Cost-Effective Coastal Habitat Mapping:
Detecting Intertidal Polychaete Aggregations
with Low-Altitude Photogrammetry
Renata M. S. Alves, Carl Van Colen, Marijn Rabaut, Alain De Wulf, Magda Vincx, and Cornelis Stal
Abstract
Intertidal polychaete aggregations may be protected in the
European Union under the Habitats Directive framework
as reef habitats. Remote reef mapping remains challeng-
ing due to severe and dynamic conditions, as well as cover
and spatial resolution requirements. This study (1) evalu-
ated kite aerial photography and low-altitude digital pho-
togrammetry to map and monitor intertidal aggregations
of a sessile tube-building polychaete, Lanice conchilega
(L. conchilega), and (2) developed a protocol for its remote
identification. Monthly campaigns yielded 12 aerial im-
age sets which were processed using structure-from-motion
into high-precision digital terrain models and orthophoto
mosaics. Maximum likelihood classification distinguished
L. conchilega from bare sediment with an accuracy of 70%
± 23.2%. Aggregations were delineated by extracting ele-
ments of positive elevation from local difference models. The
method has proven useful to detect high-value aggregations,
distinguishing these consistently. Nevertheless, systematic
biases were present during delineation, and further charac-
terisation of reference aggregations may improve detection.
Introduction
Hard compact substrata of biogenic or geogenic origin, set on
solid and soft bottoms, and arising from the sea floor in the
sublittoral and littoral zone are protected in the European
Union under the Habitats Directive framework as reef habitats
(92/43/EEC) (Hendrick and Foster-Smith
implementation of the Directive is often
tions in large-scale identification and m
types (Evans 2006). Unsurprisingly, remote sensing meth-
ods have recently been employed to detect and differentiate
organisms from their surroundings (e.g., Dekker, Brando, and
Anstee 2005; Mumby
et al.
1997; Sanchez-Hernandez Boyd,
and Foody 2007). However, remote mapping and monitoring
of reefs remains challenging since potential methods must
resist severe and dynamic climatic conditions at the coast
(Goodman, Purkis, and Phinn 2013) as well as cover large ar-
eas at very high spatial resolutions (i.e. < 0.5m)—e.g., Degraer
et al.
(2008), Hendrick and Foster-Smith (2006), and Rabaut,
Vincx, and Degraer (2009).
Lanice conchilega
(
L. conchilega
)
(Pallas 1766) is a sessile
tube-building worm abundant on European coasts (Godet
et
al.
2008). It forms tube aggregations that can reach up to 30
000 ind·m
-2
(Alves
et al.
2017), protrude up to 16 cm in height
(Rabaut, Vincx, and Degraer 2009), and spread across approx.
15 m
2
in area (Degraer
et al.
2008). Past certain densities,
these aggregations significantly attenuate water flow (Fried-
richs, Graf, and Springer 2000), increasing sedimentation
(Borsje
et al.
2014) and modulate local habitat conditions
(Rabaut, Vincx, and Degraer 2009). This positively affects
macrobenthic abundance and species richness, as well as epi-
and hyperbenthic abundance (De Smet
et al.
2015), and the
development of surficial photosynthesising microorganisms
(Passarelli
et al.
2012). These effects may grant
L. conchilega
aggregations reef status (Rabaut, Vincx, and Degraer 2009),
making them eligible for conservation (Godet
et al.
2008).
Current monitoring frameworks incorporate mapping
technologies that enable not only habitat mapping but also in-
ferences on how organisms impact the ecosystem (Maes
et al.
2016). Recently developed guidelines to evaluate polychaete
aggregations as reef habitats focus on their likeness to a set of
reef-like features closely related to their ability to modulate
aspects of the environment (see Hendrick and Foster-Smith
2006; Rabaut, Vincx, and Degraer 2009). Features include av-
gregation relative elevation, aggregation
sediment consolidation, fragmentation,
galef’s index, and longevity (Rabaut,
Vincx, and Degraer 2009). However, feature evaluation and
identification through current guidelines remains restricted
due to difficulties in assessing large areas for long-term moni-
toring (Godet
et al.
2008). Low-altitude photogrammetry is a
very promising option due to its ultra-high spatial resolution
and robustness (Aber, Marzolff, and Ries 2010), and allows
the construction of digital elevation models (
DEMs
) and ortho-
photos. Coastal applications include assessing spatial pat-
terns in species distribution along coastlines (e.g., Bryson
et
al.
2013; Guichard, Bourget, and Agnard 2000; Mumby
et al.
2004) and further offshore (e.g., Magome
et al.
2007).
To the best of our knowledge, low-altitude photogramme-
try methods have yet to be employed in assessing the distri-
bution of polychaete aggregations on coastal environments.
The present study aimed to (1) evaluate the use of kite aerial
photography (
KAP
) and low-altitude digital photogrammetry
Photogrammetric Engineering & Remote Sensing
Vol. 85, No. 12, December 2019, pp. 899–905.
0099-1112/19/899–905
© 2019 American Society for Photogrammetry
and Remote Sensing
doi: 10.14358/PERS.85.12.899
Renata M. S. Alves (corresponding author) is with Biosciences
Institute of São Paulo University, Landscape Ecology and
Conservation Laboratory and Ghent University, Marine
Biology Research Group (Biology Department).
Carl Van Colen is with Ghent University, Marine Biology
Research Group (Biology Department).
Marijn Rabaut is an Independent Expert – Marine and
Renewables.
Alain De Wulf is with Ghent University, 3D Data Acquisition
Research Unit (Geography Department).
Magda Vincx is with Ghent University, Marine Biology
Research Group (Biology Department).
Cornelis Stal is with Ghent University, 3D Data Acquisition
Research Unit (Geography Department) and University College
Ghent, Department of Real-estate and Applied Geomatics.
PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
December 2019
899
