Incongruent long‐term trends of a marine consumer and primary producers in a habitat affected by nutrient pollution

Abstract

AbstractNutrient pollution, defined as excess nitrogen and phosphorus related to human activities, is a widespread anthropogenic impact that decreases water quality and limits light penetration in estuarine habitats. These factors can lead to shifts in primary producer communities, but the association between these changes and marine consumer patterns of abundance and growth is relatively unknown. Understanding these relationships is necessary for herbivorous species of conservation concern like the green turtle (Chelonia mydas). We used generalized additive models to analyze concurrent 18‐yr data sets of seagrass cover, drift macroalgae cover, juvenile green turtle abundance, and growth to (1) evaluate long‐term trends in primary producer cover and green turtle demographics; and (2) gain insights into their interrelatedness at a study site known to be affected by nutrient pollution throughout the study period. We found that seagrass cover sharply declined during our study period while macroalgae cover was more resilient with trends varying over time. Juvenile green turtle abundance slowly declined throughout the study period with altered intra‐annual trends, while growth rates remained relatively stable. The incongruence of these trends suggests that relationships between marine consumers and nutrient pollution are more complex than effects on food availability alone. Our results focus on the green turtle but highlight the need for greater understanding of relationships between marine consumer species and nutrient pollution at both local and regional scales.