Microphytobenthos and benthic macroalgae determine sediment organic matter composition in shallow photic sediments

A K Hardison,M N Waters,E A Canuel,B Veuger,I C Anderson,C R Tobias

doi:10.5194/bg-10-5571-2013

Abstract

Abstract. Microphytobenthos and benthic macroalgae play an important role in system metabolism within shallow coastal bays. However, their independent and interactive influences on sediment organic matter (SOM) are not well understood. We investigated the influence of macroalgae and microphytobenthos on SOM quantity and quality in an experimental mesocosm system using bulk and molecular level (total hydrolyzable amino acids, THAA; phospholipid linked fatty acids, PLFA; pigment) analyses. Our experiment used an incomplete factorial design made up of two factors, each with two levels: (1) light (ambient vs. dark) and (2) macroalgae (presence vs. absence of live macroalgae). Over the course of the 42-day experiment, total organic carbon (TOC) and total nitrogen (TN) increased under ambient light by 173 ± 14 and 141 ± 7%, respectively, compared to in the dark (78 ± 29 and 39 ± 22%). THAA comprised a substantial fraction of SOM (~ 16% of TOC, 35% of TN) and followed TOC and TN accumulation patterns. Mole percent composition of the THAA pool indicated that SOM was composed of more labile organic material (e.g., L-glutamic acid, phenylalanine) under ambient light conditions while SOM in dark treatments was more degraded, with higher proportions of glycine and D-alanine. PLFA content, which represents viable biomass, made up ~ 1% of TOC and contained high levels of algal fatty acids in the light, particularly PLFA derived from diatoms. In the presence of microphytobenthos (i.e., light and macroalgae treatments), SOM lability increased, resulting in the observed increases in bacterial PLFA concentrations. Macroalgae, which were added to half of the light treatments, decreased SOM accumulation compared to light treatments without macroalgae, with TOC and TN increasing by only 130 ± 32 and 94 ± 24%, respectively. This decrease likely resulted from shading by macroalgae, which reduced production of microphytobenthos. The presence of macroalgae decreased SOM lability as well, which resulted in diminished buildup of bacterial biomass. By the final day of the experiment, principal component analysis revealed that sediment composition in treatments with macroalgae was more similar to dark treatments and less similar to light treatments without macroalgae. Overall, microphytobenthos and benthic macroalgae fundamentally altered SOM quality and quantity, which may have notable ecological consequences for shallow-water systems such as increased hypoxia/anoxia, sulfide accumulation, enhanced mineralization and/or stimulated denitrification.

Highlights

Concentrations of benthic chlorophyll a were higher for light treatments without macroalgae (“Light”) and with macroalgae (“Macro”) than for the dark treatment (“Dark”; Fig. 1b)
total nitrogen (TN) for all samples began at ∼ 14 μmol N gdw−1 and increased throughout the experiment, reaching levels of 35.2 ± 1.0, 28.4 ± 5.0, and 19.8 ± 3.1 μmol N gdw−1 on Day 42 for Light, Macro, and Dark treatments, respectively
TOC increased from ∼ 144 μmol C gdw−1 on Day 0 to 404.0 ± 20.6, 340.7 ± 67.0, and 248.6 ± 40.2 μmol C gdw−1 for Light, Macro, and Dark treatments, respectively, on Day 42

Summary

Introduction

In Waquoit Bay, MA, for example, ephemeral populations of green (Cladophora) and red (Gracilaria) macroalgae replaced Zostera marina seagrass when nutrient loadings increased six-fold (Hauxwell et al, 2003) The mechanisms underlying this shift in community structure relate to differences among plant types in nutrient uptake and growth strategies (Sand-Jensen and Borum, 1991; Nielsen et al, 1996). Macroalgae affect other primary producers directly through shading and/or competition for nutrients Because of their location at the sediment surface or floating just above the sediments, macroalgae may reduce the amount of light available for microphytobenthos, thereby decreasing or inhibiting microphytobenthic production (Sundback and McGlathery, 2005; Tyler et al, 2003; Valiela et al, 1997). In addition to their effects on light, macroalgae may out-compete microphytobenthos for water column nutrients, when microphytobenthos are nutrient limited, which may occur in sandy sediments during warm months (Nilsson et al, 1991; Sundback and McGlathery, 2005)

Objectives

Methods

Results

Conclusion