dynamics. , 2000, Trends in ecology & evolution.

[40]  C. Folke,et al.  Coral reef disturbance and resilience in a human-dominated environment. , 2000, Trends in ecology & evolution.

[41]  J. McClintock,et al.  Differential survivorship, reproduction, growth and nutrient allocation in the regular echinoid Lytechinus variegatus (Lamarck) fed natural diets , 1998 .

[42]  David M. Lodge,et al.  From Greenland to green lakes : Cultural eutrophication and the loss of benthic pathways in lakes , 2003 .

[43]  J. Finn,et al.  Nitrogen loading alters seagrass ecosystem structure and support of higher trophic levels , 2002 .

[44]  M. Vanderklift,et al.  Sources of variation in consumer-diet δ15N enrichment: a meta-analysis , 2003, Oecologia.

[45]  T. Ebert.  Negative Growth and Longevity in the Purple Sea Urchin Strongylocentrotus purpuratus (Stimpson) , 1967, Science.

[46]  R. Rosenberg,et al.  Macrobenthic succession in relation to organic enrichment and pollution of the marine environment , 1978 .

[47]  M. Régis.  Microstructure adaptative des radioles deParacentrotus lividus (Echinodermata: Echinoidea) en milieu eutrophisé par des eaux usées , 1986 .

[48]  E. Sala,et al.  Fishing, trophic cascades, and the structure of algal assemblages : evaluation of an old but untested paradigm , 1998 .

[49]  B. Fry,et al.  Detection of inconspicuous epiphytic algae supporting food webs in seagrass meadows , 1984, Oecologia.

[50]  J. Rasmussen,et al.  Comparison of aquatic food chains using nitrogen isotopes. , 1996, Proceedings of the National Academy of Sciences of the United States of America.

[51]  James W. Fourqurean,et al.  Overgrazing of a large seagrass bed by the sea urchin Lytechinus variegatus in Outer Florida Bay , 1999 .

[52]  Carlos M. Duarte,et al.  Submerged aquatic vegetation in relation to different nutrient regimes , 1995 .

[53]  G. Polis,et al.  THE ECOLOGY AND EVOLUTION OF INTRAGUILD PREDATION: Potential Competitors That Eat Each Other , 1989 .

[54]  L. Chauvaud,et al.  Marine eutrophication and benthos: the need for new approaches and concepts , 2002 .

[55]  G. Polis,et al.  Food Web Complexity and Community Dynamics , 1996, The American Naturalist.

[56]  Susan L. Williams.  Experimental Studies of Caribbean Seagrass Bed Development , 1990 .

[57]  R. Peters,et al.  Ecosystem differences in the trophic enrichment of 13C in aquatic food webs , 1997 .

[58]  S. Carpenter,et al.  Catastrophic shifts in ecosystems , 2001, Nature.

[59]  L. Pomeroy,et al.  Caught in the food web: complexity made simple? , 2001 .

[60]  D. Lirman,et al.  Destruction of Florida Bay seagrasses by a grazing front of sea urchins , 1999 .